Infrared spectrum and absorption coefficient of the cofactor flavin in water

Abstract

Flavins play a key role as redox cofactors of enzymes involved in important metabolic processes. Moreover, they undergo photochemical reactions as chromophores in sensors of blue light or magnetic field in many organisms. The reaction mechanisms of flavoproteins have been investigated by infrared spectroscopy and theoretical studies. However, basic information on flavins in the infrared spectral range has been missing, such as absorption spectra in water and absorption coefficients. Here, the cofactors flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) were investigated in aqueous medium by Fourier transform infrared spectroscopy. Transmission and attenuated total reflection (ATR) configuration were employed in direct comparison. Absorption spectra in the range of 920–1800 cm −1 were determined after accurate subtraction of the contributions from the water vibrations. The important carbonyl vibrations were resolved at 1661 and 1712 cm −1. The absorption spectra may serve as a reference for theoretical and experimental studies on the effect of the microenvironment on the flavin cofactor. Furthermore, the molar absorption coefficient of FAD at 1547 cm −1 was determined to 2200 L mol −1 cm −1 with an integral absorption coefficient of ∼50,000 L mol −1 cm −2. These values are prerequisite for the determination of reaction yields in flavoproteins from reaction-induced difference spectra.