Apr 13, 2007

Infrared spectroscopy and theoretical studies on gas-phase protonated leu-enkephalin and its fragments: direct experimental evidence for the mobile proton

Journal of the American Chemical Society
Nick C PolferBéla Paizs

Abstract

The gas-phase structures of the protonated pentapeptide Leu-enkephalin and its main collision-induced dissociation (CID) product ions, b4 and a4, are investigated by means of infrared multiple-photon dissociation (IR-MPD) spectroscopy and detailed molecular mechanics and density functional theory (DFT) calculations. Our combined experimental and theoretical approach allows accurate structural probing of the site of protonation and the rearrangement reactions that have taken place in CID. It is shown that the singly protonated Leu-enkephalin precursor is protonated on the N-terminus. The b4 fragment ion forms two types of structures: linear isomers with a C-terminal oxazolone ring, as well as cyclic peptide structures. For the former structure, two sites of proton attachment are observed, on the N-terminus and on the oxazolone ring nitrogen, as shown in a previous communication (Polfer, N. C.; Oomens, J.; Suhai, S.; Paizs, B. J. Am. Chem. Soc. 2005, 127, 17154-17155). Upon leaving the ions for longer radiative cooling delays in the ion cyclotron resonance (ICR) cell prior to IR spectroscopic investigation, one observes a gradual decrease in the relative population of oxazolone-protonated b4 and a corresponding increase in N-term...Continue Reading

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Mentioned in this Paper

Biochemical Pathway
Exploration With a Probe
Carboxy-Terminal Amino Acid
Abnormal Fragmented Structure
Imines
Theoretical Study
Protons
Enkephalin, Leucine
Infrared Spectrophotometry
Peptide Fragments

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