Search

Infrared multiple photon dissociation action spectroscopy was performed on the AlaOrn b(2)(+) and AlaAlaOrn b(3)(+) fragment ions from ornithine-containing tetrapeptides. Infrared spectra were obtained in the fingerprint region (1000 – 2000 cm(−1)) using the infrared free electron lasers at the Centre Laser Infrarouge d’Orsay (CLIO) facility in Orsay, France and the Free Electron Lasers for Infrared eXperiments (FELIX) facility in Nijmegen, the Netherlands. A novel terminal ornithine lactam AO(+) b2(+) structure was synthesized for experimental comparison and spectroscopy confirms that the b2(+) fragment ion from AOAA forms a lactam structure. Comparison of experimental spectra with scaled harmonic frequencies at the B3LYP/6-31+G(d,p) level of theory shows that AO(+) b(2)(+) forms a terminal lactam protonated either on the lactam carbonyl oxygen or the N-terminal nitrogen atom. Several low-lying conformers of these isomers are likely populated following IRMPD dissociation. Similarly, a comparison of the experimental IRMPD spectrum with calculated spectra shows that AAO(+) b(3)(+)-ions also adopt a lactam structure, again with multiple different protonation sites, during fragmentation. This study provides spectroscopic confirmation for the lactam cyclization proposed for the “ornithine effect” and represents an alternative b(n)(+) structure to the oxazolone and diketopiperazine/macrocycle structures most often formed.

Protonated ferulic acid and its principle fragment ion have been characterized using infrared multiple photon dissociation spectroscopy and electronic structure calculations at the B3LYP/6-311 + G(d,p) level of theory. Due to its extensively conjugated structure, protonated ferulic acid is observed to yield three stable fragment ions in IRMPD experiments. It is proposed that two parallel fragmentation pathways of protonated ferulic acid are being observed. The first pathway involves proton transfer, resulting in the loss of water and subsequently carbon monoxide, producing fragment ions m/z 177 and 149, respectively. Optimization of m/z 177 yields a species containing an acylium group, which is supported by a diagnostic peak in the IRMPD spectrum at 2168 cm(-1). The second pathway involves an alternate proton transfer leading to loss of methanol and rearrangement to a five-membered ring.

The structure of the proton-bound lysine dimer has been investigated by infrared multiple photon dissociation (IRMPD) spectroscopy and electronic structure calculations. The structures of different possible isomers of the proton-bound lysine dimer have been optimized at the B3LYP/6-31 + G(d) level of theory and IR spectra calculated using the same computational method. Based on relative Gibbs free energies (298 K) calculated at the MP2/aug-cc-pVTZ//B3LYP/6-31 + G(d) level of theory, LL-CS01, and followed closely (1.1 kJ mol(-1)) by LL-CS02 are the most stable non-zwitterionic isomers. At the MP2/aug-cc-pVTZ//6-31 + G(d) and MP2/aug-cc-pVTZ//6-31 + (d,p) levels of theory, isomer LL-CS02 is favored by 3.0 and 2.3 kJ mol(-1), respectively. The relative Gibbs free energies calculated by the aforementioned levels of theory for LL-CS01 and LL-CS02 are very close and strongly suggest that diagnostic vibrational signatures found in the IRMPD spectrum of the proton-bound dimer of lysine can be attributed to the existence of both isomers. LL-ZW01 is the most stable zwitterionic isomer, in which the zwitterionic structure of the neutral lysine is well stabilized by the protonated lysine moiety via a very strong intermolecular hydrogen bond. At the MP2/aug-cc-pVTZ//B3LYP/6-31 + G(d), MP2/aug-cc-pVTZ//6-31 + G(d) and MP2/aug-cc-pVTZ//6-31 + G(d,p) levels of theory, the most stable zwitterionic isomer (LL-ZW01) is less favored than LL-CS01 by 7.3, 4.1 and 2.3 kJ mol(-1), respectively. The experimental IRMPD spectrum also confirms that the proton-bound dimer of lysine largely exists as charge-solvated isomers. Investigation of zwitterionic and charge-solvated species of amino acids in the gas phase will aid in a further understanding of structure, property, and function of biological molecules.