One type of gamma-turn, rather than the other gives rise to chain-reversal in proteins

In inverse gamma-turns, although giving rise to a kink in the chain, rarely occur within beta-hairpins and are seldom situated at a position of reversal, by 180 degrees, in chain direction.

Abstract

Gamma-turns may be defined by a hydrogen bond between the carbonyl group of one amino acid residue and the amino group of the acid two residues ahead in the sequence. They occur as two types, inverse gamma-turns and classic gamma-turns (classic gamma-turns are usually called just gamma-turns but we prefer to add the adjective classic to distinguish them from the word gamma-turn, referring collectively to both). Of the two, classic gamma-turns are less common and are considered by all authors to be extreme rarities in proteins. However, we find that a number do occur in a sample of proteins of known three-dimensional structure. One occurs at the edge of the second hypervariable region of the light chain in some immunoglobulins. All classic gamma-turns except one are associated with a reversal in the main chain direction. In most cases, the turn lies at the loop end of a beta-hairpin. By contrast, inverse gamma-turns, although giving rise to a kink in the chain, rarely occur within beta-hairpins and are seldom situated at a position of reversal, by 180 degrees, in chain direction.