NMR Shieldings and Electron Correlation Reveal Remarkable Behavior on the Part of the Flavin N₅ Reactive Center

Abstract

NMR calculations on oxidized lumiflavin using density functional theory (DFT) reveal that methods including electron correlation show significant improvements in the flavin carbon and nitrogen NMR shieldings over uncorrelated Hartree−Fock (HF) results. In particular, the GIAO B3PW91//B3PW91 cc-pVTZ NMR shielding of the important N5 reactive center shows an improvement of 60.7 ppm over GIAO HF//B3PW91 cc-pVTZ, bringing it to within 6.1 ppm of the solvent-corrected experimental value. While the N5 center becomes dramatically shielded upon going from HF to correlated NMR shieldings, with the exception of C10a, all other NMR resonances are systematically deshielded. The nitrogens N1, N3, and N10 become deshielded by 23−26 ppm, and carbons become deshielded by an average of 5.2 ppm, bringing them into better agreement with experiment. Similar results are obtained with the significantly smaller 6-311G(d,p) basis set. The effects of electron correlation through the flavin geometry are also evident. The geometries calculated by B3LYP, B3PW91, and MP2 correlated methods using the 6-311G(d,p) basis lead to systematic NMR deshielding for all flavin heavy atoms in comparison with those obtained at the HF geometry. On average, the flavin nitrogens become deshielded by more than 10 ppm and carbons by almost 4 ppm. Similar changes, but of smaller magnitude, are observed for the NMR shieldings of fully reduced flavin, indicating that electron correlation is less important in that flavin state. The magnitude of the dramatic NMR shielding increase of several electrophilic centers upon flavin reduction is well captured by DFT methods. In particular, the GIAO B3PW91/cc-pVTZ N5 NMR shielding difference between oxidized and reduced flavin is 299.6 ppm, which compares well to the solvent-corrected experimental shielding difference of 299.1 ppm. The N5 center is also remarkable for its large decrease in shielding, by 33.4 ppm (98.0 ppm deshielding of σ11), upon protonation of the oxidized flavin at N1, a process which activates N5 for electrophilic attack.

Spectroscopic Properties of Lumiflavin: A Quantum Chemical Study

“…The experimental data for the carbon and nitrogen atoms were obtained from previous studies of the oxidized form of tetraacetylriboflavin . The conversion of the measured chemical shift values to chemical shieldings is described in ref . Since chloroform can only donate a hydrogen bond but not accept it, Walsh and Miller have corrected the values for N 1 and N 5 by −12.5 ppm.…”

“…The conversion of the measured chemical shift values to chemical shieldings is described in ref . Since chloroform can only donate a hydrogen bond but not accept it, Walsh and Miller have corrected the values for N 1 and N 5 by −12.5 ppm. Further, it is also important to note that the experimental shifts were measured in chloroform.…”

“…The NMR chemical shielding values are given in Table . The experimental data for the carbon and nitrogen atoms were obtained from previous studies of the oxidized form of tetraacetylriboflavin . The conversion of the measured chemical shift values to chemical shieldings is described in ref .…”

15N Solid-State NMR as a Probe of Flavin H-Bonding

“…42 Likewise, previous SSNMR studies of a model flavin demonstrated that very different CSPVs clearly differentiate between the N5 and N1 sites, consistent with N5 and N1’s distinct electronic properties and reactivities. 30,43 …”

Hydrogen bond-free flavin redox properties: managing flavins in extreme aprotic solvents

“…17 These experiments confirmed earlier ab initio calculations that predicted that several of the principal components of the chemical shift tensor, in particular the σ 11 components of the N(1) and N(5) tensors, strongly reflect the H-bond accepting lone pairs of the flavin. 18 In order to perform solid state NMR experiments in frozen solution, a molecule must be soluble to at minimum 100 mM concentration in a solvent that freezes above -70° C. In order for a solvent to dissolve a model flavin it must be able to interact favorably with some part of it. An additional parameter of importance is the ease and expense of constructing the molecule with isotopic labels at the appropriate positions.…”