Examination of the new α-(2′Z-fluoro)vinyl trigger with lysine decarboxylase: The absolute stereochemistry dictates the reaction course

The first examination of a terminal α-fluorovinyl trigger in an amino acid decarboxylase active site is reported. To investigate the enantiospecificity of inactivation with this new AADC trigger, an enantioselective synthesis of l-α-(2′Z-fluoro)vinyllysine and its d-antipode has been developed. Control of stereochemistry is achieved through introduction of the amino acid side chain via alkylation of a chiral vinylglycine-derived dienolate. Facial selectivity is conferred by a trans-2′(β-naphthyl)-2′-propylcyclohexyl ester auxiliary, available in both antipodal forms (Comins protocol). The alkylation employs a new electrophile, N-p-methoxybenzyl-N-(2′-trimethylsilylethanesulfonyl)-4-iodobutylamine, for convergent installation of the lysine side chain. Vinyl to 2′-fluorovinyl interconversion then provides l- and d-α-(2′Z-fluoro)vinyllysine in 97-99% ee, as demonstrated by chiral HPLC. Both time-dependent enzyme kinetics and ¹⁹F NMR reveal striking differences in the behavior of these two antipodes in the lysine decarboxylase active site. The l-antipode displays time dependent inactivation (t_1/2 = 3 ± 1 min; K_I = 86 ± 22 μM), whereas the d-antipode behaves as a substrate, being completely turned over to α-(2′Z-fluoro)vinylcadaverine. Titration of LDC with varying amounts of l-α-(2′Z-fluoro)vinyllysine provides an estimate of 20 ± 3 for the partition ratio for this antipode. 19F NMR provides a more detailed account of the inactivation with the l-antipode, revealing that 1 in 3.4 turnovers of this mechanism-based inhibitor results in errant protonation (required by design), with 1 in 5 errant protonation events leading to LDC inactivation. This gives an overall partition ratio of 16 ± 2. Fluoride-selective electrode measurements are in agreement with ¹⁹F NMR estimates of [fluoride] released.