| Abstract
| - Hydrophobic hydration was studied by the microwave dielectricmethod, which has been developed to separatethe rotational relaxation of restrained water by hydrophobic sidechains from the dielectric spectrum of anamino acid solution. Measurements were taken with a precisionmicrowave network analyzer and athermostated glass cell at 20.0 ± 0.01 °C with an open-endflat-surface coaxial probe. Examined aminoacids were glycine, alanine, α-aminobutylic acid, norvaline, valine,norleucine, isoleucine, leucine, andphenylalanine. The present method assumes that the dielectricspectrum of an amino acid solution isapproximated with that of a solute/water emulsion containing two kindsof spherical solutes having differentDebye-type relaxation frequencies. We found that in straight alkylside chains of tested amino acids, each−CH2− restrains three water molecules on average andits hydration shell has a relaxation frequencyfc around5.5 GHz, while branched alkyl side chains restrain less than thestraight ones and their hydration shells havea slightly lower fc of 4.1−4.4 GHz. Thefc of the hydration shell of a phenylalanineside chain is 4.0 GHzand the phenyl group restrains nearly six water molecules, indicating arather hydrophilic nature. The staticdielectric constant of the hydrophobic hydration shell was found to bearound 110, which is higher than thatof bulk water.
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