Nuclear magnetic resonance of synthetic polypeptides

High resolution n.m.r. spectroscopy of synthetic polypeptides has already been extensively applicd to a study of conformations and conformational transitions, particularly the helix-coil transition. Since every atom ofan amino acid residue can, in principle, be made to yield a spectrum, the potential of the method for investigating the finer details of conformation far exceeds that of optical methods. The main chain proton resonances have been shown to be sensitive not only to the helix content (and helix sense in the case of poly aspartate esters) but also to the polydispersity of thc sample, giving a direct indication of the conformational heterogeneity at all points through the transition. Earlier suggestions that multiple rxCH and NH peaks were the rcflection of a slow step in the conversion of a helical residuc into a solvated coil state, have now been shown tobe unfounded. The rxCH shiftdifference bctween thetwoconformational states has bcen found not to depend on the nature ofthe polypeptide, but rather tobe a function ofthe solvent system used; in all cases so far observed however, the helical rxC/l was upfield ofthe coil. In a situation for which ORD/CD data were ambiguous (poly L tyrosine) this upfield displacement of the peak was itself taken as indicating a coil to helix transition. The same criterion for the existence of a transition has also been applied in the study of the conformationsofracemicDLcopolypeptidesandthemethodshowntobeparticularly appropriate for these polymers. The main chain rcsonances of poly L alanine have long been the subject of discussion. due both to the presence of unusual additional peaks (that are now seen to be in part an end effect) and to the fact that multiple rt.CH resonances arenot observed through thc helix-coil transition. It has been shown that this is a direct consequencc of the low cooperativity of transition in this case, and other instances ofrelated behaviour have been detailed. The ability of n.m.r. to study the conformation of the sidechain in addition to that of the backbone, has been exploited the most fully for poly ß benzyl L aspartate. This polymer was chosen since it has been calculated that the left handed helix sense is a consequence of specific interactions that involve a strongly preferred sidechain conformation. Analysis of the vicinal and gern spin-coupling led to the result that no such rigidity exists in the sidechain and therefore that a reappraisal is required ofthe interactions leading to this unusual helix sense. In AB copolymers, whether block or random, n.m.r. can be used to follow the behaviour ofthe A and the B components separately. This has been illustrated with a number of glutamate-aspartate copoJymers and led to conclusions regarding both helix sense and also the details of the copolymerization process not readily available by other methods.