A conformational flexibility scale for amino acids in peptides

The end-to-end collision frequency of short polypeptides labeled with a fluorescent probe (DBO) at one end and an efficient contact quencher (Trp) at the other end (see scheme) provides an absolute measure of the time scale of conformational changes in short structureless peptides as a function of the amino acid type. A general correlation with secondary structure propensity applies, with the most flexible amino acids being most abundant in β turns and the most rigid ones in β sheets.