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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 deduced2,3. 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.

IT is now well established that the principal primary reducing species formed in the radiolysis of water and aqueous solutions are the negative polarons1. Their occurrence in these media has been directly demonstrated by observation of their transient optical absorption spectrum in pulsed radiolysis experiments2,3. Although their direct observation by electron spin resonance or optical absorption spectroscopy has not been possible in the case of pure ice, trapped at defect sites, they have been characterized in alkaline ice at 77° K by both these techniques4,5. Their reactions with Brönsted acid solutes present in ice to form hydrogen-atoms (identified through their characteristic electron spin resonance spectra) both in the presence and absence of various competing electron scavengers have been reported previously6,7. In this communication we describe their reactions with metal cations such as Mg++, Zn++, and Cd++to form lower oxidation states which are normally unstable. Evidence for the occurrence of Zn+and Cd+, but not Mg+, in aqueous solutions had been previously obtained8in pulsed radiolysis investigations.