Nuclear-magnetic-resonance investigation of 15N-labeled flavins, free and bound to Megasphaera elsdenii apoflavodoxin.

Flavin derivatives, enriched with 15N (approximately equal to 95%) at the four nitrogen atoms of the isoalloxazine ring, have been investigated in the oxidized and the two-electron reduced state by the 15N nuclear magnetic resonance technique. The measurements were conducted with aqueous and chloroform solutions of flavin. A comparison of the chemical shifts of the N(1) and N(5) atoms of oxidized flavin in the two solvents revealed that these atoms are sensitive indicators for possible hydrogen-bridge formation to these atoms. The N(5) atom of oxidized flavin resonates at low field and shifts about 300 ppm upfield upon reduction. A pKa of 6.8 was determined from pH-dependent 15N NMR measurements of the two-electron reduced flavin molecule. In addition it is also shown that reduced flavin in aqueous solution possesses a more coplanar structure than in chloroform solution. The 15N chemical shifts of flavin bound to Megasphaera elsdenii apoflavodoxin indicate that various hydrogen bridges are formed between the prosthetic group and the apoprotein. Especially the N(1) atom of the prosthetic group in the oxidized state seems to form a strong hydrogen bond with the apoprotein. In the reduced state the prosthetic group is bound in the anionic form and possesses an almost coplanar structure. These results are in agreement with published crystallographic data on the related flavodoxin from Clostridium MP. Where possible 15N-1H, 15N-15N and 13C-15N coupling constants were determined. Some of the coupling constants are useful parameters for the elucidation of the planarity of free and protein-bound flavin and for the evaluation of the interaction between flavin and apoprotein. Spin-lattice relaxation measurements show that the relaxation of the 15N(3)H group of flavin is predominantly determined by dipole-dipole interaction. The calculated rotational correlation times of flavin in two different solvents were determined and are in good agreement with published results.