Fluorinated Tryptophan Derivatives for Photo-CIDNP NMR

Given the large hyperfine couplings commonly observed in 19F 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 19F 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 1H and 19F 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 19F and 1H 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.