NMR shieldings and electron correlation reveal remarkable behavior on the part of the part of the flavin N₅ reactive center

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 C_10a, 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 particularly, 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.