Asymmetric synthesis of conformationally restricted L-arginine analogues as active site probes of nitric oxide synthase

Using the catalytic asymmetric Sharpless carbamate aminohydroxylation, conformationally restricted L-arginine and L-homoarginine derivatives (5-8) were prepared in good enantiomeric excess to investigate the binding requirements of L-arginine-based compounds with nitric oxide synthase. The L-arginine derivatives (5 and 6) inhibited both the inducible and neuronal isoforms of nitric oxide synthase with little isoform selectivity (5, IC50 = 42 and 144 mu M, 6, 8 and 12 mu M, respectively). The guanidine-containing compound (5) did not act as a nitric oxide producing substrate for nitric oxide synthase. The ability of these compounds to interact with the enzyme supports the idea that L-arginine-based inhibitors bind to the enzyme in a folded conformation. The L-homoarginine derivatives (7 and 8) did not interact with the enzyme as either substrates or inhibitors. The two-carbon L-arginine homologue (9), prepared from L-phenylalanine, demonstrated the greatest isoform selective inhibition of the compounds examined (IC50(iNOS) = 19 and IC50- (nNOS) = 147 mu M, IC50(nNOS)/IC(50)i(NOS) = 7.7). These results suggest isoform selective inhibition may be related to the folded conformations required for binding of these higher L-arginine homologues.