19 F NMR relaxation studies of fluorosubstituted tryptophans.

We present ¹⁹ F longitudinal and transverse relaxation studies for four differently fluorosubstituted L-tryptophans, which carry single F atoms in the indole ring, both in the context of the free amino acid and when located in the cyclophilin A protein. For the free 4F-, 5F-, 6F-, 7F-L-Trp, satisfactory agreement between experimentally measured and calculated relaxation rates was obtained, suggesting that the parameters used for calculating the rates for the indole frame are sufficiently accurate. We also measured and calculated relaxation rates for four differently ¹⁹ F-tryptophan labeled cyclophilin A proteins, transferring the parameters from the free amino acid to the protein-bound moiety. Our results suggest that ¹⁹ F relaxation data of the large and rigid indole ring in Trp are only moderately affected by protein motions and provide critical reference points for evaluating fluorine NMR relaxation in the future, especially in fluorotryptophan labeled proteins.