Refined molecular and crystal structure of silk I based on Ala-Gly and (Ala-Gly)(2)-Ser-Gly peptide sequence

The molecular and crystal structure of one of the crystalline modifications of Bombyx mori, silk I, was determined by x-ray diffraction method. Cell dimensions are essentially the same as those found in the synthetic model peptide poly(L-Ala-Gly). The (straight phi, psi) values of L-Ala and Gly in the repeating unit are (-112 degrees, -6 degrees ), and (71 degrees, -99 degrees ) respectively, which are in the Bridge and the forth quadrant regions of the Ramachandran map, respectively. The observed molecular conformation in the present study has a "crank-shaft" or a S-shaped zigzag arrangement, leading to a remarkable agreement of observed and calculated structure amplitudes for both dipeptide and hexapeptide sequences, and has a reasonable hydrogen bond networks. Obtained (straight phi, psi) values are quite different from those reported by Lotz and Keith, even though overall appearances are quite similar to each other. In spite of intra- and intermolecular hydrogen-bond networks, silk I structure changes easily to the silk II by a mechanical deformation. This fragility may be due to the above peculiar crank-shaft conformation deduced from the alternating structure of alanine and glycine.