FTIR characterization of heterocycles lumazine and violapterin in solution: Effects of solvent on anionic forms

Abstract

Fourier transform infrared (FTIR) spectra have been obtained from solution samples of the heterocycles uracil, lumazine, and violapterin and reveal interpretable carbonyl stretching frequencies. Spectra of conjugate bases of lumazine and violapterin demonstrate decreases in these carbonyl stretching frequencies upon ionization. Based on isotopic shifts from amide deuterated analogs, semiempirical QCFF/PI calculations were used to assign the vibrational frequencies in the region 1100–1800 cm−1 observed from samples in dimethylsulfoxide (DMSO) and aqueous solutions to specific normal modes. The observed deuterium shifts and the calculations suggest that, in some cases, N—H bending motions are coupled to the C=O stretching motions of the pyrimidine ring. These data suggest that for lumazine anions a change in solvent can significantly change the mixing of the N—H bending and C=O stretching vibrational motions. This implies that vibrational analysis for lumazine species in relatively noninteracting media like nonpolar solvents, mulls or pellets cannot necessarily be transferred to the system when it is dissolved in a polar, hydrogen‐bonding solvent such as water. Although other explanations can be offered, our vibrational analysis suggests that the changes in normal mode composition of the predominantly C=O stretching vibrations of lumazine anion on going from dimethylsulfoxide to water solution are consistent with a change in the predominant tautomer of the heterocycle. This change appears to correspond to a shifting of the location of the remaining acidic proton to a different ring nitrogen atom. This interpretation is of interest in view of recent ab initio calculations which suggest that proton shifts may occur during the hydroxylation of lumazine as mediated by the enzyme xanthine oxidase. © 1998 John Wiley & Sons, Inc. Biospectroscopy 4: 235–256, 1998

Resonance Raman Studies of Xanthine Oxidase: the Reduced Enzyme−Product Complex with Violapterin

“…H/D Isotopic Shifts. As seen in previous vibrational studies of lumazine and violapterin prepared in both H 2 O and D 2 O, , correlation of vibrational modes between protonated and deuterated species is made difficult because of changes in heterocycle mode composition upon deuteration, along with strong coupling of NH/ND bending motions shifting from the higher energy modes (1350 cm -1 and higher) for the protonated species to modes with energies below 1250 cm -1 upon deuteration. It is evident from the mode plots of the protonated model shown in Figure S (column 1) in the Supporting Information that nearly every mode has some amount of NH bending character and that this coupling is appreciable in all modes above 1350 cm -1 (Q17−Q38).…”

Vibrational Spectra of Lumazine in Water at pH 2−13: Ab Initio Calculation and FTIR/Raman Spectra

“…has demonstrated that the anion isomer of lumazine in aqueous solvent is different from that in dimethyl sulfoxide (DMSO). 36 Again, because it is a substrate for enzymatic oxidation and therefore exposed to environments of highly inhomogeneous protonating power, this aspect of lumazine chemistry deserves careful analysis. The problem, unfortunately, becomes complicated somewhat by methodological and procedural limitations of the electronic structure calculations.…”

“…It is possible, however, that there is not a single most stable lumazine anion or not always the same isomer in every medium. For example, a previous study has demonstrated that the anion isomer of lumazine in aqueous solvent is different from that in dimethyl sulfoxide (DMSO) …”

A new composite hydrogel combining the biological properties of collagen with the mechanical properties of a supramolecular scaffold for bone tissue engineering

“…Indeed, the peak from GNF at 1,635 cm −1 is shifted to higher wavelength, 1,638 cm −1 , and the amide I band from collagen from 1,656 to 1,658 cm −1 (Figure a1). These shifts could be correlated to chemical interactions such as hydrogen bonding between carbonyl and nitrogen species, as already described in the literature (Michaud et al, ). In order to confirm these observations, the theoretical spectrum (addition of collagen and GNF spectra as described in experimental part) was compared to the experimental one as seen in Figure a2.…”