Hydrogen bonding effects on the conformational changes of polyglutamates containing long flexible side chains

The FT-IR spectra for three polyglutamates containing different side chains, poly(γ-methyl α, l-glutamate) (PMLG), poly(γ-hexyl α, l-glutamate) (PHLG), and poly(γ-stearyl α, l-glutamate) (PSLG), were measured and analyzed. Experimental data show that longer side chains induce weaker hydrogen bonds between the carbonyl groups and the amide linkages of the α-helical backbone. Simulated infrared spectra strongly support the infrared spectra with remarkable accuracy. Variable temperature FT-IR measurements on PHLG and PSLG were also performed to examine the effect of temperature on the conformational behavior of the side chains. In the case of PHLG, no perceptible conformational change has been found with increasing temperature. On the other hand, the changes of the characteristic vibrational bands have been found for the infrared spectra of PSLG after the temperature of phase transition. The relative broadness of several bands over 60°C suggests that the increased mobility of the long side chains affects the conformation of the α-helical backbone.