Amino acid distribution and solvent accessibility. The incidence of three different classes of amino acids as a function of distance from the membrane center. A) The subset of residues that have accessibility lower or equal to 25% (buried). B) The subset of residues that have accessibility higher than 25% (exposed). The coloring differentiates between polar, intermediate and hydrophobic residues. The two lower figures show the log of the difference in substitution rates between the globular and membrane regions plotted against the biological hydrophobicity value for each amino acid type. In C) the rate for buried sites is shown while in D) the rate for exposed sites is shown.