Formation and Spectroscopy of a Tryptophan Radical Containing Peptide in the Gas Phase

摘要

Phenoxyl and indolyl radicals are ubiquitous in biology. For example, indolyl derivatives are known to possess antioxidant properties as radical scavengers. Tryptophan radicals participate in electron and radical transfers in proteins, and they have been recognized in numerous enzymes as catalytically relevant redox agents. Radical transfers occur either by electron transfer followed by proton release to create a neutral radical, or by H-atom transfer with the simultaneous transfer of electrons and protons. Distinguishing between neutral and cationic radical forms in a protein is crucial because the identity of the radical determines the details of its reactivity. The combination of spin resonance spectroscopic techniques and density functional theory (DFT) theoretical results is currently the most advanced approach for elucidating the nature of radicals. Optical properties have also undergone extensive investigation. For tryptophan, radical spectra of the relevant protonated and deprotonated forms in solution are obtained from pulse radiolysis experiments and are usually used as reference spectra for characterizing tryptophan radicals in proteins. However, it has been shown that electronic transitions in tryptophan radicals are highly environment-dependent (in particular for structurally defined environments). Moreover, the difficulty of modeling the influence of noncovalent contacts on the spectra of these radicals may limit the interpretation of these experiments.