Radiation effects of poly(L-glutamic acid) and poly(L-lysine) in the helix-coil transitional state

The degradation of poly(L-glutamic acid) and poly(L-lysine) by X-rays and ultraviolet radiation has been studied at various pH values, including the helix-coil transition region. It was found that X-rays cause maximum degradation in the transition region for both polypeptides, indicating that the efficiency of degradation is affected not only by the secondary structure but also by some other factors related to the transition. On the contrary, the degradation by monochromatic ultraviolet 2537 A radiation showed a strong dependence on helix content of PLGA, suggesting that degradation occurs more effectively in the helical form than in the coil form. Theoretical calculations were made to approximate the pH dependence of degradation, assuming different cross sections for the peptide bonds in helix, coil, and junctions between helical and coil regions.