5-Fluoro-L-tryptophan

5-Fluoro-L-tryptophan is a competitive inhibitor of the vesicular glutamate transporter (VGLUT). Generally regarded as a fluorinated analogue of tryptophan, it is frequently used as a fluorescent probe or a nuclear magnetic resonance (NMR) probe in protein structure studies.

Exogenous 5-fluoro-Trp is incorporated into proteins in normal protein synthesis. Since ¹⁹F is a useful reporter group, this provides a method for studying enzyme mechanisms by NMR.

5-Fluoro-L-tryptophan serves as a ligand in 19F NMR binding experiments with human serum albumin and as an amino acid analogue in spectrophotometric titration of the a2b2 complex, where the absorbance difference between 470 and 412 nm is monitored.

5-Fluoro-L-tryptophan is obtained by resolving 5-fluoro-DL-tryptophan: the N-(trifluoroacetyl) derivative is formed with ethyl trifluoroacetate and triethylamine in methanol, digested with carboxypeptidase A to remove the D isomer, deprotected with 1 M aqueous piperidine, and recrystallized from chloroform-methanol (1:1)[1]. It is also synthesized from 5-fluoroindole and L-serine or O-acyl-L-serines such as O-benzoyl-L-serine in the reaction catalyzed by tryptophan synthase, including the intact a2b2 complex[2-3].

5-Fluoro-Trp is nonspecifically cytotoxic. It is believed this is due to malfunctioning enzymes that have had replacements of Trp residues by 5-fluoro-Trp. However, at least one case is known where 5-fluoro-Trp substitution leads to significantly greater catalytic activity.

Recrystallise it from EtOH, aqueous EtOH or AcOH. Also purify it by passage through a Dowex AG1x2 (acetate form) column and recrystallise the L-enantiomer (from enzymic enrichment) from H2O/EtOH, m 158-163o(dec), [] D -8.3o (c 2.5, N NaOH). [Coy et al. Biochemistry 13 3550 1974, Beilstein 22/14 V 116.]

[1] Miles, E. W., Phillips, R. S., Yeh, H. J. C., Cohen, L. A. (1986). Isomerization of (3S)-2,3-dihydro-5-fluoro-L-tryptophan and of 5-fluoro-L-tryptophan catalyzed by tryptophan synthase: studies using fluorine-19 nuclear magnetic resonance and difference spectroscopy. Biochemistry, 25(15), 4240–4249. https://doi.org/10.1021/bi00363a011
[2] Ahmed, S. A., Martin, B., Miles, E. W. (1986). .beta.-Elimination of indole from L-tryptophan catalyzed by bacterial tryptophan synthase: a comparison between reactions catalyzed by tryptophanase and tryptophan synthase. Biochemistry, 25(15), 4233–4240. https://doi.org/10.1021/bi00363a010
[3] Phillips, R. S. (1987). Reactions of O-acyl-l-serines with tryptophanase, tyrosine phenol-lyase, and tryptophan synthase. Archives of Biochemistry and Biophysics, 256(1), 302–310. https://doi.org/10.1016/0003-9861(87)90450-4