Synthesis and solution properties of poly(trans-5-ethylproline). A slow mutarotating substituted polyproline

An optically active polypeptide, poly(trans-5-ethylproline) (PT5EP) was synthesized and its solution properties were observed to investigate the conformational changes with various conditions. The trans-5-ethyl substitution on polyproline showed noticeable perturbed effects on the conformations of the polypeptide as well as mutarotation of the polypeptide in solution. Circular dichroism (CD) spectra suggested that the polypeptide existed in a poly(L -proline) form-I-type helix and mutarotated slowly to an intermediate conformation in which some of the amide bonds had rotated to a trans conformation. In trifluoroethanol (TFE) solution the polymer took more than 20 days to change from a form-I-type helix conformation, in which CD bands for D -PT5EP are at 199.5 ± 1.0 (positive), 115.5 ± 0.5 (negative), and around 238 nm (positive), to an intermediate conformation. Upon addition of trifluoroacetic acid (TFA) to a TFE solution, the polymer was transformed to form-II-type polymers. Even a greater change in conformation was observed in a solution of TFA or in LiClO₄-TFE. The overall change of the intensity ratio R_CD of positive to negative CD bands of D -PT5EP was from 0.6–0.7 to 30. Reverse mutarotation toward the original form I was observed when n-butyl alcohol, water, or THF was added to a solution containing the form II polymer. A blue shift of the UV spectra and a change in the NMR spectrum also supported the concept of this conformational change.