Conformations and mutarotations of poly(5-methyl-L-proline)
Optically active poly( cis -5-methyl-L-proline) and poly( trans -5-methyl-L-proline) were synthesized from their respective N-carboxy-anhydrides (NCA). Analysis of the i.r., u.v., n.m.r., ORD, and CD spectra of the polymers showed that two different conformations of each polymer exist in the solid and in solution in appropriate solvents. Form I and form II of poly( trans -5-methyl-L-proline) were found to have the same conformations as the form I and form II of poly-L-proline, respectively. Form B of poly( cis -5-methyl-L-proline) seemed to have a somewhat different conformation from form II of poly-L-proline. The changes in viscosity during the mutarotations of the two polymers showed that the mutarotation entailed cis-trans isomerization of amide bonds in the polymers. By following the kinetics of the mutarotations it was found that the activation energies for poly( cis -5-methyl-L-proline) was 22 kcal per residue mole, whereas E a of poly( trans -5-methyl-L-proline) was 33 kcal per residue mole. The excess 10 kcal for the latter polymer was attributed partially to the steric barrier between the methyl group at the trans -5-position and the carbonyl group in the polymer.