Possible contribution of ionic clustering to molecular packing of collagen

Abstract

AN analysis of the primary structure of the α1 chain of a collagen demonstrated that electrostatic and hydrophobic interactions between collagen molecules were maximal when the rod-like molecules were staggered by 234n amino acid residues, where n is 0, 1, 2, 3 or 4 (ref. 1). It was not possible, however, to determine what value of n was most likely. This information would help to select from among possible molecular packing models^2,3. Doyle et al.⁴ re-examined the electrostatic interactions in an attempt to determine n. Since amino acid side chains of opposite charge frequently occur close to each other in the sequence¹, they considered that these potential intrachain ion pairs might function as dipoles with approximately the same direction in the molecule as in the sequence. If antiparallel dipoles on adjacent molecules were attractive (Fig. 1a) and parallel dipoles were repulsive, they found that the net number of attractions was maximal at n = 1 and negative at other values of n. They concluded that dipole–dipole interactions were important in stabilising the native fibril and that the Smith microfibril⁵ was favoured over other packing models since it is essentially a helix of collagen molecules staggered by 234 residues between nearest neighbours.