Flavin Radical Chelates as Studied by Electron Spin Resonance and Isotopic Substitution.

The complexes formed between Zn2+ or Cd2+ and half-reduced flavins (isoalloxazines) in neutral aqueous as well as non-aqueous media were studied by electron spin resonance. Formation of 1:1 radical chelates between anionic radicals and Zn2+ as well as Cd2+ were found to occur. The stability constants at 20° for the chelates were estimated. A number of flavin derivatives were investigated. The spin density distribution was mapped by means of isotopic substitutions (²H, 15N, 64Zn, 67Zn, 111Cd, and 114Cd). The spin distribution within the ligand is similar to that found earlier for the anionic flavin radical. Spin delocalization to the metals was observed and hyperfine coupling constants for magnetic isotopes determined by direct measurements and spectra simulations. The difference in spin delocalization between Zn and Cd is discussed. [ABSTRACT FROM AUTHOR]