Energetics from slow infrared multiphoton dissociation of biomolecules

Photodissociation kinetics of the protonated pentapeptide leucine enkephali n measured using a cw CO2 laser and a Fourier-transform mass spectrometer a re reported. A short induction period, corresponding to the time required t o raise the internal energy of the ion population to a (dissociating) stead y state, is observed. After this induction period, the dissociation data ar e accurately fit by first-order kinetics. A plot of the log of the unimolec ular dissociation rate constant, k(uni), as a function of che log of laser power is linear at low laser powers (<9 W, k(uni) < 0.05 s(-1)), but tapers off at high laser power (9-33 W, k(uni) = 0.05-7 s(-1)). The entire measur ed dissociation curve can be accurately fit by an exponential function plus a constant. The experiment is simulated using a master equation formalism. In the model, the laser radiation is described as an energetically flat-to pped distribution which is spatially uniform. This description is consisten t with experimental results which indicate that ion motion within the cell averages out spatial inhomogeneities in the laser light. The model has seve ral adjustable parameters. The effect of varying these parameters on the ca lculated kinetics and power dependence curves is discussed. A procedure for determining a limited range of threshold dissociation energy, E-o, which f its both the measured induction period and power dependence curves, is pres ented. Using this procedure, E-o of leucine enkephalin is determined to be 1.12-1.46 eV. This result is consistent with, although less precise than, v alues measured previously using blackbody infrared radiative dissociation. Although the blackbody dissociation results were used as a starting point t o search for fits of the: master equation model to experiment, these result s demonstrate that it is, in principle, possible to determine a limited ran ge of E-o from slow infrared multiphoton dissociation data alone.