Polymer and Materials Science
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
The effect of covalently crosslinked loops on the entropy of polypeptide chains is discussed. Since the available theory applies to single, independent loops, this theory has been extended to cases where the chains are multiply crosslinked and, therefore, dependent. Multiply crosslinked loops are present in many proteins, e.g., ribonuclease. It is shown that, whereas in general there is a significant difference between the entropy computed with the present treatment of dependent chains and that incorrectly computed by applying the theory of independent chains to multiply crosslinked loops, the difference in the two procedures is not very great for ribonuclease (and well within the uncertainty of our knowledge of the residue conformational entropy) because the chains of ribonuclease are only slightly dependent even though they are multiply crosslinked. Specifically, estimates of the entropy losses per residue are: -0.45 e.u. for chains in a single, independent loop, -0.58 to -0.66 e.u. for dependent chains in multiply crosslinked loops, and -0.51 e.u. for a simplified model of ribonuclease. With this information, a treatment is given of a helix-coil transition in two chains, assumed to be polyglycine, connected at each end by disulfide bonds. The results of a previous theory for short chains are combined here with the results for the entropy loss on loop formation. It is seen that the presence of the loop stabilizes the helical forms (because of the lowering of the entropy of the random state) and broadens the transition (because of the increased importance of intermediate states).
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