ENDOR and related EMR methods applied to flavoprotein radicals

• Published in: Applied magnetic resonance 2007-09, Vol.31 (3-4), p.457-470
• Main Authors: Medina, M., Cammack, R.
• Format: Article
• Language: English
• Subjects: Binding sites; Biological activity; Chemical reactions; Couplings; Electron paramagnetic resonance; Electron spin; Electron transfer; Electronic structure; Oxidation; Protonation; Substrates; Superhigh frequencies
• ISSN: 0937-9347; 1613-7507
• DOI: 10.1007/BF03166596

Flavoproteins are involved in a wide range of biological processes, owing to the versatility of the isoalloxazine moiety of flavin, which can undergo both one- and two-electron reactions with the formation of three oxidation states. Paramagnetic semiquinone radical states are stabilised in some flavoproteins and appear as transient intermediates in the reaction of many others. The apoprotein controls the reactivity of flavin semiquinones, including redox potentials, protonation states and access to substrates. Most flavoproteins are involved in oxidation-reduction processes, but some catalyze different types of reactions involving radical intermediates. Anionic and neutral flavin radicals are found in flavoproteins and are distinguished by their line widths in X-band electron paramagnetic resonance. Electron-nuclear double resonance, electron spin echo envelope modulation and hyperfine sublevel correlation spectroscopy make it possible to observe biological electron transfer and catalysis at the level of the electronic structure of the intermediate states. They provide information about the protein environment of flavin semiquinone radicals and their interactions with nearby nuclear and electron spins. Hyperfine couplings, particularly to the 8-methyl protons on the flavin ring, are a sensitive probe of perturbations of the flavin environment. They demonstrate differences in polarity of the flavin binding site and changes that occur in flavoenzymes during binding of substrates.