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J. Phys. Chem. A 2003, 107, 5342-5351
Binary Phases of Aliphatic N-Oxides and Water: Force Field Development and Molecular
Dynamics Simulation

Kristine M. Kast,†,‡ Ju
1 rgen Brickmann,† Stefan M. Kast,*,†,§ and R. Stephen Berry§
Physikalische Chemie I, Technische UniVersita¨t Darmstadt, Petersenstrasse 20, 64287 Darmstadt, Germany,and Department of Chemistry, The UniVersity of Chicago, 5735 South Ellis AVenue, Chicago, Illinois 60637 ReceiVed: October 29, 2002; In Final Form: April 23, 2003 Aliphatic N-oxides as cosolvents with water play an important role in stabilizing and destabilizing the structureof biopolymers such as cellulose and proteins. To allow for detailed microscopic investigations, an empiricalforce field to be used in molecular simulations is developed for two N-oxide species, N,N,N-trimethylamine-N-oxide (TMAO) and N-methylmorpholine-N-oxide (NMMO). The intra- and intermolecular force field isparametrized mainly on the basis of quantum-chemical calculations and is tested against available experimentalspectroscopic, crystallographic, and liquid state data. Special emphasis is put on the identification of transferablepotential terms in order to guide future parametrization of other species. By construction, the force field iscompatible with widely used potential functions for proteins and carbohydrates. With the resulting parameterset, molecular dynamics simulations are carried out on binary mixtures of water and N-oxides, revealingstructural features and the influence of intramolecular N-oxide flexibility. Limitations and possible extensionsof the presented models are also discussed.
I. Introduction
TMAO-water interaction potential comprises a modified Cou-lomb term and a r-10/r-4/r-2 expression covering dispersion The unique properties of solvent mixtures as compared to and repulsion where r means the site-site distance and has not pure phases play an increasingly important role for industrial been tested with respect to its performance for reproducing applications, e.g., for tuning solubility or reactivity by introduc-ing the mole fraction as an additional control variable. Further- condensed phase experimental data. The force field was then more, the biochemical relevance of dissolved compounds in used in MD simulations of a single TMAO molecule in water.20 water is an important aspect of current research on protein Zou et al. applied the force field with some adjustments in stability and biomolecular recognition. Tertiary aliphatic N- simulations at finite TMAO concentrations.19 They found some oxides are remarkable species in these respects: Some are evidence regarding changes of water-water structure and known as good cosolvents with water for dissolving cellulose dynamics due to the N-oxide presence and related this result to fibers,1-6 increasing the reactivity of the swollen cellulose the protein stabilization effect. Besides this early TMAO force material for further derivatization in pollution-free industrial field, quite recently a model potential function for studying fiber processing. For instance, N-methylmorpholine-N-oxide intramolecular H-bond dynamics in picolinic acid N-oxide has (NMMO) in water dissolves cellulose, whereas N,N,N-tri- been constructed and applied to the computational treatment of methylamine-N-oxide (TMAO) does not.7 On the other hand, TMAO abounds in marine organisms as an osmolyte counter- In this work, a force field for two prototypical N-oxides, acting protein denaturation provoked by urea and related osmotic TMAO and NMMO, is developed and tested for its capability water stress8,9 or by high-pressure conditions.10 TMAO even to reproduce experimental data. It is intended to be balanced in appears to play a role in possible therapies for Alzheimer’s the sense of satisfying several requirements: (1) The functional disease.11 Explanations on the molecular level for these phe- form and the parameters should be compatible with common nomena are only beginning to surface.12-19 water models and biopolymer force fields such as CHARMM23-25 To investigate into the molecular mechanism of these effects for proteins and extensions for carbohydrates;26 (2) the force by computational methods such as molecular dynamics (MD) field should be as simple as possible to avoid overly expensive simulation techniques, a force field for mixed solvents composed computations for simulating the solvent, yet account for of water and N-oxide species is required that is also compatible intramolecular flexibility; (3) certain terms in the force field with available biopolymer potential energy functions. Noto et that represent topologically similar units should be attributed al.20 were the first who constructed a force field for a rigid identical, i.e., transferable parameters guiding future parametri- TMAO model in the presence of an aqueous environment based zation of other N-oxide species; (4) it should be applicable for on quantum-chemical calculations of TMAO and a single water a range of different situations such as various concentrations.
molecule within the Hartree-Fock (HF) approximation. The Because experimental information about N-oxide systems is verylimited, the parametrization relies mainly on quantum-chemical * To whom correspondence should be addressed. E-mail: kast@ calculations. The adequate approximations, like basis set and pc.chemie.tu-darmstadt.de. Phone: +49 6151 165397. Fax: +49 6151164298.
inclusion of electron correlation, have been outlined in the past † Technische Universita¨t Darmstadt.
by some of us for a number of different N-oxide/water systems.27 ‡ Present address: T-Systems GEI GmbH, Goebelstr. 1-3, 64293 Although several parameters can be directly deduced from these resources, the parametrization of a solvent force field containing Binary Phases of Aliphatic N-Oxides and Water J. Phys. Chem. A, Vol. 107, No. 27, 2003 5343
flexible molecular entities constitutes a major challenge. In thenext sections, we first describe the model function and thestrategy toward useful parameters along with the results. Themodel is validated by comparison with crystallographic andspectroscopic as well as liquid state data from experiments andis finally applied to equimolar N-oxide/water mixtures, revealingliquid-phase structural features and the influence of intramo-lecular flexibility.
Figure 1. Structure and site indices for TMAO (a) and NMMO (b).
II. Force Field Parametrization
structure and energetics of N-oxide hydrates from ab initio andexperimental crystal data. Second, the torsional potentials were (a) Outline. Force field development, particularly for complex
fitted to quantum-chemical barriers, and finally, the remaining condensed phase systems, is a challenging task, often guided valence force field terms were determined by adjusting to more by experience in conducting the appropriate steps than structural and vibrational results again from ab initio calcula- by straightforward recipes; for recent reviews, see refs 28-30 tions. The final potential was then tested against experimental The model potential used in this work has the form condensed phase properties by MD simulations. The force fieldis largely inspired by the CHARMM approach,23-25 in particular we use, at least as initial estimates, known parameters from the CHARMM force field whenever possible. By construction of the consecutive parametrization steps, the nonbonded parameters influence the intramolecular ones and not vice versa, so some (r - rac)2 + ∑ ∑kabcd[1 + cos(- τabcd)] + compatibility to other force fields based on pairwise site-site intermolecular interactions can be expected. Parameters that turn out to be very similar upon individual optimization of similar molecular groups will be set equal if possible without significantloss of accuracy, thereby allowing for the identification of basic where the first three terms define an intramolecular valence force building blocks to be used in other N-oxides. The key develop- field and the last term the nonbonded contributions, including ments involve the region around the N-O bond that is difficult intermolecular interactions. Superscripts a-d denote atomic types by requirements of topological equivalence, and subscripts Quantum chemical ab initio calculations were performed with i-l refer to particular atomic site indices. The intramolecular the Gaussian suite of programs.33 The appropriate level of theory valence force field consists of harmonic terms for bond has been analyzed in depth in an earlier study:27 Although the stretching (site distance rij, force constant kr, and equilibrium HF approximation with the 6-31G** basis set is suitable for distance r0) and angle bending (bend angle R, force constant pure compounds, water complex properties need computations kR, and equilibrium angle R0), and a torsional potential defined on the MP2 level (Møller-Plesset perturbation theory to second over cosines of the dihedral angle τ (multiplicity n, phase τ0, order) for correctly representing experimental H-bond energies.
and torsional parameter ,n that is just half the energy barrier).
N-oxide/water complex interactions energies were corrected by For fine-tuning the normal frequencies, additional Urey-Bradley the basis set superposition error (BSSE) according to Boys and terms are introduced comprising a harmonic potential along the Bernadi.34 MD simulations were conducted in the isothermal- distance between the first and the third atom of a bend angle.
isobaric (NpT) ensemble35,36 at a pressure of 1 bar and various The nonbonded potential is described basically by the sum of temperatures, using a time step of 1 fs and periodic boundary Coulomb interaction (nonpolarizable partial site charges q, conditions throughout and applying distance constraints when dielectric constant 0) and a Lennard-Jones term (well depth , necessary.37,38 The numerical parametrization work was done contact distance σ; standard Lorentz-Berthelot combinations for a large part with a dynamical simulated annealing optimiza- ab ) ( a b)1/2, σab ) (σa + σb)/2) (b) Partial Charges. Based on results of earlier ab initio
investigations,27 atomic site charges were determined by fitting 4 ab[(σab)12 - (σab)6] to the electrostatic potential (ESP charges) rather than frompopulation analysis. The latter (see also ref 20) yields quite for all atoms pairs in different molecules as well as in the same unphysical values particularly for the central N atom that carries molecule if they are separated by three or more bonds.
a positive formal charge. We used gas phase results instead of, Throughout, nonbonded interactions except for intramolecular for instance, quantum-chemical reaction field techniques to allow distances are modified by multiplication with23,32 (1 - (rij/rc)2)2 for electronic polarization due to the environment in order to using a truncation distance rc discussed later. For reasons of maintain compatibility with the parametrization strategy com- computational performance, this form has been used for both monly used for solute species. Furthermore, for such a nonpo- the Lennard-Jones and the Coulomb term; the energetic differ- larizable force field as used in this work, we would need ence as compared to applying more elaborate and computa- representative environment models for a broad range of molar tionally more demanding Lennard-Jones shifting techniques32 ratios between solvent and cosolvent. To allow for rotatable is negligibly small for such strongly polar systems.
methyl groups the hydrogen charges were averaged, also for An appropriate strategy for finding suitable parameters for the NMMO methylene groups. Table 1 shows the dipole the model compounds TMAO and NMMO (the structure and moments for both the HF and the MP2 level of theory together site numbering is shown in Figure 1) consists of several with result from the respective point charge distribution, and consecutive stages: First, the site charges were determined from experimental values40,41 for TMAO. As can be seen, dipole quantum-chemical ab initio calculations of isolated N-oxides; moments taken directly from the wave function are quite similar the remaining nonbonded parameters were adjusted to represent for HF and MP2, and the HF result is closer to the experimental 5344 J. Phys. Chem. A, Vol. 107, No. 27, 2003
TABLE 1: Dipole Moments µ Resulting from Wave
Function and Point Charge Distributions on Various Levels
of Theory

TABLE 2: Assignment of Atom Types and Partial Charges
to the N
-Oxidesa
Figure 2. Superposition of optimized ab initio and force field structures
of dihydrates of TMAO (a) and NMMO (b).
TABLE 3: Lennard-Jones Parameters of the Atom Types
calculations are 0.045 Å for TMAO and 0.056 Å for NMMO a Subscripts a-c denote atoms in topologically equivalent methyl/ with the final Lennard-Jones parameters summarized in Table 3. The BSSE-corrected quantum-chemical and resulting forcefield interaction energies (computed on the ab initio structures) values for TMAO. However, the HF point charge result deviates are for TMAO dihydrate -20.04 and -19.57 kcal mol-1, strongly in the TMAO case. Therefore, the HF-derived point respectively, and for NMMO dihydrate -18.70 and -19.31 kcal charges were used for NMMO only and the MP2 ones for mol-1. A number of other mono- and dihydrate structures have TMAO. The final charges are given in Table 2 along with the been computed yielding a rms deviation between BSSE- assigned atom type symbols used later.
corrected MP2 energies and force field values of around 0.9 With the resulting partial charges, the potential truncation kcal mol-1, so no further parameter adjustment was deemed distance was optimized using a procedure developed by Dufner et al.:42 Forces and energies from direct summation of the It turned out that the force field dihydrate structures tend to shifted-force potential were compared with Madelung values break Cs symmetry by bending the water planes synchronously from Ewald summation43 in the case of the experimental TMAO toward the methyl/methylene groups plane upon geometry crystal structure.44 For a cutoff distance, rc, of 13 Å, directly optimization, regardless of the parameters. Because this effect summed energies deviate by less than 2% and forces less than was not observed for the ab initio structures, we can attribute it 1% from the true Madelung values. This truncation distance to a lack of flexibility, i.e., the nonpolarizability, of the model has then been used throughout. From earlier experiences with potential, expected to be large at the N-oxide oxygen. Improve- cutoff distances optimized in this way,39 we can expect structural ment could possibly be achieved by using off-site charge centers deviations with respect to results from crystal simulations on the oxygen atom, reflecting to some extent charge transfer applying the Ewald summation technique of around 1-2%.
into the water hydrogen directions. Both possible remedies, (c) Lennard-Jones Parameters. Because no explicit polar-
adding explicit polarizability49,50 or off-site charge centers for izability was taken into account, the remaining nonbonded flexible entities, would mean a significant complication of the parameters were designed to reflect the effective many-body model and higher computational demand. We therefore refrained interactions mapped on simple site-site interactions. The from such extensions and used constraints for keeping the water Lennard-Jones parameters in the N-O region were adjusted to molecules upright during geometry optimizations, mimicking geometrical and energetic reference data from ab initio calcula- the directional forces induced by the H-bonding environment tions on N-oxide hydrates. The dominant structural motif found in the experimental crystal structures can be reproduced by The ether group in the morpholine ring (-CH - geometry optimization of TMAO and NMMO dihydrates on the Lennard-Jones parameters of which were set to standard the MP2 level (see Figure 2). In this case, the internal N-oxide values of the CHARMM parameter set up to this point, needed geometries were frozen on the previous results,27 and the water some further refinement: The associated parameters were structure and potential was represented by the rigid three-site slightly modified by a Newton-Raphson51 minimization of a TIP3P model45,46 with small Lennard-Jones parameters attributed to the hydrogens.47 The parameters of the atoms in the N-Oregion including attached methyl and methylene groups were S( ) ) ∑(〈O 〉 - O )2 then adjusted by simulated annealing28,39 in order to reproduce the optimal dihydrate geometries. Each atom sort was assignedthe same parameters, taken to be transferable between TMAO that measures the deviation of thermally averaged observables Oi( ) depending on a number of parameters As a result, illustrated in Figure 2, rms deviations between values Oi,ref. In our case, the crystallographic cell parameters matched48 dihydrate structures from ab initio and force field of NMMO monohydrate act as observables, with reference Binary Phases of Aliphatic N-Oxides and Water J. Phys. Chem. A, Vol. 107, No. 27, 2003 5345
TABLE 4: Torsional Parameters kabcd a
TABLE 5: Rigid Substructure Coordinates for the Partly
Rigid TMAO Model
TABLE 6: Rigid Substructure Coordinates for the Partly
Rigid NMMO Model
a Multiplicity n ≡ 3, phase τ 0 ≡ 0; a-d denote adjacent atoms values taken from the literature52 and thermal averages obtained from MD simulations with the rigid molecules used up to this point. Also, the necessary first and second derivatives of S with respect to nonbonded parameters were approximated by finite differences, deduced from a number of 48 ps simulations at 298.15 K and 1 bar with small variations of the Lennard-Jones parameters applied to a crystal section comprising 384 water and NMMO molecules. It turned out that the cell parameters model to be examined and contrasted with the fully flexible are only weakly sensitive to a variation of the ether group model in more detail later. We do not consider the inherently nonbonded parameters, so the optimization was terminated after quantal nature of methyl group rotations in our model. The rigid a single Newton-Raphson step. The resulting parameters are substructure coordinates are given in Tables 5 and 6; the remaining constraint distances are d(N-H (d) Torsional Parameters. Starting from HF-optimized
N-oxide geometries with imposed local C methyl groups, the ab initio torsional energy profile was 11-H12) ) d(H12-H13) ) d(H11-H13) ) 1.772 computed on the MP2 level by varying the O-N-C-H dihedralangle in steps of 30°, keeping all other coordinates fixed. The (e) Bond and Angle Parameters. Keeping all parameters
corresponding force field barriers, to be expressed by the determined so far fixed, the remaining bond stretching and angle torsional potential terms, were then computed from the differ- bending terms were adjusted with respect to minimizing the ence of ab initio energies and the model potential known so deviations between quantum-chemical and force field N-oxide far. To this end, inclusion of 1-4 Coulomb plus Lennard-Jones optimal structures and normal vibrations. Using again HF/6- as well as only 1-4 Lennard-Jones interactions, both without 31G** N-oxide geometries, the normal frequencies were further scaling, were compared. The energy barriers obtained determined and scaled by the empirical value of 0.893.53 The are very similar for TMAO and NMMO in both cases, including force field parameters were optimized again by simulated only 1-4 Lennard-Jones terms (TMAO 0.540 kcal mol-1, annealing;28,39 structural data in the form of atomic distances NMMO 0.537 kcal mol-1) or including both Lennard-Jones and and vibrational data were weighted equally, and normal modes Coulomb interactions (TMAO 0.506 kcal mol-1, NMMO 0.506 were assigned by maximizing the overlap between model and kcal mol-1). To allow for transferability to other N-oxides with different charge distributions, torsional parameters obtained from Starting with TMAO, it turned out that the inclusion of Urey- the calculations including only 1-4 Lennard-Jones contributions Bradley terms is essential for a reliable representation of were derived for the further parametrization process.
structure and normal mode spectrum. With the resulting Assuming equal contributions to the total barrier, the indi- parameter set given in Tables 7 and 8, the rms deviation between vidual terms were distributed (for instance one H-C-N-C and matched force field and ab initio geometry is 0.005 Å, the rms two H-C-N-O terms for each of three hydrogen atoms in frequency deviation is ca. 50 cm-1. The spectrum is depicted one methyl group, the result multiplied by three gives the total in Figure 3 together with experimental infrared-spectroscopic barrier) and averaged for TMAO and NMMO. Force field data by Kuroda and Kimura.54 More detailed frequency infor- contributions to the dihedral potential within the NMMO mation along with other experimental sources55,56 is summarized morpholine ring were described by standard CHARMM and in Table 9, showing excellent agreement. The normal mode carbohydrate26 parameters for similar torsions. In this way, the quality can be directly attributed to the global optimization methyl group rotation is correctly described in all cases, whereas technique applied, and any simple local optimization with the torsion contribution to the intraring flexibility of NMMO starting values taken from similar fragments fails.
influences the normal vibrations that are optimized in the next Keeping parameter transferability to other N-oxides in mind, section by adjusting the remaining parameters. All torsional as many TMAO bond and angle terms as possible were used without change for the N-O region in NMMO. Furthermore, The set of intermolecular and torsional potential terms the intraring torsional potentials were not modified in anticipa- allowing only for methyl group rotation constitutes a partly rigid tion that the spectral adjustment could be accomplished solely 5346 J. Phys. Chem. A, Vol. 107, No. 27, 2003
TABLE 7: Harmonic Bond Stretching Parameters: Force
Constants k ab

ab
and Equilibrium Distances r0
by varying the morpholine bend angle potentials. For correctlyreproducing the angle between the N-O axis and the ring, itturned out that the O - released, thereby dropping some transferability of the intramo-lecular parameters of the N-O region. This is related to thestrong electrostatic oxygen-oxygen repulsion that could againin principle be compensated by off-site oxygen charge centersor explicit polarizability. Transferable values were maintainedfor the parameters of the O - bond potentials as well as for the parameters of the N-C - angle potential and the corresponding N-H1 Urey-Bradleyterm. New NMMO parameters had to be derived for the O - N-C1 angle potential and for all bond and angle potentials thatimply other morpholine atoms besides nitrogen. Additionally,Urey-Bradley distance potentials for all 1-3 atom pairs exceptfor O - C2 and N-C3 were necessary. With the final parameter set given in Tables 7 and 8, the structural rms deviation between Figure 3. Scaled ab initio and force field normal frequencies of TMAO
(top, with experimental IR spectrum54) and NMMO (bottom). Dashed
force field and ab initio results is 0.004 Å, and the rms frequency deviation is ca. 48 cm-1. The NMMO spectrum is also depictedin Figure 3, indicating the excellent quality of the intramolecular optimized N-oxide geometry with local C3 methyl symmetry, where only the methyl groups are free to rotate under the actionof the torsional potential, and the fully flexible model using all III. Molecular Dynamics Simulation Results
The complete force field was tested by MD simulations of (a) Crystal Structures. Unit cells of NMMO,52 NMMO
condensed phases of N-oxide/water systems, both crystalline monohydrate,52 di-NMMO-pentahydrate,57 and TMAO dihy- and liquid ones, at various conditions for which experimental drate58 were multiplied to yield reasonable simulation boxes (a, data are available. Two model instances were taken into b, c multiples were for NMMO: 4 × 5 × 7, NMMO‚H2O: 2 account: The partly rigid model based on the HF/6-31G**- × 6 × 4, 2NMMO‚5H2O: 3 × 6 × 2, TMAO‚2H2O: 4 × 3 × TABLE 8: Harmonic Angle Bending and Associated Urey-Bradley 1-3 Stretching Parameters: Force Constants
kabc ac
ac a
R /k
and Equilibrium Displacements R
/r
a a-c denote adjacent atoms constituting a bend angle.
Binary Phases of Aliphatic N-Oxides and Water J. Phys. Chem. A, Vol. 107, No. 27, 2003 5347
TABLE 9: Vibrational Frequencies of TMAO: Computed
density deviation of more than 20%. Given the quality of agree- (HF/6-31G**, FF: Force Field) and Experimental: (a)
ment for all other phases studied, one might speculate that the Gigue`re and Chin,55 (b) Kuroda and Kimura,54 and (c)
experimental crystal structure is flawed: The experimental struc- Choplin and Kaufmann56
ture consists of antiparallel TMAO layers perpendicular to the a axis; the smallest site-site distance observed between the layers is 2.951 Å (hydrogen-hydrogen) while within the layers the shortest distance is 1.615 Å (also hydrogen-hydrogen). Such an anomalously large gap does not exist in the other densely packed N-oxide and N-oxide hydrate structures.
(b) Density of Liquid Mixtures. A number of simulation
for various molar ratios of N-oxide/water and two different temperatures have been carried out from which the average densities were obtained. These could be compared with experi- mental values.7 For the 1:5 N-oxide/water ratio, a total of 840 molecules were used, 1034 for the 1:10 mixture, and 1120 for 1:15. The TMAO 1:10 system corresponds roughly to a 4 M, and the TMAO 1:15 system corresponds to a 3 M solution.19 Starting with randomly placed molecules, the systems were equilibrated for 400 ps at 1000 K and constant volume at the expected density, and for another 40 ps at the specified temperature and 1 bar followed by 100 ps NpT sampling runs.
The results are summarized in Table 11.
The agreement between experimental and computational results is generally good, and even better so for NMMO. TMAO solutions tend to be systematically denser than obtained experimentally, whereas NMMO solutions are less dense, TABLE 10: Results of NpT Simulations and Experimental
although to a lesser percentage. Accounting for full flexibility Crystal Data of NMMO and NMMO Monohydrate (exp., ref
shows an albeit small yet notably systematic effect for the 52), Di-NMMO Pentahydrate (exp., ref 57), and TMAO
solutions: The densities in general slightly increase. The smaller Dihydrate (exp., ref 58) at 293.15K and 1 Bara
the N-oxide concentration, the smaller the density deviations as expected, because for small molarity the properties of the water model dominate that is optimized for bulk properties.
Further optimization of the N-oxide models should focus on the N-O oxygen polarization: As we have seen during the parametrization, with the present point charge model, water molecules do not keep the correct orientation relative to the N-O group. This feature is most likely responsible for the slight (c) Liquid Structure of Equimolar Mixtures. We finally
turned to conditions for which no experimental information is available but that are most important for the problem of cellulose solubility: Equimolar mixtures of NMMO and water do dissolve cellulose, whereas analogous TMAO solutions do not.7 We cannot expect to explain these phenomena from structural properties of the solvent alone, but the results will serve as a reference for characterizing the influence of a solute to be studied in the future. Simulation systems were prepared a Cell parameters a-c, monoclinic angle , average density F.
analogously to the dilute solutions: For TMAO/water, 600molecules were used at 1 bar and a temperature of 533.15 K, 3). The systems were simulated at the experimental conditions above the melting point of the amorphous monohydrate of of 293.15 K and 1 bar for 100 ps after 150 ps of equilibration.
474.15 K as reported by Hattori.59 For the NMMO/water Table 10 shows the simulations results for cell parameters and mixtures, 440 molecules were simulated at 1 bar and 373.15 density along with the experimental values.
K, the temperature used in industrial cellulose processing, also The agreement is good, and the differences between partly above the experimental melting point of 345.15 K.2 The rigid and fully flexible models are marginal. The densities are sampling time was 300 ps for each system.
on average improved upon using the fully flexible systems and In contrast to the dilute solutions, we observe in the equimolar deviate on average by around 1% from the experimental values.
case a rather strong dependence on the chosen model, more The largest discrepancies are observed for the monoclinic angle visible for TMAO: The average density from the partly rigid in the NMMO cases. One can again expect that the single local- model is 0.672 g cm-3, and for the fully flexible one, it is 0.764 ized point charge on the N-oxide oxygen is responsible for this g cm-3. For NMMO, we have 1.065 (partly rigid) and 1.093 g effect: The true charge distribution is slightly shifted off the cm-3 (fully flexible). Accounting for full flexibility obviously morpholine ring, accounting for which would induce a change increases the density (by 12% for TMAO and 2.5% for NMMO), in the relative NMMO orientations. Simulation details of pure as seen before for the dilute systems to a lesser extent. Inter- TMAO are not given here. All cell parameters agree quite well and intramolecular degrees of freedom strongly couple under with the experimental results44 except for the a axis, yielding a these conditions, unexpectedly larger so for TMAO.
5348 J. Phys. Chem. A, Vol. 107, No. 27, 2003
TABLE 11: Average Densities G from NpT Simulations and Experimental Data7 of Liquid Mixtures of TMAO or NMMO and
Water at Various Temperatures T
and Molar Ratios N-Oxide/Water
Figure 4. Radial distribution functions of the water oxygen OW around
Figure 5. Radial distribution functions of the water oxygen OW and
the N-oxide atoms O1, N, C1, and O2 (only for NMMO), for TMAO/ hydrogen HW around the N-oxide oxygen O1, for TMAO/water (top) water (top) and NMMO/water (bottom).
This phenomenon should also be reflected by the liquid O2 distance of 4.27 Å found for one of the NMMO structure that is analyzed here in terms of radial distribution monohydrate structures obtained from earlier ab initio calcula- functions, g(r), for various site pairs. In the case of N-oxide/ tions27 where the water molecule is positioned above the water mixtures, the formation of solvent shells of water around morpholine ring bridging both NMMO oxygens. A slight the polar N-O bond as well as the positions of N-oxides around influence of flexibility can be observed in these g functions.
each other are of great importance. The water oxygen OW In Figures 5 and 6 (top), the distribution of water sites around distribution around the atoms of the functional N-oxide group the oxygen atoms in TMAO and NMMO is shown. For the O1, N, C1 (and for NMMO also O2 in the morpholine ring) is oxygen atoms O1 of the N-O group (Figure 5), in both systems, shown in Figure 4. Concerning the N-O oxygen, two solvent two solvent shells of the water oxygen OW and three peaks for shells at around 2.8 and 5.2 Å can be found which appear to be the corresponding hydrogen atoms HW can be observed, again more pronounced in the NMMO case because of the lower more pronounced for NMMO. The corresponding H-bond temperature. The distribution around the carbon atom C distances for HW O1 of about 1.9 Å are in good agreement rotatable methyl groups shows for TMAO two weak maxima with the results of ab initio calculations of N-oxide monohy- at around 3.2 and 5.0 Å. For NMMO, the solvation shell is drates.27 In Figure 5, the first two peaks for HW and the first characterized by a single predominant peak. The distribution OW peak can be assigned to the same water molecule. As the around the nitrogen atom appears for both systems as a broader first HW peak is much larger than the second one, an H-bond maximum at around 4.0 Å. For the ring oxygen O2 of NMMO with a more mobile water molecule can be assumed. The also two solvent shells can be identified with the second peak position of this water molecule corresponds to the optimized at ca. 4.8 Å as the more pronounced one. This value resembles structure of the monohydrates for TMAO and NMMO.27 Binary Phases of Aliphatic N-Oxides and Water J. Phys. Chem. A, Vol. 107, No. 27, 2003 5349
Figure 6. Radial distribution functions of the water oxygen OW and
Figure 7. Radial distribution functions of the N-oxide atoms O1, N,
hydrogen HW around the NMMO ring oxygen O2 (top) and of the and C1 around the N-oxide oxygen O1, for TMAO/water (top) and NMMO atoms O1, N, and C1 around the NMMO atom O2 (bottom).
By calculating the average amount of OW atoms in a region Finally, in analogy to the distribution of OW (Figure 4), the 1, for each of the N-oxides, more than one water molecule in the direct neighborhood of the N-O group distribution of the methyl group’s atoms O1, N, and C1 around can be found. For the partly rigid TMAO, we have 1.12 water O1 is shown in Figure 7. Comparing the results for TMAO and molecules, whereas the flexible model yields 1.20. For NMMO, NMMO, both show more than one N-oxide coordinated with the average number of surrounding water molecules increases the O1 atom. The distribution of the NMMO molecules shows from 1.28 to 1.32 upon switching to full flexibility. Water is markedly more pronounced differences between the partly rigid apparently more tightly bound to the N-O group if flexible and the flexible model than the TMAO distribution due to the molecules are present, more so for TMAO than for NMMO.
flexible ring system. The onset of the peaks at ca. 3 Å point to To clarify the physical nature of this effect, we will have to the existence of bridging water molecules between the N-oxides.
look at intra-/intermolecular cross correlation functions and IV. Concluding Remarks
vibrational/librational mode coupling, for which much largersimulation times will be necessary.
The present paper aimed at the development and assessment In the case of the morpholine ring oxygen atom O2 (Figure of an empirical force field for aliphatic N-oxides as important 6, top), more water molecules can be found in the second solvent cosolvents for water with interesting and still unexplained shell than in the first one, and it is also spread more broadly.
properties with respect to the stabilization of biomolecules in This is a hint for a water position directly above the morpholine solution. We have focused on the following key issues: (i) The ring. Compared to the N-O oxygen O1, a much smaller number thorough derivation of potential parameters from a variety of of water molecules can be found in the O2 region, and the peaks sources by advanced parametrization techniques, (ii) the iden- for the partly rigid and the flexible model show slight differ- tification of basic building blocks guiding future parametrization ences. This effect is even more obvious in the distribution of of related species, (iii) providing simulation results as reference atoms of the N-O group (O1, N, and C1) around the ring oxygen material for studies of solvated molecules, and (iv) an assess- O2 (Figure 6, bottom). For the carbon C1 of the rotating methyl ment of the influence of molecular flexibility as a likely source group, only a weak solvent shell can be observed around O2.
of genuine N-oxide/water mixture properties.
The peaks for O1 and N are similarly high and broad. For O1 The potential function derived for the two prototypical on the other hand, there exists another solvent shell that is lower N-oxide species TMAO and NMMO yields single molecule and than the first one in the flexible model. The similar height of condensed phase properties in good agreement with available the main peaks for O1 and N and the distance between those experimental data, given the requirements of simplicity and peaks corresponding to the length of the N-O bond hint at an transferability outlined in the Introduction. Because of the simple O1 N orientation mediated by water molecules.
functional form chosen, standard parameter combination rules 5350 J. Phys. Chem. A, Vol. 107, No. 27, 2003
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Acknowledgment. This work was supported by the Deutsche
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