Spatial Recognition of Arginine in the Tropocollagen Macromolecule

Abstract

TROPOCOLLAGEN macromolecules, in appropriate reaction conditions, pack into the segment long spacing form (SLS) with the molecules arranged in parallel array and with like features in register¹. SLS collagen thus provides an intensified and laterally extended picture of the tropocollagen molecule from which it is possible to determine by electron microscopy, after appropriate staining, axial distribution of those regions of the molecule rich in polar amino-acids. Comparison of the positions of these regions with the distribution observed in native (640 Å periodic) collagen fibrils allows the broad features of the fibril-assembly mechanism to be deduced^2,3. To date the staining has been non-selective, presumably involving the binding of phosphotungstic acid (PTA) to the guanidino and amino side chains of arginine, lysine and hydroxylysine, or the uptake of chromium complexes and uranyl cations by the carboxylic groups of glutamic and aspartic acids⁴. Bensusan et al. claimed to have located the position of arginine residues using PTA staining of SLS prepared from solutions of collagen which had been 87 per cent acetylated to block the ε-amino group of lysine⁵. They argued that with so few histidines in the collagen molecule, the bonds observed after this treatment must reflect the position of the arginyl residues.