Time-resolved electron paramagnetic resonance study of photoionization of tyrosine anion in aqueous solution

Photoionization of the amino acid tyrosine in basic water was studied by ti me-resolved electron paramagnetic resonance (TREPR) at X-band (9.5 GHz), Ph otoionization of deprotonated tyrosine leads to a spin-polarized emissive/a bsorptive chemically induced dynamic electron polarization (CIDEP) spectrum produced by the radical pair mechanism, with the tyrosyl radical in emissi on and the solvated electron in absorption, which implies a triplet precurs or. The exchange interaction, J, is found to be negative for this radical p air. The triplet photoionization channel is determined to be monophotonic. The singlet channel of photoionization of deprotonated tyrosine is seen onl y upon addition of the electron acceptor 2-bromo-2-methylpropionic acid (BM PA) to the sample. The singlet channel is isolated by performing TREPR on a sample containing tyrosine, BMPA and a triplet quencher (2,4-hexadienoic a cid). This channel is also found to be monophotonic.