FIGURE 1.

Considering the surface area of amino acids allows a direct comparison of interface propensities and pairwise residue contact propensities. (a) The interface propensity of arginine is the ratio of arginine frequency at the interface relative to its frequency at the surface. The pairwise residue contact propensity between arginine and glutamate is the frequency of their contacts at the interface relative to their frequency at the interface. These two propensities are not directly comparable because the terms used in their derivation are in different units (amino acid frequencies for the former and normalized contact frequencies for the latter). (b) Considering the surface area of amino acids makes interface propensity and contact propensity directly comparable because the same measure (i.e., area fraction) is used to derive all terms. The interface propensity of arginine becomes its fractional area at the interface relative to the surface and the arginine–glutamate contact propensity becomes the fractional area of arginine–glutamate contacts relative to the fractional interface area occupied by arginine