Water binding energies of [Pb(amino acid-H)H2O] complexes determined by blackbody infrared radiative dissociation

摘要

The water binding energies (E0) of eight deprotonated Pb~(2+)-amino acid (Aa) complexes of the form [Pb(Aa-H)H2O]~+ (Aa = Gly, Ala, Val, Leu, Ile, Phe, Glu, and Lys) were determined using blackbody infrared radiative dissociation (BIRD). A Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer was used to trap ions generated by electrospray ionization (ESI) in a "zero"-pressure (~10~(-10) torr) cell where dissociation can only occur by absorption of thermally generated photons. Since the [Pb(Aa-H)H2O]~+ complexes have relatively few vibrational degrees of freedom (36-78) and are within the slow-exchange kinetic limit, the master equation was solved to extract meaningful threshold dissociation energies and thermal unimolecular dissociation rate constants (k_(uni)). The master equation analysis uses variable reaction coordinate transition state theory (VRC-TST) to minimize the Rice-Ramsperger-Kassel-Marcus (RRKM) dissociation rate constants. The determined water binding energies range From 76.6 to 113.6 kJ mol~(-1), and agree well with 0 K dissociation energies calculated using the B3LYP/6-3G(d,p) and MP2(full)/ 6-311++G(2d,2p)//B3LYP/6-3G(d,p) methods. The relative strengths of the binding energies reflect the known structural isomers (A-, B-, C-, and D-type) of these [Pb(Aa-H)H2O]~+ complexes.