IR spectroscopic and quantum-chemical manifestation of flavin mononucleotide’s isoalloxazine ring protonation

Ю. Н. Близнюк Институт радиофизики и электроники им. А. Я. Усикова НАН Украины
Д. А. Песина Институт радиофизики и электроники им. А. Я. Усикова НАН Украины
Д. М. Глибицкий Институт радиофизики и электроники им. А. Я. Усикова НАН Украины
М. О. Семенов Институт радиофизики и электроники им. А. Я. Усикова НАН Украины
А. В. Шестопалова Институт радиофизики и электроники им. А. Я. Усикова НАН Украины

Keywords: infrared spectroscopy, flavin mononucleotide, protonation, quantum-chemical calculation, nitrogen atom

Abstract

Flavin mononucleotide (FMN) is the prosthetic group of many oxidoreductases performing redox function in cells. The coenzyme reversible reduction reaction involvs isoalloxazine ring which may take two electrons and two protons upon reduction. The spectra of dried films of flavin mononucleotide grown from neutral (pH=7,0) and weakly acidic (pH=6,0) solutions were obtained in absorption region of C=C, C=N atoms of isoalloxazine ring (1450–1650 cm^-1) using infrared (IR) spectroscopic method. The high-frequency shifts of absorption bands of intra-ring vibrations in films grown from weakly acidic (pH=6,0) solution were found. These high-frequency shifts were explained by the protonation of N₁ and N₅ nitrogen atoms of the ring. The performed quantum-chemical calculation using the density functional theory (DFT) confirmed that the N₁ and N₅ atoms have significant negative charges. The obtained quantum-chemical data on the charges of the isoalloxazine ring atoms and its bond orders correlate with the frequency shifts of the absorption bands of protonated atoms of intra-ring vibrations.

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