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

Abstract

The water binding energies (E₀) of eight deprotonated Pb²⁺–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/6-31+G(d,p) and MP2(full)/6-311++G(2d,2p)//B3LYP/6-31+G(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.