[pH dependence and EPR spectra of Fe-NO complexes with purines and pyrimidines]

Equilibria between different types of Fe(I)-dinitrosyl complexes with nucleobases in solution were studied by means of EPR spectroscopy. Computer simulation and 15NO isotopic substitution were used in order to make easier the interpretation of complicated EPR patterns. The pH dependence of the purine and pyrimidine complexes was investigated. Several EPR signals, under slow exchange conditions, were present in the range of pH values of biological significance. Four types of complexes were identified on the basis of the nuclear hyperfine structure: B' = where two purine molecules were bound to iron via N-7 in the imidazole ring; B'' = where two mercapto-base molecules were bound to iron via S-; B''' = where one mercapto-base molecule was bound to iron via S- and another via pyrimidine-nitrogen; B* = where two pyrimidine molecules were bound to iron via pyrimidine-nitrogen.