Amino acid propensities revisited.

Statistical analysis of amino acid patterns in approximately 160,000 alpha-helices in experimentally determined structures revealed di-, tri-, and tetrapeptides, whose frequencies deviate most from the statistical model. Importantly, some sequences were never found in alpha- helices. This fact was detected initially with tripeptides, where nearly 1% of the possible sequences were never seen in the helical segments. For tetrapeptides, this effect is very strong and significant; almost 43% of the possible sequences never appear in alpha-helices. It is possible that there are some steric and energetic restrictions that do not allow these tetrameric amino acid sequences to form alpha-helical structure.