substituted proline residues
Springer Online Journal Archives 1860-2000
Chemistry and Pharmacology
Abstract The kinetics of isomerization of the helical forms of three oligoprolines was determined by far-ultraviolet CD spectropolarimetry and kinetic analysis by singular value decomposition. ZRA (Pro3-X-Pro2-Y-Pro2-Z-Pro3) and ZRA2 (Pro7-X-Pro2-Y-Pro2-Z-Pro7) bear large redox-active substituents on proline residues X, Y, and Z, but P9 (Pro9) does not. All three peptides formed a stable proline-II helix in water. In acetonitrile, both ZRA2 and P9 were converted into a proline-I helical form but ZRA remained predominantly in the proline-II helical form. Evidently, in order to undergo substantial proline II→I isomerization, an oligoproline chain containing large substituents needs to have a segment of consecutive unsubstituted proline residues that is sufficiently long to form a stable proline helix. Biexponential kinetics (A→B, k1 = ∼3.3 × 10-4 s-1; B→C, k2 = ∼0.8 × 10-4 s-1) were observed for the proline II→I isomerization of ZRA2 and P9 in acetonitrile and for the proline I→II isomerization of ZRA2 in water, which provides evidence for the growth and decay of a major kinetic intermediate.
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