π-HB turns encompassing multiple a) β- and c) α-turns. The hydrogen bond in π-turn is indicated by a solid line with two arrow-heads, whereas the dashed lines show the possible β- and α-turns – a maximum of three in (a) and two in (c). b,d) Some representative examples, in which hydrogen bonds are shown as thick dotted lines and Cα-Cα distances are shown as dashed lines with the values of the distances in Å. Residue numbers involving each π-turn are shown in parentheses, preceded by the conformational designation ('A', 'a'). The PDB code of each structure is indicated at the bottom. b) Two examples of π-HB turns are shown with types AAAa (left) and AAAA (right), with two overlapping type-I β-turns involving (i)-(i+3) and (i+1)-(i+4) interactions in the former and only one β-turn involving (i)-(i+3) interaction in the latter. d) Two examples of the occurrence of α-turns within π-HB turn of conformation AAAa. The first one shows two α-turns involving (i)-(i+4) and (i+1)-(i+5) interactions; the next has only (i)-(i+4) interaction.
Dasgupta and Chakrabarti BMC Structural Biology 2008 8:39 doi:10.1186/1472-6807-8-39