Effects of anions on the zwitterion stability of Glu, His and Arg investigated by IRMPD spectroscopy and theory

Abstract

Interactions of halide anions (Cl −, Br −, and I −) with glutamic acid (Glu), histidine (His), and arginine (Arg) and their effects on stabilizing the zwitterionic form of these amino acids were investigated using infrared multiple photon dissociation (IRMPD) spectroscopy between 850 and 1900 cm −1 and hybrid density functional theory. The IRMPD spectra of Glu·X − and His·X − each have a diagnostic carbonyl stretching band at ∼1750 cm −1 from a carboxylic acid group, indicating that the nonzwitterionic form of these amino acids is most stable. In contrast, a broad band at ∼1625 cm −1 for Arg·X −, consisting of the antisymmetric stretch of a carboxylate group and hydrogen bonded NH bends, clearly shows that Arg is zwitterionic in these complexes. There are many similarities between these spectra and those of cationized amino acids, which aid in spectral interpretation. Cl − and Cs + are of comparable size, and attachment of either ion to these amino acids has little effect on the frequencies of these diagnostic carbonyl stretches. The coordination of cations to these amino acids is different from that of anions, resulting in a favorable alignment of the dipole moment of the carbonyl group with the electric field of ions of either polarity, which causes a redshift in this band, i.e., a Stark effect. There is a slight redshift (∼10 cm −1) in the carbonyl stretch band at ∼1750 cm −1 for Glu·X − and His·X − with decreasing anion size, consistent with both a Stark effect and with greater carboxylate character for the carboxylic acid group in complexes with the less acidic halide ions. The anion size has little effect on the structures and relative zwitterion stabilities for most of these complexes, which can be attributed to the large size of the halide anions investigated compared to that of the alkali metal cations where size effects are more pronounced. The spectra calculated for the lowest energy structures are generally consistent with the experimental spectra, although no single structure accounts for the many distinct bands in the IRMPD spectra of His·X −.