Modeling vibrational spectra of amino acid side chains in proteins: Effects of protonation state, counterion, and solvent on arginine C-N stretch frequencies

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 ν_C-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 ν_C-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 stretch vibrations of deprotonated EG are observed at 1600, 1566, and 1208 cm^-1. The ν_C-N bands of EG cation also show characteristic shifts of up to ∼40 cm^-1 depending on solvent and counterion conditions. The biggest effects are seen for counterion variations in the solid state, where the highest ν_C-N frequency ranges from 1695 cm^-1 for EG-carbonate down to 1652 cm^-1 for EG-bromide. In nonpolar solvent, ion pairing occurs, as evidenced by reproducible differences seen for vibrational frequencies of different EG salts, e.g., 1185 cm^-1 for the acetate vs 1179 cm^-1 for the iodide. In polar solvents (methanol, ethanol, dimethyl sulfoxide, or water), however, there is little if any difference in the vibrational frequencies of EG acetate, chloride, bromide, iodide, or phenolate, indicating independently solvated anion and cation. We conclude that when arginine's side chain is buried as part of an ion pair within a hydrophobic region of a protein, its strongly IR-absorbing ν_C-N frequencies are likely to be sensitive to perturbations such as changing the nature or position of the counterion or altering the hydrogen-bonding capacity of nearby neutral amino acids.