ON THE PROTON AFFINITY OF SOME ALPHA-AMINO-ACIDS AND THE THEORY OF THE KINETIC METHOD

A rationalization of the kinetic method for determination of proton af finities (E(pa)) has been formulated. When a proton-bound amine dimer with the general structure amine(1)-amine(2)-H+ decomposes to either a mine(1)-H+ or amine(2)-H+ the critical energies for the competing frag mentations can be calculated from a simplified version of the Marcus e quation, which is supported by published values of molecular pair prot on affinities. Consequently reaction rates of the metastable ions can be calculated from the expression k(E) = upsilon[(E-E(0))/E](s-1) and ion abundances from the expression integral(E)P(E)F(E)dE, where P(E) i s the probability of reaction and F(E) is the energy distribution func tion of the metastable ions. It is argued that for metastable ions gen erated by ionization methods such as Cl or FAB. the energy distributio n functions will be smooth and that consequently the relative ion abun dances from two competing decompositions will not depend on F(E). Mode l calculations of fragment ion abundances from metastable decompositio n of ions with the general structure pentylamine-amine(x)-H+ show a li near relationship between the logarithm to the ratio: /(amine(x)H(+))/ (pentylamine-H+) and the E(pa) of amine(x). This provides a rationaliz ation of the kinetic method that avoids any introduction of a thermody namic temperature. Determination of the fragment ion abundances from d ecomposition of metastable protonated clusters with the general struct ure alpha-amino acid-amine(x)-H for 17 different alpha-amino acids gav e the following E(pa)/kcal mol(-1) values: Ser. 217.2; Val, 218.1; Asp , 218.1; Leu, 218.7; lle, 219.2; Thr, 219.2; Phe, 219.9; Tyr, 220.7; M et, 221.0; Asn, 222.1; Glu, 222.3; Pro, 222.4; Trp, 223.5; Gin, 226.9; Lys, 228.7; His, 230.5; Arg, > 242.8.