Effect of Hydrophobicity of Amino Acids on the Structure of Water

Abstract

The structure of water in various amino acid solutions was examined by Raman spectroscopy and 1H NMR. Analyses of the relative intensity (C value) of the O−H stretching Raman band corresponding to an in-phase collective vibration of O−H oscillators that are connected by hydrogen bonds revealed that the structure of water in solutions of various amino acids (Gly, Ala, Ser, Val, Leu, Ile, His, Asn) at neutral pH did not differ from one another significantly, which shows that the structure of water is little affected by the side chains. Deionization of carboxylate or ammonium ions induced appearance of the effects of side chains on the structure of water around them. That is, the structure of water in aqueous solutions of amino acids with nonpolar side chain was enhanced, and the probability of hydrogen bonding between water molecules increased by forming water clathrate around the side chain. On the other hand, the structure of water in the solutions of amino acids with polar or charged side chain was destroyed. Moreover, the motion of water molecules around both hydrophilic and hydrophobic side chains was revealed to be restricted by spin−lattice relaxation time (T1) measurements of hydrogen atoms in water molecules. Electrostatic interaction or hydrogen bonding between the amino acid and water molecule is responsible in the former case, and the formation of water clathrate around a hydrophobic moiety is important in the latter case. The deviations (|Cx − Cw|) of the values of C of amino acid solutions (Cx) from that of pure water (Cw) were well correlated with the solvent-accessible surface area of the side chain to water and increased in the order Leu > His > Ile > Val > Asn > Ser, Ala, Gly. The C values of amino acids, the physical meaning of which is clearer than those of indices obtained by thermodynamic methods, might be used as a novel index for relative hydrophobicities of amino acids.