Completing the Picture: Determination of 13C Hyperfine Coupling Constants of Flavin Semiquinone Radicals by Photochemically Induced Dynamic Nuclear Polarization Spectroscopy

We investigate the electronic structure of flavin semiquinone radicals in terms of their 13C hyperfine coupling constants. Photochemically induced dynamic nuclear polarization (photo-CIDNP) spectroscopy was used to study both the neutral and anionic radical species of flavin mononucleotide (FMN) in bulk aqueous solution. Apart from universally 13C-labeled FMN, partially labeled isotopologues are used to increase sensitivity for nuclei exhibiting very small hyperfine couplings and to cope with spectral overlap. In addition, experimental findings are supported by quantum chemical calculations, and implications for the spin density distribution in free flavin radicals are discussed.

Molecular features toward high photo-CIDNP hyperpolariztion explored through the oxidocyclization of tryptophan

Photo-chemically induced dynamic nuclear polarization (photo-CIDNP) is a promising solution to the inherent lack of sensitivity in NMR spectroscopy. It is particularly interesting in biological systems since it operates in water, at room temperature, and it can be repeated if the bleaching of the system can be controlled. However, the photo-CIDNP signal enhancement is well below those of other hyperpolarization techniques. While DNP, PHIP, and SABRE reach polarization enhancements of 10³ to 10⁴-fold, photo-CIDNP enhancement is typically only one order of magnitude for ¹H and two orders of magnitude for ¹³C in the amino-acids tryptophan and tyrosine. Here we report on a photo-oxidation product of tryptophan that is strongly photo-CIDNP active under continuous wave light irradiation. In conjunction with the dye Atto Thio 12, a ¹H signal enhancement of 120-fold was observed on a 600 MHz spectrometer, while at 200 MHz the enhancement was 380-fold. These enhancements in signal to noise correspond to a reduction in measurement time of 14 400-fold and 144 400-fold, respectively. The enhancement for ¹³C is estimated to be over 1200-fold at 600 MHz which corresponds to an impressive measurement time reduction of 1 440 000-fold. This photo-CIDNP active oxidation product of tryptophan has been identified to be 3α-hydroxypyrroloindole. The reasons for its improved signal enhancement compared to tryptophan have been further investigated.

Elucidating organic reaction mechanisms using photo-CIDNP spectroscopy

CIDNP (chemically induced dynamic nuclear polarization) arises in radical pairs or biradicals but is detected in the diamagnetic reaction products. Hence, it can be used not only to identify and characterize both types of species but also to establish the pathways connecting precursors, paramagnetic intermediates and products, and to employ the polarizations as labels to individual nuclei. Recent applications of CIDNP to elucidate the mechanisms of photochemical reactions are reviewed, which illustrate all these facets.