Abstract
Beinert and Sands (1) have observed that free radicals occur in reduced diphosphopyridine nucleotide-cytochrome c reductase systems in the presence of reduced diphosphopyridine nucleotide at low temperature and that the radical concentration is a function of the DPN:DPNH mole ratio. The radicals appear to be flavin semiquinone species. Numerous attempts in this laboratory to observe radicals in DPNH-cytochrome c reductase systems at temperatures above O”r by the electron spin resonance (ESR) technique have been uniformly unsuc- cessful. Experiments reported here suggest that the existence of an icy environment for the enzyme is of crucial importance for radical formation. The temperature dependency of the ESR signal of the radical from - 150” to - 5” is discussed with regard to the possible types of spin states that may exist over the temperature range studied.