Amino Acid Propensities are Position-dependent Throughout the Length of α-Helices

The 20 commonly occurring amino acids have been shown to have distinct position-dependent, helix-forming propensities near the ends of α-helices. Here, we show that the amino acids also have very strong position-dependent propensities throughout the length of a helix. Most helices are amphiphilic, and they have a strong tendency to both begin and end on the solvent-inaccessible face of the helix. These position-specific propensities should provide valuable parameters to guide de novo protein design, and should allow more precise prediction of helical topology in natural proteins.