The kinetic method was used to determine the proton affinities of methyl ea
ters of several saturated fatty acids. Decompositions of the proton-bound d
imers of the methyl esters, AHB(+) were observed under different conditions
with two instruments. The proton affinities (PAs) of the methyl esters inc
rease continually with increasing carbon number in the acid. Equilibrium an
d initial rate experiments were performed with a Fourier transform ion cycl
otron resonance mass spectrometer on the methyl ester of the C-22 saturated
acid (methyl behenate). These experiments give values for PA (methyl behen
ate) that are perhaps slightly lower than those obtained with the kinetic m
ethod. The Pas of the methyl esters of the fatty acids could be correlated
with the equation: PA (ester) = (40.0 +/- 2.5)*log(n) + (784.7 +/- 3.9) kJ/
mol or PA (ester) = (864 +/- 2) - (479 +/- 41)/n, where it = number of atom
s in the molecule. Proton affinities of smaller sets of 1-alkylamines and 1
-alkanols can be fit to similar equations. (C) 2000 American Society for Ma
ss Spectrometry.