Factors affecting light energy conversion in dual fluorophore-nitroxide molecules in solution and a protein

Donor-acceptor structures capable of retaining the charge-photoseparated state during the time long enough for secondary chemical reactions of these charges to occur attract special interest from the viewpoint of the problem of light energy utilization. We proposed dual fluorophore-nitroxide compounds (FNO^•) as systems for the conversion of light energy to chemical energy. In these systems, the fluorophore segment in the excited singlet state serves as an electron donor, and the nitroxide segment is an electron acceptor. In FNO^•, the photo- and chemical reduction of nitroxide results in the drastic decay of the electron spin resonance (ESR) signal from the nitroxide and the parallel enhancement of fluorescence. The same groups allow one to measure the factors affecting the electron transfer, namely, molecular dynamics and micropolarity of the medium in the vicinity of the donor (by fluorescence technique) and acceptor (by ESR) moieties. We demonstrate that in the dual probes the nitroxide segment is photoreduced to hydroxylamine in solution and in such nanoscale structures as serum albumins. The photoreduction occurs by very weak reducing agents (glycerol, ethanol, ethylene glycol, etc.) without a violation of the fluorophore structure. Therefore, photochemical reactions in the dual compounds with the formation of a reducing agent as hydroxyl amine can be considered as processes of light energy transfer. The nitroxide segment tethered to the donor-bridge-acceptor triad affects the photoseparated charge recombination via the mechanism of spin catalysis. Proficiency of the dual compounds for developing energy conversion systems can be extended by an optimal choice of the participants of the photochemical and -physical processes.