A Helix Propensity Scale Based on Experimental Studies of Peptides and Proteins

AbstractThe average globular protein contains 30% α-helix, the most common type of secondary structure. Some amino acids occur more frequently in α-helices than others; this tendency is known as helix propensity. Here we derive a helix propensity scale for solvent-exposed residues in the middle positions of α-helices. The scale is based on measurements of helix propensity in 11 systems, including both proteins and peptides. Alanine has the highest helix propensity, and, excluding proline, glycine has the lowest, ∼1kcal/mol less favorable than alanine. Based on our analysis, the helix propensities of the amino acids are as follows (kcal/mol): Ala=0, Leu=0.21, Arg=0.21, Met=0.24, Lys=0.26, Gln=0.39, Glu=0.40, Ile=0.41, Trp=0.49, Ser=0.50, Tyr=0.53, Phe=0.54, Val=0.61, His=0.61, Asn=0.65, Thr=0.66, Cys=0.68, Asp=0.69, and Gly=1