The influence of flavo nucleotide (FMN) dimerization on the efficiency of the FMN triplet states generation

The efficiency of triplet state generation in flavin mononucleotide (FMN) water solutions at various pigment concentrations illuminated at two wavelengths (380 nm and 430 nm) was established by the Laser Induced Optoacoustic Spectroscopy (LIOAS) method. It was found that the efficiency of triplet generation by the singlet–triplet intersystem crossing after illumination at 430 nm was usually lower than that after illumination at 380 nm. The yield of triplet generation in FMN at low concentrations in solutions at 380 nm illumination is rather high, about 0.6 but its value is diminished with the increase in the sample temperature and the dye concentration and depends also on the wavelength of excitation. The increase in FMN dimers content gives an increase in fast internal nonradiative conversion of excitation and a decrease in the yield of the fluorescence. LIOAS measurements of the sample and the reference dye in buffered water solutions at 3 °C suggest that only a small part of the excitation energy is used for some structural changes of FMN or possibly for the changes in FMN interactions with its surrounding solvent molecules. The changes in both types should ensue some modification in the volume of the absorbing species giving nonthermal contributions to LIOAS signal. The LIOAS results were corrected for FMN volume changes. The different yields of triplet states generation obtained by two wavelengths of illumination are not only due predominantly to various contributions to absorption from dimeric FMN but also can be partially related to different grade of the superposition of n–π∗ and π–π∗ transitions in a region of 380 nm and 430 nm absorption. The absorption at 430 nm band is due predominantly to π–π∗ transition, whereas in 380 nm region the n–π∗ transition could have higher contribution which can exhibit different efficiency of triplet generation than π–π∗ transition. For the samples with low FMN concentrations the yields of fluorescence of FMN excited at these two regions are similar, but at higher dye concentrations they are different.