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

摘要

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.