Hydration of amino acids: FTIR spectra and molecular dynamics studies.

The hydration of selected amino acids, alanine, glycine, proline, valine, isoleucine and phenylalanine, has been studied in aqueous solutions by means of FTIR spectra of HDO isotopically diluted in H2O. The difference spectra procedure and the chemometric method have been applied to remove the contribution of bulk water and thus to separate the spectra of solute-affected HDO. To support interpretation of obtained spectral results, molecular dynamics simulations of amino acids were performed. The structural-energetic characteristic of these solute-affected water molecules shows that, on average, water affected by amino acids forms stronger and shorter H-bonds than those in pure water. Differences in the influence of amino acids on water structure have been noticed. The effect of the hydrophobic side chain of an amino acid on the solvent interactions seems to be enhanced because of the specific cooperative coupling of water strong H-bond chain, connecting the carboxyl and amino groups, with the clathrate-like H-bond network surrounding the hydrocarbon side chain. The parameter derived from the spectral data, which corresponds to the contributions of the population of weak hydrogen bonds of water molecules which have been substituted by the stronger ones in the hydration sphere of amino acids, correlated well with the amino acid hydrophobicity indexes.

Systematic study of hydration patterns of phosphoric(V) acid and its mono-, di-, and tripotassium salts in aqueous solution.

Fourier transform infrared (FTIR) spectroscopy of the OD band of HDO molecules has been applied to perform a systematic study of various phosphate forms in the order of decreasing protonation: H3PO4, KH2PO4, K2HPO4, K3PO4. HDO isotopically diluted in H2O has been prepared by adding adequate amounts of D2O to aqueous solutions in ordinary water. The difference spectra procedure has been applied to remove the contribution of bulk water and thus to separate the spectra of solute-affected HDO. The position at maximum of the principal anion-affected HDO band for potassium phosphates moves in the order KH2PO4 (2478 cm(-1))>K2HPO4 (2363 cm(-1))>K3PO4 (2301 cm(-1)), that is, decreases with increasing solute basicity and charge. The number of moles of water affected by one mole of solute (N) equals 11.0, 13.8 and 16.2, respectively. Phosphoric acid affects statistically 13.9 water molecules and appears to be a "structure making" solute in water. The isotopic substitution with deuterium occurs also on the phosphate anions and phosphoric acid. The thus formed P-O-D groups interact with water molecules via strong hydrogen bonds and the relative strength of this interaction increases with increasing solute acidity. The plausible assignments of OD bands of HDO have been confirmed by calculating equilibrium structures of small aqueous clusters of the studied individual utilizing density functional theory. Further interpretation of the energetic and structural properties of hydrating water is enabled by calculating intermolecular interaction energy of water and probability distributions for interatomic oxygen-oxygen distance.

Hydration of carboxylate anions: infrared spectroscopy of aqueous solutions.

Hydration of carboxylate ions was studied in aqueous solutions of sodium salts by means of FTIR spectroscopy using the HDO molecule as a probe. The quantitative version of the difference spectra method has been applied to determine the solute-affected water spectra. They display two-component bands of affected HDO at ca. 2550 and 2420 cm(-1). These bands are attributed to the -COO(-) group of the R-COO(-) ion (R = H, CH(3), C(2)H(5)), because water molecules surrounding the substituent R behave roughly as molecules in the bulk phase. For the studied carboxylates the net water structure making effect is observed, which increases with electron-donor ability of R, by means of changing the relative intensity of solute-affected HDO component bands. The observed splitting of the carboxylate-ion-affected HDO band is unique for these anions. The experimental results were confronted with DFT-calculated structures of small gas-phase and polarizable continuum model (PCM) solvated aqueous clusters to establish the structural and energetic states of carboxylate ions hydrates. This was achieved by comparison of the calculated optimal geometries with the interatomic distances derived from HDO band positions. Different possibilities have been considered to explain the peculiar spectral results. The plausible explanation assumes symmetry breaking of the carboxylate ion induced by interaction with water solvent: C-O bond lengths of RCOO(-) and electric charge localization become unequal. It is demonstrated by nonequivalent interaction of oxygen atoms of the RCOO(-) anion with water molecules. Taking into account only the energetic effect, the phenomenon is explained by the anticooperative H-bond formation of the carboxylate group with water molecules, which increases with the electron-donor ability of the substituent R. In this interaction two water molecules play an important part, as appears from the calculated clusters. They interact with oxygen atoms of the RCOO(-) ion, forming a cooperative system, within which solvent molecules are nonequivalent with respect to H-bond formation with both proton-accepting sites of the solute. This additionally enhances solvent-induced symmetry breaking of carboxylate anion. Strongly hydrogen-bonded solvent is more effective in inducing symmetry breaking; thus, increasing the temperature decreases the splitting of the carboxylate-ion-affected water, as experimentally observed.

Hydration of simple amides. FTIR spectra of HDO and theoretical studies.

The hydration of formamide (F), N-methylformamide (NMF), N,N-dimethylformamide (DMF), acetamide (A), N-methylacetamide (NMA), and N,N-dimethylacetamide (DMA) has been studied in aqueous solutions by means of FTIR spectra of HDO isotopically diluted in H2O. The difference spectra procedure has been applied to remove the contribution of bulk water and thus to separate the spectra of solute-affected HDO. To facilitate the interpretation of obtained spectral results, DFT calculations of aqueous amide clusters were performed. Molecular dynamics (MD) simulation for the cis and trans forms of NMA was also carried out for the SPC model of water. Infrared spectra reveal that only two to three water molecules from the surrounding of the amides are statistically affected, from among ca. 30 molecules present in the first hydration sphere. The structural-energetic characteristic of these solute-affected water molecules differs only slightly from that in the bulk and corresponds to the clathrate-like hydrogen-bonded cage typical for hydrophobic hydration, with the possible exception of F. MD simulations confirm such organization of water molecules in the first hydration sphere of NMA and indicate a practical lack of orientation and energetic effects beyond this sphere. The geometry of hydrogen-bonded water molecules in the first hydration sphere is very similar to that in the bulk phase, but MD simulations have affirmed subtle differences recognized by the spectral method and enabled their understanding. The spectral data and simulations results are highly compatible. In the case of F, NMF, and A, there is a visible spectral effect of water interactions with N-H groups, which have destabilizing influence on the amides hydration shell. There is no spectral sign of such interaction for NMA as the solute. The energetic stability of water H-bonds in the amide hydration sphere and in the bulk fulfills the order: NMA > DMA > A > NMF > bulk > DMF > F. Microscopic parameters of water organization around the amides obtained from the spectra, which have been used in the hydration model based on volumetric data, confirm the more hydrophobic character of the first three amides in this sequence. The increased stability of the hydration sphere of NMA relative to DMA and of NMF relative to DMF seems to have its origin in different geometries, and so the stability, of water cages containing the amides.

Hydration of simple carboxylic acids from infrared spectra of HDO and theoretical calculations.

The hydration of carboxylic acids in dilute aqueous solutions is important for our understanding of their functioning in the biochemical context. Here we apply vibrational spectra of HDO isotopically diluted in H(2)O to study this phenomenon, using the difference spectra method for analysis and interpretation of the results. The spectra of HDO affected by formic, acetic, and propionic acid display characteristic component bands, significantly red-shifted from the bulk HDO band position. The appearance of these component bands is linked with isotopic substitution on the carboxylic acid molecule, which forms a short and strong hydrogen bond with a water molecule. Additionally, a charge separation due to the proton transfer in the neutral form of the complex leading to a contact ion pair formation may be inferred from the affected HDO spectra. Apart from the contraction of the principal acid-water hydrogen bond, it results in other major structural changes in the hydration shell, as revealed by density functional theory (DFT) calculations of optimal geometries of aqueous clusters of the studied acids.

Hydration of aprotic donor solvents studied by means of FTIR spectroscopy.

The paper attempts to explain the mutual influence of nonpolar and electron-donor groups on solute hydration, the problem of big importance for biological aqueous systems. Aprotic organic solvents have been used as model solutes, differing in electron-donating power. Hydration of acetonitrile, acetone, 2-butanone, and triethylamine has been studied by HDO and (partially) H2O spectra. The quantitative version of difference spectra method has been applied to determine solute-affected water spectra. Analysis of the data suggests that solvent-water interaction via the donor center of the solute is averaged between water-water interactions around the solute. Such behavior can be simply explained by the model of solute rotating in a cavity of water structure, which is formed by clathratelike hydrogen-bonded water network. On the basis of the band shape of solute-affected HDO spectra and the corresponding distribution of intermolecular distances, the criterion for hydrophobic type hydration has been proposed. From that point of view, all the studied solutes could be treated as hydrophobic ones. The limiting band position and the corresponding intermolecular distance of affected water, gained with increasing electron-donating power of solutes, has been inferred from the data obtained. These observations are important for interpretation of vibrational spectra of water as well as for volumetric measurements of solutions. The simple model of hydration, proposed to better justify the results, connects the values obtained from the methods providing microscopic and macroscopic characteristics of the system studied.