Kinetics of isomerization for the proline helical forms of two oligoproline redox arrays by circular dichroic spectropolarimetry

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 (Pro(3)-X-Pro(2)-Y-Pro(2)-Z-Pro(3)) and ZRA2 (Pro(7)-X-Pro(2)-Y-Pro(2)-Z-Pro(7)) bear large redox-active substituents on proline residues X, Y, and Z, but P9 (Pros) 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, k(1) = similar to 3.3 x 10(-4) s(-1); B-->C, k(2) = similar to 0.8 x 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.