Extensions to amino acid description.

A new set of amino acid descriptor scales was recently introduced. The new scales relate to amino acid side chain (i) rigidity and (ii) flexibility. These two properties were found to be orthogonal to each other. In this study, (i) the understanding of their meaning is improved, (ii) their utility further corroborated, and (iii) the scope of their applicability is broadened. Notably, the rigidity and flexibility scales are benchmarked against previous amino acid description precedence and found to contribute (1) new information, and (2) direct interpretation. The suggested description extensions were tested using empirical data from peptide description and in quantitative structure-activity relationships (QSAR).