MOLECULAR CONNECTIVITY DESCRIPTORS OF THE PHYSICOCHEMICAL PROPERTIES OF THE ALPHA-AMINO-ACIDS

The molecular connectivity index combinations used to describe the phy sicochemical properties of the alpha-amino acids were specifically ana lyzed in terms of their statistical meaning and, especially, of their Q (quality) value. Combinations of ordinary connectivity indexes provi de a good description of nearly every physicochemical property examine d but especially of the isoelectric point, side-chain volume, and crys tal density of the alpha-amino acids. Some features of these combinati ons of connectivity indices are as follows: (i) the combination with t he maximum number of molecular connectivity indexes does not always sh ow the best quality; (ii) successive inclusion of the next good index to the best combination of indexes does not always produce the next be st combination of indexes, and (iii) even highly intercorrelated index es can contribute to the best combination of indexes. It was also dete cted 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 s et of two simple connectivity indexes. Orthogonal indexes, obtained fr om 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 t he specific rotation and solubility of the alpha-amino acids is, inste ad, achieved by the aid of the nonconnectivity Kidera, Konishi, Oka, O oi; and Scheraga indexes.