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

Abstract

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 densityfunctional 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 CdO 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 CdO 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.

Structure of sodiated octa-glycine: IRMPD spectroscopy and molecular modeling

“…This has been extended to cationized peptides recently [38,39,[41][42][43]. Most cases of cationized peptides for which a SB structure was established contain arginine, a highly basic residue that favors proton transfer [38,39,41]. Only when a divalent ion was attached to the AA dipeptide was a SB structure found to be favored, while the alkali cationized di-and tripeptides favored a CS isomer [15,42,43].…”

“…The salt bridge versus charge solvation structures of cationized amino acids have been a matter of intense research using IRMPD spectroscopy in the last few years [38]. This has been extended to cationized peptides recently [38,39,[41][42][43]. Most cases of cationized peptides for which a SB structure was established contain arginine, a highly basic residue that favors proton transfer [38,39,41].…”

“…The free O-H stretch has been previously identified for smaller protonated or cationized amino acids and peptides [33][34][35][36]. The COH bend has been used as a diagnostic tool as well [15,[37][38][39][40][41][42]. Among peptidic N-Hs, NH 5 interacts weakly with CO 1 with an amide A (N-H stretching) frequency at 3433 cm Ϫ1 , lying just in between the experimental bands.…”

Application of Neural Networks in Diagnostics of Chemical Compounds Based on their Infrared Spectra

“…This is the so-called "finger-print" interpretation method [6]. The cons and pros of this identification method is that there is no need and at the same time no learning opportunities to recognize spectra of chemical compounds.…”

Rotational Spectrum of Tryptophan

“…[1][2][3][4][5] "Action" spectra and double-resonance techniques are in widespread use to detect different conformations of biomolecules, and, in some cases, identify their structures. [6][7][8][9][10] Because the majority of these techniques use multiphoton resonantly enhanced ionization, they have been applied to molecules containing UV chromophores. Rotational spectroscopy, considered as the most powerful technique for structural characterisation, is not constrained by the need of a chromophore, but it had somehow lagged behind other gas-phase spectroscopic methods in the study of solid biomolecules.…”