PROTON AFFINITIES AND PHOTOELECTRON-SPECTRA OF PHENYLALANINE AND N-METHYLPHENYLALANINE AND N,N-DIMETHYLPHENYLALANINE - CORRELATION OF LONE-PAIR IONIZATION ENERGIES WITH PROTON AFFINITIES AND IMPLICATIONS FOR N-METHYLATION AS A METHOD TO EFFECT SITE-SPECIFIC PROTONATION OF PEPTIDES

A Fourier transform ion cyclotron resonance (FT-ICR) technique for mea suring gas-phase proton affinities is presented which utilizes collisi onal dissociation of proton-bound clusters by off-resonance translatio nal excitation. A simplified RRKM analysis relates unimolecular dissoc iation rates to proton affinities. This technique is used to measure v alues for the proton affinities of phenylalanine and N-methyl- and N,N -dimethylphenylalanine of 220.3, 223.6, and 224.5 kcal/mol, respective ly (relative to the proton affinity of NH3 = 204.0 kcal/mol). The prot on affinity measured for phenylalanine is in excellent agreement with reported literature values. The photoelectron spectra of these three m olecules are also presented and analyzed. Assignments of bands to spec ific ionization processes are aided by comparison with model compounds such as methyl-substituted amines and 2-phenylethylamines. These data are employed to examine the correlation of adiabatic nitrogen lone pa ir ionization energies with gas-phase proton affinities for phenylalan ine, N-methylphenylalanine, and N,N-dimethylphenylalanine in compariso n to correlations for other amino acids and selected aliphatic amines. Although amine nitrogen methylation increases the potential for local izing charge at the amine terminus of protonated peptides by increasin g the gas-phase proton affinity, the present study establishes that th e increase is not sufficient to compete with protonation of some of th e more basic side chains in peptides.