Temperature dependence of the CN stretching vibration of a nitrile-derivatized phenylalanine in water

Nitrile-derivatized amino acids have recently been proposed to serve as infrared (IR) probes of local environment of proteins. In this Letter, we presented results regarding the temperature dependence of the CN stretching vibration of a nitrile-derivatized phenylalanine, PheCN, in water. Two-dimensional (2D) infrared correlation analysis revealed that at least three species, corresponding to PheCN molecules with either free or hydrogen-bonded CN groups, exist. This microheterogeneity was further studied by a global curve-fitting procedure, which yielded: ΔH=−4.7 kcal mol−1, ΔS=−15 cal K−1 mol−1, and ΔCp=−79 cal K−1 mol−1, for the transition from the free to hydrogen-bonded species.