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Conformational energy studies of β‐sheets of model silk fibroin peptides. I. Sheets of poly(Ala‐Gly) chains

A new model structure is proposed for the silk I form of the crystalline domains of Bombyx mori silk fibroin and the corresponding crystal form of poly(L‐Ala‐Gly). It was deduced from conformational energy computations on stacked sheet structures of poly(L‐Ala‐Gly). The novel sheet structure contains interstrand hydrogen bonds but is composed of antiparallel polypeptide chains whose conformation differs from that of the antiparallel β‐sheets that constitute the silk II structure. The strands of the new sheet have a two‐residue repeat, in which the Ala residues adopt a right‐handed and the Gly residues a left‐handed sheet‐like conformation. The computed unit cell is orthorhombic, with cell dimensions a = 8.94 Å, b = 6.46 Å, and c = 11.26 Å. The model accounts for most spacings in the observed fiber x‐ray diffraction patterns of silk I and of the silk‐I‐like form of poly(L‐Ala‐Gly), and it is consistent with nmr and ir spectroscopic data. As a test of the computations, the well‐established β‐sheet structure of silk II and the corresponding form of poly(L‐Ala‐Gly) have been reproduced. The computed energies for the two forms of poly(L‐Ala‐Gly) indicate that the silk‐II‐like form is more stable, by about 1.0 kcal/mol per residue. The main difference between the two structures is the orientation of the Ala side chains of neighboring strands in each sheet. In the Pauling–Corey β‐sheet and in the silk II form, referred to as an “in‐register” structure, the Ala side chains of every strand point to the same side of a sheet. In the silk I structure, referred to as “out‐of‐register,” the side chains of Ala residues in adjacent strands point to opposite sides of the sheet.