Fluorinated Tryptophan Derivatives for Photo-CIDNP NMR

High Resolution Image Download MS PowerPoint Slide Given the large hyperfine couplings commonly observed in 19 F nuclei within radicals, fluorinated amino acids are promising candidates for investigating the solvent exposure of amino acids and redox-active cofactors in proteins, as well as for identifying photogenerated intramolecular spin-correlated radical pairs in proteins through photochemically induced dynamic nuclear polarization (photo-CIDNP) nuclear magnetic resonance (NMR). By analyzing their photo-CIDNP properties in solution, this work aims to establish the foundation for using fluorinated tryptophan derivatives in photo-CIDNP, including protein studies. Although fluorinated tyrosine derivatives have been used in several photo-CIDNP studies, there are hardly any data for fluorinated tryptophan derivatives. We assume this is due to the significant line broadening of 19 F resonances of fluorinated tryptophan derivatives when flavin mononucleotide (FMN) is used as a photosensitizer. We attribute this to a photochemical reaction occurring between FMN and the fluorinated tryptophans. The broadening can be avoided by using fluorescein as a photosensitizer. The hyperfine couplings of 1 H and 19 F nuclei in the fluorinated tryptophan radicals of the commercially available derivatives 4-fluorotryptophan, 5-fluorotryptophan, 6-fluorotryptophan, and 7-fluorotryptophan were probed by geminate photo-CIDNP. Time-resolved photo-CIDNP was used to investigate the microsecond kinetics of 19 F and 1 H photo-CIDNP polarization in the 6-fluorotryptophan radical, revealing strongly different time evolutions due to the nuclei’s distinct paramagnetic relaxation. Experimental considerations for incorporation into proteins and for photo-CIDNP of such proteins are discussed.