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 degrees 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 beta-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. P2(1)2(1)2(1), a = 9.212 angstroms, b = 19.055 angstroms, c = 32.247 angstroms, d = 1.157 g/cm3.