Crystal structure of cyclic (APGVGV)2, an analog of elastin, and a suggested mechanism for elongation/contraction of the molecule

Tropoelastin is a complex polymeric protein composed primarily of repeating segments of Val‐Pro‐Gly‐Gly, Val‐Pro‐Gly‐Val‐Gly, and Ala‐Pro‐Gly‐Val‐Gly‐Val that occurs in connective tissue and arteries. It has rubber‐like extensible properties. A synthetic cyclic dodecapeptide, with a double repeat of the hexapeptide sequence, has been shown to undergo a reversible inverse temperature transition; that is, crystals grow at 60°C and dissolve in the mother liquor upon cooling. An x‐ray crystal structure analysis established that the cyclic backbone formed an elongated loop with a Pro‐Gly, type II beta turn at both ends. Six internal cross strand NH···OC hydrogen bonds form between six NH donors and four OC acceptors where two of the carbonyl O atoms are bifurcated acceptors. As a result, the molecule is pulled up into a corrugated profile. The corrugated loops form extended β‐sheets by additional intermolecular hydrogen bonds. An analysis of the dome region in a corrugated sheet suggests a reversible mechanism for extending and contracting the length of the whole molecule, akin to the motion of opening and closing an umbrella, caused by the motion of a water molecule with its associated hydrogen bonds acting as spokes. Crystal parameters: C44H72N12O12˙3H2O, sp. gr. P212121, a = 9.212 Å, b = 19.055 Å, c = 32.247 Å, d = 1.157 g/cm3. © 2005 Wiley Periodicals, Inc. Biopolymers, 2005