Hydrophobic hydration analysis on amino acid solutions by the microwave dielectric method

Hydrophobic hydration was studied by the microwave dielectric method, which has been developed to separate the rotational relaxation of restrained water by hydrophobic side chains from the dielectric spectrum of an amino acid solution. Measurements were taken with a precision microwave network analyzer and a thermostated glass cell at 20.0 ± 0.01°C with an open-end flat-surface coaxial probe. Examined amino acids were glycine, alanine, α-aminobutylic acid, norvaline, valine, norleucine, isoleucine, leucine, and phenylalanine. The present method assumes that the dielectric spectrum of an amino acid solution is approximated with that of a solute/water emulsion containing two kinds of spherical solutes having different Debye-type relaxation frequencies. We found that in straight alkyl side chains of tested amino acids, each -CH₂- restrains three water molecules on average and its hydration shell has a relaxation frequency f_c around 5.5 GHz, while branched alkyl side chains restrain less than the straight ones and their hydration shells have a slightly lower f_c of 4.1-4.4 GHz. The f_c of the hydration shell of a phenylalanine side chain is 4.0 GHz and the phenyl group restrains nearly six water molecules, indicating a rather hydrophilic nature. The static dielectric constant of the hydrophobic hydration shell was found to be around 110, which is higher than that of bulk water.