Infrared spectroscopic study on hydration and chiral interaction of temperature-responsive polymer with L-proline...

We studied the hydration of a temperature-responsive polymer containing L-proline moieties (poly(acryloyl-L-proline methyl ester), PAProM) by using infrared spectroscopy. Red shifts of ν(C-H) bands and blue shifts of amide and ester carbonyl bands of PAProM during temperature-induced phase separation indicate that the alkyl, amide, and ester groups are partially dehydrated. The population of the amide carbonyls forming hydrogen bonds (H-bonds) with two water molecules decreased from 63 to 33%, while that of the ester carbonyls forming one H bonding decreased from 100 to 84%. We labeled the methyl groups of PAProM by introducing deuterium (poly(acryloyl-L-proline methyl-d3 ester, PAProMd3) to clarify hydration change of the labeled groups. Red shifts of three ν(C-D) bands appearing at 2000-2200 cm(-1) clearly showed that the methyl groups at the end of side chains also dehydrated as well as the alkyl groups on the main chain. As for the effects of additives, methanol raised the phase separation temperature (Tp) of PAProM. The IR spectra show that the average number of H bonds to the amide and ester carbonyls decreases with increasing methanol concentration and that the water molecules surrounding the alkyl groups of PAProM are replaced by methanol molecules. The increase in Tp suggests that the favorable effect of the latter is superior to the unfavorable effect of the former. On the other hand, malic acid (MA) reduced Tp of PAProM. Moreover, a chiral interaction occurs; that is, Tp was lower in the presence of d-isomer than L-isomer. The analysis of the amide band revealed that the d-isomer associates more effectively with the amide carbonyls of PAProM than the L-isomer does.