Conformation of amino acid side-chains in proteins.

We have analysed the side-chain dihedral angles in 2536 residues from I9 protein structures. The distributions of x1 and xz are compared with predictions made OII the basis of simple energy calculations. The x1 distribution is trimodal; the qposition of the side-chain (~rans to Ha), which is rare except in serine, the t position (trams to the amino group), and the g+ position (trans to the carbonyl group), which is preferred in all residues. Characteristic xz distributions are observed for residues with a tetrahedral y-carbon, for aromatic residues, and fol aspartic acid/asparagine. The number of configurations actually observed is small for all types of side-chains, with SO”,b or more of them in only one or two configurations. We give estimates of t,he experimental errors on x1 and xz (3” t,o 16”, depending on the type of the residue), and show that the dihedral a,ngles remain within 15” to 18” (standard drviabion) from the configurations \vith t)he lowest calculated energies. The distribution of the side-chains among t,he permitted configurations varies slightly with the conformation of the main &ail\. and with the position of the residue relative to the protein surface. Configurations that are rare for exposed residues are even rarer for buried residues, suggesting that, while the folded structure puts little strain on side-chain conformations, the side-chain positions with the lowest energy in the unfolded st,ructure are chosen preferentially during folding.