Carbon-13 nuclear magnetic resonance study on the dynamics of the conformation of reduced flavin

Several flavin model compounds in the reduced state have been investigated by 13C NMR techniques. The NMR spectra were recorded in dependence of temperature, in the range of 30 to -100 degrees C. The results show that the activation barrier for the ring inversion ("butterfly" motion) is too low to be observed directly. In order to be able to detect the barrier of the ring inversion, it was coupled with a side-chain rotation. In this way, the intrinsic barrier for the ring inversion is increased by the barrier of the side-chain rotation, which allowed detection of the former barrier. It is shown that the intrinsic barrier for the ring inversion is less than 20 kJ/mol. Moreover, it is shown that previous results of Tauscher et al. [Tauscher, L., Ghisla, S., & Hemmerich, P. (1973) Helv. Chim. Acta 56, 630-649] are incorrect and nitrogen inversion is not observed. Symmetry arguments in the dynamic processes are discussed. From the low activation barrier for the ring inversion, it can be concluded that the conformation of the reduced flavin can be easily influenced upon binding to apoflavoproteins. This aspect might be of importance in the regulation of the function of the flavin prosthetic group in biological systems.