Investigation of hydration of alpha-amino acids by means of absorption millimeter spectroscopy

The hydration indexes for 19 protein a-amino acids are measured by means of absorption millimeter spectroscopy (AMS) at 31.42 GHz. The plot of the hydration indexes on the area of surface of aliphatic amino acid molecules accessible fbr water is a straight line located above the points corresponding to aromatic or polar amino acids. The contribution of nonpolar groups in the hydration index is greater than that of polar groups provided that their accessible surface areas are equal. The contribution to hydration of -OH and -CONH2 groups in Ser, Gln, and Asn coincides in sign with that of pure hydrophobic hydration but the value of the contribution is significantly smaller. The change in mobility of water molecules, which is the basis of the AMS method, may serve as the physicochemical foundation for the construction of a new hydrophobicity scale for amino acids comparable with the already existing scales.