CONFORMATIONAL PROPERTIES OF POLY(L-PROLINE) AND POLY(GAMMA-HYDROXY-L-PROLINE) .1. CIS IMIDE BONDS AND EVIDENCE FOR 2 TRANS CONFORMERS

The conformations of poly(L-proline) and poly(γ-hydroxy-L-proline) chains were analyzed from the results of proton and carbon-13 high-resolution NMR spectroscopy and the characteristic ratios in pure solvents. The absence of cis residues in the poly(L-proline) chain in trifluoroethanol, coupled with its low characteristic ratio in this solvent, requires the population of another trans conformer in the vicinity of Φ =-50°. This result is consistent with one group of conformational energy calculations. The results for each of the polymers in D2O, where the presence of cis residues is established, can be given a consistent interpretation in accord with the above. It is also shown that in concentrated aqueous salt solution poly(γ-hydroxy-L-proline) undergoes trans → cis isomerization about the imide bond as had been established for poly(L-proline). Previous reports to the contrary can be attributed to the fact that the signal-to-noise ratio in the spectrum was too low. © 1979, American Chemical Society. All rights reserved.