Average Hydration Number Research Articles

Response of Ionic Hydration Structure and Selective Transport Behavior to Aqueous Solution Chemistry during Nanofiltration.

The effect of aqueous solution chemistry on the ionic hydration structure and its corresponding nanofiltration (NF) selectivity is a research gap concerning ion-selective transport. In this study, the hydration distribution of two typical monovalent anions (Cl- and NO3-) under different aqueous solution chemical conditions and the corresponding transmembrane selectivity during NF were investigated by using in situ liquid time-of-flight secondary ion mass spectrometry in combination with molecular dynamics simulations. We demonstrate the inextricable link between the ion hydration structure and the pore steric effect and further find that ionic transmembrane transport can be regulated by breaking the balance between the hydrogen bond network (i.e., water-water) and ion hydration (i.e., ion-water) interactions of hydrated ion. For strongly hydrated (H2O)nCl- with more intense ion-water interactions, a higher salt concentration and coexisting ion competition led to a larger hydrated size and, thus, a higher ion rejection by the NF membrane, whereas weakly hydrated (H2O)nNO3- takes the reverse under the same conditions. Stronger OH--anion hydration competition resulted in a smaller hydrated size of (H2O)nCl- and (H2O)nNO3-, showing a lower observed average hydration number at pH 10.5. This study deepens the long-overlooked understanding of NF separation mechanisms, concerning the hydration structure.

Structure of Aqueous KNO3 Solutions by Wide-Angle X-ray Scattering and Density Functional Theory.

The structure of aqueous KNO3 solutions was studied by wide-angle X-ray scattering (WAXS) and density functional theory (DFT). The interference functions were subjected to empirical potential structure refinement (EPSR) modeling to extract the detailed hydration structure information. In aqueous KNO3 solutions with a concentration from 0.50 to 2.72 mol·dm-3, water molecules coordinate K+ with a mean coordination number (CN) from 6.6 ± 0.9 to 5.8 ± 1.2, respectively, and a K-OW(H2O) distance of 2.82 Å. To further describe the hydration properties of K+, a hydration factor (fh) was defined based on the orientational angle between the water O-H vector and the ion-oxygen vector. The fh value obtained for K+ is 0.792, 0.787, 0.766, and 0.765, and the corresponding average hydration numbers (HN) are 5.2, 5.1, 4.8, and 4.5. The reduced HN is compensated by the direct binding of oxygen atoms of NO3- with the average CN from 0.3 ± 0.7 to 2.6 ± 1.7, respectively, and the K-ON(NO3-) distance of 2.82 Å. The average number of water molecules around NO3- slightly reduces from 10.5 ± 1.5 to 9.6 ± 1.7 with rN-OW = 3.63 Å. K+-NO3- ion association is characterized by K-ON and K-N pair correlation functions (PCFs). A K-N peak is observed at 3.81 Å as the main peak with a shoulder at 3.42 Å in gK-N(r). This finding indicates that two occupancy sites are available for K+, i.e., the edge-shared bidentate (BCIP) and the corner-shared monodentate (MCIP) contact ion pairs. The structure and stability of the KNO3(H2O)10 cluster were investigated by a DFT method and cross-checked with the results from WAXS.

In Situ Characterization of Dehydration during Ion Transport in Polymeric Nanochannels.

The transport of hydrated ions across nanochannels is central to biological systems and membrane-based applications, yet little is known about their hydrated structure during transport due to the absence of in situ characterization techniques. Herein, we report experimentally resolved ion dehydration during transmembrane transport using modified in situ liquid ToF-SIMS in combination with MD simulations for a mechanistic reasoning. Notably, complete dehydration was not necessary for transport to occur across membranes with sub-nanometer pores. Partial shedding of water molecules from ion solvation shells, observed as a decrease in the average hydration number, allowed the alkali-metal ions studied here (lithium, sodium, and potassium) to permeate membranes with pores smaller than their solvated size. We find that ions generally cannot hold more than two water molecules during this sterically limited transport. In nanopores larger than the size of the solvation shell, we show that ionic mobility governs the ion hydration number distribution. Viscous effects, such as interactions with carboxyl groups inside the membrane, preferentially hinder the transport of the mono- and dihydrates. Our novel technique for studying ion solvation in situ represents a significant technological leap for the nanofluidics field and may enable important advances in ion separation, biosensing, and battery applications.

First-Principles Simulations of CuCl in High-Temperature Water Vapor.

Experimental data suggest that the solubility of copper in high-temperature water vapor is controlled by the formation of hydrated clusters of the form CuCl(H2O)n, where the average number of water molecules in the cluster generally increases with increasing density [Migdisov, A. A.; et al. Geochim. Cosmochim. Acta 2014, 129, 33-53]. However, the precise nature of these clusters is difficult to probe experimentally. Moreover, there are some discrepancies between experimental estimates of average cluster size and prior simulation work [Mei, Y. Geofluids 2018, 2018, 4279124]. We have performed first-principles Monte Carlo (MC) and molecular dynamics (MD) simulations to explore these clusters in finer detail. We find that molecular dynamics is not the most appropriate technique for studying aggregation in vapor phases, even at relatively high temperatures. Specifically, our MD simulations exhibit substantial problems in adequately sampling the equilibrium cluster size distribution. In contrast, MC simulations with specialized cluster moves are able to accurately sample the phase space of hydrogen-bonding vapors. At all densities, we find a stable, slightly distorted linear H2O-Cu-Cl structure, which is in agreement with the earlier simulations, surrounded by a variable number of water molecules. The surrounding water molecules do not form a well-defined second solvation shell but rather a loose network of hydrogen-bonded water with molecular CuCl on the outside edge of the water cluster. We also find a broad distribution of hydration numbers, especially at higher densities. In contrast to previous simulation work but in agreement with experimental data, we find that the average hydration number substantially increases with increasing density. Moreover, the value of the hydration number depends on the choice of cluster definition.

Mercury exchange in zeolites Na-A and Na-Y studied by classical molecular dynamics simulations and ion exchange experiments

Classical molecular dynamics simulations have been employed to study the exchange of Na + for Hg 2 + in zeolite Na-A, with a Si/Al ratio of 1, and zeolite Na-Y, with Si/Al ratios of 2 and 5, in dry and hydrated conditions within the temperature range 330 – 360 K, to understand factors underpinning the performance of zeolites for water decontamination. A classical forcefield based on DFT energies has been developed for the interaction between the Hg 2 + ions and the zeolite O atoms. In terms of water diffusion, zeolite Na-A shows the lowest calculated diffusivity, followed by zeolite Na-Y (Si/Al=2) and Na-Y (Si/Al=5), as a consequence of differing pore dimensions and extra-framework ion loadings. In the absence of speciation anions, the Hg 2 + ions are consistently adsorbed at the supercage windows in both the LTA and FAU framework types. The reduced pore size of zeolite A leads to an average hydration number per Hg 2 + ion of ¡1.0, whilst the wider pore of zeolite Y exerts less steric hindrance, and thus the Hg 2 + hydration number reaches values between 1.0 and 2.0 in zeolite Y. These observations might indicate that Hg 2 + ions are more strongly immobilized in zeolite A than in zeolite Y. Preliminary measurements of mercury removal using these zeolites, as synthesized from bauxite and kaolin, seem to support these findings. • A new set of interatomic potentials is provided to describe the interaction between Hg 2+ and O. • The hydration of Hg 2+ is affected by the Si/Al ratio and size of the supercage windows. • Smaller supercage windows and lower Si/Al ratio should be more effective exchanging Hg 2+ . • Preliminary experiments show Na-A is more robust than Na-Y for the removal of Hg 2+ .

Solvent fluctuations in the solvation shell determine the activation barrier for crystal growth rates

Solution crystallization is a common technique to grow advanced, functional crystalline materials. Supersaturation, temperature, and solvent composition are known to influence the growth rates and thereby properties of crystalline materials; however, a satisfactory explanation of how these factors affect the activation barrier for growth rates has not been developed. We report here that these effects can be attributed to a previously unrecognized consequence of solvent fluctuations in the solvation shell of solute molecules attaching to the crystal surface. With increasing supersaturation, the average hydration number of the glutamic acid molecule decreases and can reach an asymptotic limit corresponding to the number of adsorption sites on the molecule. The hydration number of the glutamic acid molecule also fluctuates due to the rapid exchange of solvent in the solvation shell and local variation in the supersaturation. These rapid fluctuations allow quasi-equilibrium between fully solvated and partially desolvated states of molecules, which can be used to construct a double-well potential and thereby to identify the transition state and the required activation barrier. The partially desolvated molecules are not stable and can attach spontaneously to the crystal surface. The activation barrier versus hydration number follows the Evans-Polanyi relation. The predicted absolute growth rates of the α-glutamic acid crystal at lower supersaturations are in reasonable agreement with the experimental observations.

Poly (aryl ether ketone)/polymeric ionic liquid with anisotropic swelling behavior for anion exchange membranes

Dimensionally stable and high level of conductivity is of importance for alkaline exchange membrane (AEM) applications. However, the main challenge for AEMs is to control swelling behavior and construct ion transport “high way” simultaneously. Here we report a series of self-crosslinked membranes using poly (aryl ether ketone) as rigid matrix and imidazolium-based polymeric ionic liquid as hydroxyl transporting groups. The obtained membranes possess anisotropic swelling behavior: no more than restricted 4% in-plane swelling ratio indicate excellent dimensional stability; while, obvious through-plane swelling guaranteed abundant water absorption in the hydrophilic domains, which benefited sufficient hydroxide ion dissociation and improved ion conductivity. The swelling ratio of through-plane can be more than tenfold of in-plane. This result quantifies the dramatic influence of flexible polymeric ionic liquid on specific swelling behavior of membranes for the first time. Moreover, the distinct microscopic phase separation endows the membranes possessing high ion conductivity (0.067 S cm−1). The introduction of long-chain polymeric ionic liquid not only facilitate continuous pathway, but also reduce the dependency on λ (average hydration number vs. positive group). After treatment in alkaline conditions, the membranes still exhibit more than 8 MPa tensile strength.

CuCl Complexation in the Vapor Phase: Insights from Ab Initio Molecular Dynamics Simulations

We investigated the hydration of the CuCl0 complex in HCl-bearing water vapor at 350°C and a vapor-like fluid density between 0.02 and 0.09 g/cm3 using ab initio molecular dynamics (MD) simulations. The simulations reveal that one water molecule is strongly bonded to Cu(I) (first coordination shell), forming a linear [H2O-Cu-Cl]0 moiety. The second hydration shell is highly dynamic in nature, and individual configurations have short life-spans in such low-density vapors, resulting in large fluctuations in instantaneous hydration numbers over a timescale of picoseconds. The average hydration number in the second shell (m) increased from ~0.5 to ~3.5 and the calculated number of hydrogen bonds per water molecule increased from 0.09 to 0.25 when fluid density (which is correlated to water activity) increased from 0.02 to 0.09 g/cm3 (fH2O 1.72 to 2.05). These changes of hydration number are qualitatively consistent with previous solubility studies under similar conditions, although the absolute hydration numbers from MD were much lower than the values inferred by correlating experimental Cu fugacity with water fugacity. This could be due to the uncertainties in the MD simulations and uncertainty in the estimation of the fugacity coefficients for these highly nonideal “vapors” in the experiments. Our study provides the first theoretical confirmation that beyond-first-shell hydrated metal complexes play an important role in metal transport in low-density hydrothermal fluids, even if it is highly disordered and dynamic in nature.

Deconstructing Free Energies in the Law of Matching Water Affinities.

The law of matching water affinities (LMWA) is explored in classical molecular dynamics simulations of several alkali halide ion pairs, spanning the size range from small kosmotropes to large chaotropes. The ion-ion potentials of mean force (PMFs) are computed using three methods: the local molecular field theory (LMFT), the weighted histogram analysis method (WHAM), and integration of the average force. All three methods produce the same total PMF for a given ion pair. In addition, LMFT-based partitioning into van der Waals and local and far-field electrostatic free energies and assessment of the enthalpic, entropic, and ion-water components yield insights into the origins of the observed free energy profiles in water. The results highlight the importance of local electrostatic interactions in determining the shape of the PMFs, while longer-ranged interactions enhance the overall ion-ion attraction, as expected in a dielectric continuum model. The association equilibrium constants are estimated from the smooth WHAM curves and compared to available experimental conductance data. By examining the variations in the average hydration numbers of ions with ion-ion distance, a correlation of the water structure in the hydration shells with the free energy features is found.

Structure of aqueous potassium pentaborate solution

The structure of aqueous potassium pentaborate solution has been confirmed by species distribution calculation, synchrotron X-ray scattering, density function theory calculation. The kinds of species and the transformation relationship were investigated by species distribution calculation. The radial distribution function and the structure parameters were given out by synchrotron X-ray scattering experiment. The hydration distance of K–O(H2O) is ∼0.294nm with a average hydration number of 8.0. The B–O(H2O) distance in hydrated anion [B3O3(OH)4(H2O)n]− were determined to be ∼0.37nm with the hydration number of 4–5. The structure and interaction energy of the first hydration sphere of [K(H2O)n]+ and [B3O3(OH)4(H2O)n]− were also studied by density functional theory calculation.

Hydration and aggregation of lysozyme by extended frequency range depolarized light scattering

Extended frequency range depolarized light scattering is a spectroscopic technique operating in the GHz–THz range that, applied to aqueous solutions of biomolecules, is able to disentangle the dynamics of the solute from that of water, and relaxation processes of bulk from those of hydration water. Experiments performed on aqueous solution of a variety of biological systems of different nature, such as small hydrophobic and hydrophilic molecules, amino acids, dipeptides, and proteins, have shown that a significant increase in the dynamical retardation and in the extent of the perturbation of water surrounding solute molecules occurs at increasing chemical complexity of the solute. The behavior of aqueous solutions of lysozyme is here analyzed in detail, as a function of solute concentration. Our results provide evidence of a dynamical perturbation extending over more than three water layers in diluted solutions. We find a strong reduction in the average hydration number at increasing solute concentration that cannot be explained by the random superposition of hydration layers among lysozyme molecules in close proximity. This behavior is consistent with the formation of clusters in solution.

Solvent Sharing Models for Non-Interacting Solute Molecules: The Case of Glucose and Trehalose Water Solutions

The concentration dependence of hydration numbers of molecules modelled as nearly spherical particles is studied by simple analytic and numerical approaches, in the ideal limit of absence of intermolecular interactions. It is shown that the random close-to-contact condition achieved by solute molecules, noticeably affects the average hydration numbers. Comparison with experimental results obtained by light scattering in glucose and trehalose water solutions shows a reduction of the hydration number that is twice faster than that calculated in absence of interactions, suggesting important aggregation phenomena to occur in both systems, even at relatively low solute concentration. The effect of concentration on shear viscosity is also reported, suggesting that the leading contribution to the increase of viscosity arises from hydration water.

Silver speciation in chloride-containing hydrothermal solutions from first principles molecular dynamics simulations

First principles molecular dynamics (FPMD) simulations are performed to investigate the speciation of Ag+ in chloride-containing hydrothermal solutions. We derive the stability and microscopic structures of aqua cations and chloro complexes of Ag+ under T–P conditions ranging from ambient to supercritical conditions. Our simulations show that the average hydration number for hydrated Ag+ decreases from 4 to 3 as temperature increases and the first hydration shell is very flexible, yielding rapid water exchange. AgCl2− and AgCl are the major chloro complexes in hydrothermal solutions over a wide T–P range. AgCl2− has a linear structure, which is similar to its counterpart chloro complexes of Au and Cu (i.e., AuCl2− and CuCl2−). AgCl has a trigonal planar structure (AgCl(H2O)2), which is quite different from the mono-chloro complexes of Au and Cu that have 2-fold linear structures (AuCl(H2O) and CuCl(H2O)). AgCl32− holds weak stability at ambient conditions, but is no longer stable at elevated temperatures. AgCl43− is not stable even under ambient conditions. AgCl32− and AgCl43−, therefore, should not be considered as important species in hydrothermal fluids.

Hydration structure and dynamics of K + and Ca 2+ in aqueous solution: Comparison of conventional QM/MM and ONIOM-XS MD simulations

Molecular dynamics (MD) simulations based on the conventional QM/MM scheme and ONIOM-XS method have been performed to investigate structural and dynamical properties of K + and Ca 2+ in water. Regarding the detailed analyses of the ONIOM-XS MD trajectories, the average hydration numbers for K + and Ca 2+ were found to be 6.3 and 7.6, respectively, compared with the corresponding values of 7.0 and 7.8 derived by the conventional QM/MM simulations. Together with the significant difference found in the comparison of the dynamics details, the ONIOM-XS method clearly shows its capability in predicting more reliable detailed knowledge of these hydrated ions.

Hydration Shell Structure and Dynamics of Curium(III) in Aqueous Solution: First Principles and Empirical Studies

Results of ab initio molecular dynamics (AIMD), quantum mechanics/molecular mechanics (QM/MM), and classical molecular dynamics (CMD) simulations of Cm(3+) in liquid water at a temperature of 300 K are reported. The AIMD simulation was based on the Car-Parrinello MD scheme and GGA-PBE formulation of density functional theory. Two QM/MM simulations were performed by treating Cm(3+) and the water molecules in the first shell quantum mechanically using the PBE (QM/MM-PBE) and the hybrid PBE0 density functionals (QM/MM-PBE0). Two CMD simulations were carried out using ab initio derived pair plus three-body potentials (CMD-3B) and empirical Lennard-Jones pair potential (CMD-LJ). The AIMD and QM/MM-PBE simulations predict average first shell hydration numbers of 8, both of which disagree with recent experimental EXAFS and TRLFS value of 9. On the other hand, the average first shell hydration numbers obtained in the QM/MM-PBE0 and CMD simulations was 9, which agrees with experiment. All the simulations predicted an average first shell and second shell Cm-O bond distance of 2.49-2.53 Å and 4.67-4.75 Å respectively, both of which are in fair agreement with corresponding experimental values of 2.45-2.48 and 4.65 Å. The geometric arrangement of the 8-fold and 9-fold coordinated first shell structures corresponded to the square antiprism and tricapped trigonal prisms, respectively. The second shell hydration number for AIMD QM/MM-PBE, QM/MM-PBE0, CMD-3B, and CMD-LJ, were 15.8, 17.2, 17.7, 17.4, and 16.4 respectively, which indicates second hydration shell overcoordination compared to a recent EXAFS experimental value of 13. Save the EXAFS spectra CMD-LJ simulation, all the computed EXAFS spectra agree fairly well with experiment and a clear distinction could not be made between configurations with 8-fold and 9-fold coordinated first shells. The mechanisms responsible for the first shell associative and dissociative ligand exchange in the classical simulations have been analyzed. The first shell mean residence time was predicted to be on the nanosecond time scale. The computed diffusion constants of Cm(3+) and water are in good agreement with experimental data.

Unified Effect of Hydrophobic Hydration on the Dynamics and the Structure of Water Molecules in Lower Alcohol Aqueous Solutions

After the analysis of the quasi-elastic neutron scattering (QENS) spectra of lower alcoholic aqueous solutions, we found that the parameter α, which is the number fraction of hydrophobic hydrating water molecules, has an important role in describing the dynamics and other properties of these solutions. The diffusion coefficient D was obtained from QENS spectra using the relaxing cage model, and D was found to have the same linear relation against α for ethyl alcohol, n -propyl alcohol, i -propyl alcohol, and t -butyl alcohol (with a small deviation) aqueous solutions. The excess molar volume and average hydration number of these solutions could also be expressed with common parameters, which are independent of the alcohol species.

Dielectric Relaxation and Water Activity in Aqueous Solution of D-Psicose

Hydration behavior of a rare sugar, D-psicose, was investigated based on measurements of the dielectric relaxation and the water activity in its aqueous solutions. Dielectric relaxation spectra in the frequency domain were obtained from time domain reflectometry. The dielectric relaxation time was then determined by fitting the spectra to a Cole-Cole type equation and used to evaluate the average hydration number representing the average number of water molecules forming a complex with a sugar molecule at infinite dilution. The concentration dependence of the dielectric relaxation time in aqueous solutions of all examined sugars generated a single master cur ve. The average hydration number of psicose was found to be identical with that of fructose. Only slight differences were noticeable among the concentration dependences of the water activity of monosaccharides. The equilibrium constant of hydration of sugar was calculated by considering material balances in an aqueous solution of sugar and using experimentally obtained values of the average hydration number and the water activity. The equilibrium constant values increased in the order of glucose < fructose � psicose < trehalose < sucrose < maltose. The present results suggest that both fructose and psicose interact with water in a similar manner.

Molecular dynamics study of hydrated alkali and halide ions in liquid nitrobenzene

The structure, dynamics and energetics of hydrated alkali and halides ions in bulk nitrobenzene are studied using molecular dynamics computer simulations. The number of water molecules in the ions’ hydration shell (hydration number) and their orientational distribution, hydration water residence time and several important energy terms are calculated and compared with the same quantities in bulk water. The water molecules in the first hydration shell of the ion are more tightly bound to the ion in bulk nitrobenzene than in bulk water as reflected by the longer lifetime in the former case. As the ion size increases, the average hydration number in nitrobenzene decreases, which is the reverse trend to that found in bulk water. The calculated hydration numbers are in reasonable agreement with experimental data.

Ion hydration in aqueous solutions of lithium chloride, nickel chloride, and caesium chloride in ambient to supercritical water

Neutron diffraction with an isotopic substitution technique was used to determine the structure (bond distance, hydration number, and orientational correlation of water) of ion hydration in concentrated aqueous solutions of lithium chloride, nickel chloride and caesium chloride in ambient to supercritical water (648 K, 169 MPa). The lithium ion has an average hydration number of 4.1 ± 0.3 at 298 ~ 470 K, but 2.8 ± 0.3 at 647 K. The Li–O bond distance was 2.02 ± 0.01 Å over the temperature range measured, whereas the Li–D bond length decreased from 2.60 ± 0.01 Å at 298 K to 2.49 ± 0.01 Å at 647 K, indicating a weakening of the orientational correlation of water molecules around Li + in supercritical water. The Ni–O and Ni–D distances were 2.10 ± 0.01 Å and 2.68 ± 0.01 Å, respectively, irrespective of temperature at 298 ~ 473 K. The hydration number of Ni 2+ was found to be 6.4 ± 0.3 at 298 and 373 K, but decreased to 5.0 ± 0.2 at 473 K, suggesting the ion-pairing of Ni 2+–Cl − at 473 K. The chloride ions in lithium chloride and caesium chloride solutions are hydrogen bonded with water molecules at Cl–D (D 2O) distances of 2.28 ~ 2.33 Å over a temperature range measured. With an increase in temperature, the number of hydrogen bonded Cl–D interactions in the LiCl solutions changed from 5.6 ± 0.1 at 298 ~ 350 K, 4.5 ± 0.1 at 470 K, to 2.5 ± 0.1 at 648 K, showing that there is a marked lessening of the degree of orientational correlation of water molecules around Cl − in high-temperature liquid, in particular, in supercritical water. In the caesium chloride solution, on the other hand, there is a tendency for formation of contact ion-pair Cs +–Cl − at 473 K due to a decrease in n Cl–D of 3.9 ± 0.5, as also evidence from an X-ray diffraction study.

Structure, Dynamics, and Hydration of a Collagen Model Polypeptide, (l-Prolyl-l-ProlylGlycyl)10, in Aqueous Media: a Chemical Equilibrium Analysis of Triple Helix-to-Single Coil Transition

The structure, dynamics, and hydration behavior of a collagen model polypeptide, (L-prolyl-L-prolylglycyl)(10) (PPG10), were investigated in pure water and dilute acetic acid over a wide temperature range using broadband dielectric relaxation (DR) techniques that spanned frequencies from 1 kHz to 20 GHz. All samples showed pronounced dielectric dispersion with two major relaxation processes around 3 MHz and 20 GHz. Because DR measurements sensitively probe dipoles and their dynamics, the structures and ionization states of the carboxy and amino termini of aqueous PPG10 were precisely determined from the relaxation times and strengths in the 3 MHz frequency range. In solution, PPG10 formed mixtures of monodisperse rods as triple helices with lengths and diameters of 8.6 and 1.5 nm, respectively, and monomeric random coils with radii of approximately 1.4 nm. Ionization of the C-terminus was suppressed by the addition of acetic acid in both states. The fraction of random coils (f(coil)) was found to be a function of temperature (T) and the concentration of PPG10 (c). At low temperatures, small f(coil) values were found, which increased with temperature to reach f(coil) = 1 at approximately 60 degrees C, irrespective of c. This phenomenon, well-known as a triple helix-to-single coil transition, is discussed on the basis of the chemical reaction, (PPG10)(3) <==> 3PPG10, with an equilibrium constant of K = 3(c/55.6)(2)f(coil)(3)(1 - f(coil))(-1). The standard enthalpy change evaluated from Arrhenius plots (ln K versus T(-1)) was found to change dramatically at the same transition temperature that was previously determined by using optical rotation experiments. The other major DR process, observed at approximately 20 GHz, was assigned to free and hydrated water molecules and used to determine the average hydration number (m) per PPG10. The m values for the triple helix and random coil state at 25 degrees C were evaluated to be m(th) = 60-70 and m(coil) = 250-270. The m(th) value was in reasonable agreement with the number of hydrated water molecules in crystals of (PPG10)(3) residing in the first and second hydration shells around the amino acid residues. This agreement suggests that the structure of the triple helix in crystals is very similar to that in aqueous solution, including the location of hydrated water molecules.