Polarised fluorescence in FAD excited at 355 and 450 nm in water–propylene glycol solutions

Abstract

Time-resolved polarised fluorescence in flavin adenine dinucleotide (FAD) in water–propylene glycol mixtures has been studied as a function of propylene glycol concentration using the TCSPC technique. The fluorescence was excited by picosecond laser pulses either at 450 nm or at 355 nm, the wavelengths that belong to the first and second FAD absorption bands, respectively. The global fit procedure was used for the determination of fluorescence decay times, corresponding weighting coefficients, initial anisotropy and rotational diffusion time from experiment. The experimental data manifested a double-exponential decay in the whole range of propylene glycol concentrations with the decay times τ 1 , τ 2 and weighting coefficients a 1 , a 2 . The coefficients a 1 and a 2 and the corresponding relative fluorescence quantum yields Q ( 1 ) r e l and Q ( 2 ) r e l were shown to change dramatically with addition of propylene glycol to the solution and demonstrated the domination of contribution of longer decay time at medium and high propylene glycol concentrations. The rotational diffusion time τ r o t manifested a sharp rise, up to 40 times with propylene glycol concentration that was practically proportional to solution viscosity. The initial anisotropy was determined to be equal to r 0 = 0.35 ± 0.03 and r 0 = 0.25 ± 0.02 under excitation at 450 nm and 355 nm, respectively. The dependence of fluorescence parameters on the solution viscosity and polarity was discussed taking into account FAD conformation distributions.