Mass-spectrometric and computational study of tryptophan radicals (Trp + H)˙ produced by collisional electron transfer to protonated tryptophan in the gas phase.

Hydrogen atom adducts to tryptophan were generated for the first time in the gas phase by collisional electron transfer to protonated tryptophan at 7170 eV kinetic energy. The radicals showed fast dissociations by C(α)-C(β) bond cleavage and cross-ring cleavages occurring on the 7.3 μs time scale. The mechanism of the C(α)-C(β) bond cleavage was explained by ab initio computational analysis of the radical potential energy surface. This showed spontaneous isomerization of the primary tryptophan radical by ammonium hydrogen atom migration to the carboxyl group. The stable intermediate formed by the isomerization can undergo radical-induced scission of the C(α)-C(β) bond in competition with H-atom migrations to the C-2 and C-4 positions of the indole ring. RRKM calculations of unimolecular rate constants on the B3-ROMP2/6-311++G(2d,p) potential energy surface indicated that the C(α)-C(β) bond cleavage was the fastest unimolecular reaction of the radical intermediates within the range of internal energies acquired upon electron transfer. We also report an updated G2(MP2) proton affinity of tryptophan (PA = 946 kJ mol(-1)) and hydrogen atom affinities of the tryptophan indole ring of relevance to electron-based peptide dissociations.