Infrared Fingerprint Spectroscopy and Theoretical Studies of Potassium Ion Tagged Amino Acids and Peptides in the Gas Phase

Infrared multiple-photon dissociation spectroscopy is effected on the K⁺ tagged aromatic amino acids tyrosine and phenylalanine, as well as the K⁺ tagged peptides bradykinin fragment 1-5 and [Leu]-enkephalin. The fingerprint (800-1800 cm^-1) infrared spectra of these species are compared to density-functional theory (DFT) calculated spectra to determine whether the complex is in the charge solvation (CS) or salt bridge (SB) (i.e. zwitterionic) configuration. For the aromatic amino acids the CS structure is favored and the tridentate N/O/ring structure is found to be the preferred binding geometry for K⁺. The experimental and theoretical evidence for bradykinin fragment 1-5 tagged with K⁺ suggests that the SB structure is favored; the calculations indicate a head-to-tail looped structure stabilized by a salt bridge between the protonated guanidine group and the deprotonated C-terminus, which allows K⁺ to sit in a binding pocket with five C=O electrostatic interactions. For K⁺ tagged [Leu]-enkephalin the spectroscopic evidence is not as clear. While the calculations clearly favor a CS structure and the observation of a weak carboxylic acid C=O stretching band in the infrared spectrum matches this finding, the prominence of a band at 1600 cm^-1 renders the analysis more ambiguous, and hence the presence of some salt bridge ions cannot be excluded. Another striking feature in the [Leu]-enkephalin spectrum is the high infrared activity of the tyrosine side-chain modes, which can be clearly identified from comparison to the [Tyr + K]⁺ experimental spectrum, but which is not reproduced by the DFT calculations