The effect of temperature on FMN absorption spectra in rigid poly(vinyl alcohol) matrices

Electronic absorption spectra of flavomononucleotide (FMN) in poly(vinyl alcohol) films (PVA) were measured over the concentrations ranging from 6.9 x 10(-4) to 6.8 x 10(-1) M and temperatures from 263 to 338 K. The FMN absorption spectra measurements performed at room temperature have shown two ranges of different changes as a function of dye concentration. For concentrations c<10(-1) M (range I) the spectra exhibited regular changes showing an isosbestic point, which evidences the equilibrium between monomers and dimers. However, for range II (c>1.05 x 10(-1) M) the FMN absorption spectra occurred to be almost independent of concentration and they nearly overlapped with the dimer spectrum (within the error limit). Temperature measurements have shown that the FMN absorption spectra in PVA are stable over a wide temperature range. The mean distances between FMN molecules in PVA films are calculated. For maximal concentrations (from the range II), they are below 13.1 A, whereas the mean dimensions of FMN monomers and dimers are 15.8 and 21.1 A, respectively, which indicates that the orientation of dimers and monomers in the PVA film cannot be random at high concentrations. Molecules are partly ordered, adopting approximately parallel orientation, which is in agreement with the calculations of dimer structure by molecular modelling method (MMM).