Iodide influences the sensitized one-electron photo-oxidation of Tyr and Trp residues by competition reaction

3-carboxybenzophenone (CB) is an efficient photosensitizer that can oxidize multiple amino acid side chains in peptides and proteins via electron transfer (ET) reactions yielding various radicals and radical ions. Recombination reactions of these species can yield CBH-adducts and cross-links, whereas secondary reactions can give radicals on other side chains and further products. Prevention of initial radical formation, or interception of intermediate radicals is predicted to modulate the extent of protein damage. Consequently, in this work the effect of iodide ions (I‾) on CB-photosensitized oxidation was investigated with Trp and TyrOH, as these moieties are prone to photooxidation. A scavenging effect of I‾ on the formation of TyrO^• radicals was readily detected in kinetic experiments using laser flash photolysis, whilst effects on TrpN^• radical formation remain ambiguous (due to overlap of the absorptions of transient absorption spectra of TrpN^• and CBH^• radicals). Addition of I‾ suppresses oxidation of Trp and TyrOH, with this resulting in lower concentrations of di-Trp, di-Tyr and adducts with CBH, without formation of additional products involving I‾. The effect of I‾ was also analysed for a model protein – lysozyme – with a protective effect observed against loss of activity on illumination with CB. Multiple products were identified, including adducts of CBH to Trp, TyrOH or Met. The formation of crosslinks arising from CB-mediated photo-oxidation of lysozyme was limited in the presence of I‾. Together these data indicate that I‾ modulates photodamage induced by CB to peptide and protein targets.