4 research outputs found
IR-induced conformational isomerization of a helical peptide in a cold ion trap
In this work we use laser-induced population transfer techniques to study the conformational isomerization of a helical peptide, Ac-Phe-(Ala)5-LysH+, in a cold ion trap. In one scheme, called IR-UV hole-filling spectroscopy, a single conformation is selectively excited with an IR pump laser via a distinct NH stretch vibration. After giving the vibrationally excited ions sufficient time to isomerize and re-cool in the trap, the new conformational redistribution is detected by UV photofragment spectroscopy. While we clearly observe a redistribution of the conformer populations due to isomerization, only those conformations that initially have population participate in this redistributionâwe do not form conformers that were not initially present in the trap. In a second scheme, called IR-induced population transfer spectroscopy, we determine the fractional populations of the four stable conformations of Ac-Phe-(Ala)5-LysH+ by scanning the IR laser while selectively detecting a specific conformation using UV photofragment spectroscopy
Fragmentation mechanism of UV-excited peptides in the gas phase
We present evidence that following near-UV excitation, protonated tyrosine- or phenylalanineâcontaining peptides undergo intersystem crossing to produce a triplet species. This pathway competes with direct dissociation from the excited electronic state and with dissociation from the electronic ground state subsequent to internal conversion. We employ UV-IR double-resonance photofragment spectroscopy to record conformer-specific vibrational spectra of cold peptides pre-excited to their S1 electronic state. The absorption of tunable IR light by these electronically excited peptides leads to a drastic increase in fragmentation, selectively enhancing the loss of neutral phenylalanine or tyrosine side-chain, which are not the lowest dissociation channels in the ground electronic state. The recorded IR spectra evolve upon increasing the time delay between the UV and IR pulses, reflecting the dynamics of the intersystem crossing on a timescale of âŒ80 ns and <10 ns for phenylalanine- and tyrosine-containing peptides, respectively. Once in the triplet state, phenylalanine-containing peptides may live for more than 100 ms, unless they absorb IR photons and undergo dissociation by the loss of an aromatic side-chain. We discuss the mechanism of this fragmentation channel and its possible implications for photofragment spectroscopy and peptide photostability
FranckâCondon-like Progressions in Infrared Spectra of Biological Molecules
Infrared
spectra in the NH stretch region are often used for structure
determination of gas-phase biological molecules. Vibrational couplings
complicate the structure determination process by giving rise to additional
vibrational bands along with the expected fundamental transitions.
We present an example of a strong anharmonic coupling in a biological
molecule, Ac-Phe-Ala-LysH<sup>+</sup>, which causes the appearance
of long vibrational progressions in the infrared spectrum. By analyzing
the spectra of the ground and the electronically excited state, we
determined that the coupling occurs between the NH stretch (Ï<sub>NH</sub>) and a low-frequency torsion of the phenyl ring (Ï<sub>Ï</sub>). We describe the vibrational progressions using a
BornâOppenheimer-like separation of the high-frequency stretch
and low-frequency torsion with a quartic Taylor expansion for the
potential energy surface that accounts for the equilibrium distance
and frequency change of the torsional vibration upon the NH stretch
excitation. We also demonstrate that small conformational changes
in the peptide are sufficient to break this coupling