On the proton affinity of some α-amino acids and the theory of the kinetic method

Abstract

A rationalization of the kinetic method for determination of proton affinities (E_pa) has been formulated. When a proton-bound amine dimer with the general structure amine₁–amine₂–H ⁺ decomposes to either amine₁–H⁺ or amine₂–H⁺ the critical energies for the competing fragmentations can be calculated from a simplified version of the Marcus equation, which is supported by published values of molecular pair proton affinities. Consequently reaction rates of the metastable ions can be calculated from the expression k(E)=ν[(E–E₀)/E]^{s– 1} and ion abundances from the expression ∫_EP(E)F(E)dE, where P(E) is the probability of reaction and F(E) is the energy distribution function of the metastable ions. It is argued that for metastable ions generated by ionization methods such as Cl or FAB, the energy distribution functions will be smooth and that consequently the relative ion abundances from two competing decompositions will not depend on F(E). Model calculations of fragment ion abundances from metastable decomposition of ions with the general structure pentylamine–amine_x–H⁺ show a linear relationship between the logarithm to the ratio: I(amine_x–H⁺)/(pentylamine–H⁺) and the E_pa of amine_x. This provides a rationalization of the kinetic method that avoids any introduction of a thermodynamic temperature. Determination of the fragment ion abundances from decomposition of metastable protonated clusters with the general structure α-amino acid–amine_x–H for 17 different α-amino acids gave the following E_pa/kcal mol^–1 values: Ser, 217.2; Val, 218.1; Asp, 218.1; Leu, 218.7; Ile, 219.2; Thr, 219.2; Phe, 219.9; Tyr, 220.7; Met, 221.0; Asn, 222.1; Glu, 222.3; Pro, 222.4; Trp. 223.5; Gln, 226.9; Lys, 228.7; His, 230.5; Arg, > 242.8.