[A new super-secondary protein structure: the alpha alpha-angle].

A novel super-secondary structure consisting of two consecutive alpha-helices connected by a polypeptide chain and packed approximately crosswise is considered in the paper. Such locally ordered regions of proteins are described here as alpha alpha-corners. Almost always one of the two possible "mirror-symmetrical" forms is observed in the proteins. The polypeptide chain of the alpha alpha-corner with a short connection consisting of two peptide units has ...alpha R alpha R alpha L beta beta alpha R alpha R... conformation. It is shown that amino-acid sequences coding for the alpha alpha-corners must have a strictly definite alternation of hydrophobic, hydrophilic and glycine residues. The packing of the remaining alpha-helices of a protein molecule can be obtained if the alpha alpha-corner is taken as the origin of folding.