Modeling Vibrational Spectra of Amino Acid Side Chains in Proteins: Effects of Protonation State, Counterion, and Solvent on Arginine C−N Stretch Frequencies

Abstract

The vibrational spectrum of arginine's side chain in various protein environments is modeled by measuring IR spectra of ethylguanidinium (EG) salts under varying conditions. Characteristic νC-N stretch vibrational frequencies of monoalkylguanidinium are assigned to observed IR bands at 1655−1685 cm-1, 1615−1635 cm-1, and 1170−1180 cm-1. Each of these bands is observed to downshift by 4−9 cm-1 upon [15N]2 substitution at the terminal nitrogens. Additional weaker bands from vibrations involving nitrogen motion are also discernible at ∼920, ∼1085, and ∼1440 cm-1. Deprotonation of EG−Cl is accompanied by an overall decrease in IR absorption intensity and substantial changes in the νC-N vibrational bands. The ∼1670 and ∼1180 cm-1 bands appear to shift substantially in the deprotonated state. A strong band near 1635 cm-1 remains in ethylguanidine, but this is interpreted as being due to a δ(NH2) scissor mode based on previously published assignments of the guanidine IR spectrum. New bands attributable to C−N st...