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GUANIDINE HYDROCHLORIDE AND THE CIRCULAR DICHROISM OF RANDOM COIL POLYPEPTIDES

1973; Wiley; Volume: 5; Issue: 4 Linguagem: Inglês

10.1111/j.1399-3011.1973.tb03451.x

0367-8377

Miguel Cortijo, Bhinyo Panijpan, Walter Gratzer,

Molecular spectroscopy and chirality

The circular dichroism of a series of disulphide‐free proteins in 6 M guanidine hydrochloride was measured. Despite qualitative similarity, the quantitative spread of molar residue ellipticities was considerable, and not explicable in terms of side chain chromophore contributions. It seems likely that composition‐dependence of the distribution of backbone dihedral angles is responsible for the variation. Chains with intact disulphide bonds have larger negative ellipticities. In the case of insulin, evidence is given to suggest that this arises from intra‐chain interactions, rather than the intrinsic optical activity of the disulphide chromophores. Whereas uncharged poly‐L‐lysine and poly‐L‐glutamic acid are not converted to the fully random state in 6 M guanidine hydrochloride, the much more weakly helical polyornithine and polyaspartic acid appear to be completely randomised in this solvent. This provides two uncharged random coil polymers, the CD of which is in the range of those of disordered proteins. The positive feature found in the circular dichroism of charged random poly‐L‐lysine, poly‐L‐ornithine and poly‐L‐glutamic acid ( though not poly‐L‐aspartic acid) is absent in the randomly coiled uncharged polymers. In the fully charged polypeptides, 6 M guanidine hydrochloride provokes a change in circular dichroism, similar to that induced by salts at high concentration, and indicative of the appearance of some α‐helical structure. This effect is enhanced by increasing temperature, suggesting a solvent exchange effect.