Molecular connectivity descriptors of the physicochemical properties of the .alpha.-amino acids

Abstract

The molecular connectivity index combinations used to describe the physicochemical properties of the a-amino acids were specifically analyzed in terms of their statistical meaning and, especially, of their Q (quality) value. Combinations of ordinary connectivity indexes provide a good description of nearly every physicochemical property examined but especially of the isoelectric point, side-chain volume, and crystal density of the a-amino acids. Some features of these combinations of connectivity indices are as follows: (i) the combination with the maximum number of molecular connectivity indexes does not always show the best quality, (ii) successive inclusion of the next good index to the best combination of indexes does not always produce the next best combination of indexes, and (iii) even highly intercorrelated indexes can contribute to the best combination of indexes. It was also detected that valence molecular connectivity indexes by themselves provide a rather good description of nearly every property with the exception of specific rotation of amino acids, which is better described by a set of two simple connectivity indexes. Orthogonal indexes, obtained from the ordinary molecular connectivity indexes, show in some cases the best single-index regressions and the best multi-index description of the molecular weights and relaxation rates. The best description of the specific rotation and solubility of the a-amino acids is, instead, achieved by the aid of the nonconnectivity Kidera, Konishi, Oka, Ooi, and Scheraga indexes.

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