IRMPD Spectroscopy of Biomolecules

Infrared multiphoton dissociation spectroscopy (IRMPD) is a spectroscopic method, developed in Europe during recent years, which is applicable to a wide class of molecular ions. This technique allows studying the structure of biomolecules in the gas phase through comparison between experimental IR spectra and their prediction from quantum chemistry calculations. Its main advantage is that it is quasi-universal and does not require the presence of a visible/UV chromophore as in R2PI/IR spectroscopy. We investigate ions thermalized at room temperature and produced by electrospray from solutions. Those ions are confined in a quadrupole ion trap mass-spectrometer (Bruker Esquire 3000) and illuminated by the high-power and high-repetition rate infrared beam issued from the CLIO free-electron laser facility (Orsay, France).

In order to interpret experimental IRMPD spectra, two possibilities are offered. The first one consists in a systematic search for the different potential energy minima. IR spectra are subsequently calculated for each low-energy equilibrium conformations. This is usually done within the harmonic approximation with some appropriate scaling factor. Comparison between the experimentally observed spectrum and the predicted spectra is then used to identify populated isomers and/or conformers of the studied system.

An approach closer to experimental conditions consists in simulating the dynamical behaviour of peptides through DFT-based Car-Parrinello molecular dynamics (CPMD) conducted at the experimental temperature. This task is performed in the group of M.P. Gaigeot LAMBE Evry. MD is essential for including conformational dynamics (entropic effects being taken into account without any approximation), and is thus essential for the calculation of IR spectra of complex molecular systems that display numerous energetically equivalent conformations. The main point of IR spectra calculations based on the dipole time correlation function is that all anharmonic effects are naturally taken into account, and no approximations are made. This is to be opposed to the two successive harmonic approximations usually adopted for the determination of IR spectra from static ab initio calculations (harmonic approximation of the potential energy surface at the optimized geometries and mechanical harmonic approximation for the transition dipole moments, as used in normal modes analysis). In particular, the finite temperature dynamics takes place on all accessible parts of the potential energy surface, be they harmonic or anharmonic.

Mass-selected ions absorb photons that bring them from a vi=0 to a vf=1 vibrational state. Between two photon absorptions, internal vibrational redistribution (IVR) takes place and brings back ions to their initial vi=0 vibrational state, while the acquired internal energy is redistributed over other modes. Following absorption of many photons, dissociation takes place. Infrared absorption is then monitored through detection of the ionic fragmentation yield.