Does IVR take place prior to peptide ion dissociation?

Small peptide ions are studied by time-resolved photodissociation (TRPD). Laser desorption of neutral peptides is combined with laser photoionization in an ion trap followed by thermalization, laser photodissociation, and time-of-flight mass analysis. Ionization and excitation take place through an aromatic chromophore at the C-terminus of the peptide, whereas dissociation produces the immonium ion at the N-terminus. The purpose is to uncover the role of intramolecular vibrational redistribution (IVR) in unimolecular fragmentations of peptide radical cations the excitation of which is site-selective. Whereas previous experiments concentrated on mass spectra, the avenue taken here is the determination of microcanonical rate constants. The rate constants are measured at a fairly well-defined internal energy E for two peptides possessing the same chromophore, undergoing the same fragmentation but having a different number of degrees of freedom. Experimental rate measurements in the range of ∼10²-10⁵ s^-1 will be presented for the peptides leucyl tyrosine (LeuTyr) and leucyl leucyl tyrosine (LeuLeuTyr). One-color (280.5 nm) two-photon ionization, thermalization for 1980 ms, and excitation at 579 nm of LeuTyr and LeuLeuTyr yield (4.8 ± 1.8) × 10³ and (2.9 ± 1.9) × 10² s^-1 inverse time constants, that is, rate constants, respectively. The rate constants provide a clear indication that the peptide length (i.e., its number of degrees of freedom) strongly correlates with the dissociation rate. This has been tested further through measurements at different photodissociation energies and through Rice-Ramsperger-Kassel-Marcus/quasi equilibrium theory (RRKM/QET) calculations that are demonstrated to be in good agreement with the experimental observations, indicating that the internal energy, E, is randomized. In other words, these peptides do not circumvent IVR.