Strong chiral discrimination and site-selective photocleavage of two model proteins, lysozyme and bovine serum albumin (BSA), by new pyrenyl probes are reported here. The enantiomeric pyrenyl probes D-phenylalanine- 1(1-pyrene)methylamide (PMA-D-Phe) and L-phenylalanine- 1(1-pyrene)methylamide (PMA-L-Phe) were synthesized by coupling the carboxyl function Of D-phenylalanine or L-phenylalanine with the amino group of I(I-pyrene)methylamine. Binding affinities of the two enantiomers with the proteins were quantitated in absorption titrations. BSA indicated 10-fold selectivity for PMA-D-Phe, and the binding constants for the L- and D-enantiomers were 3.8 x 10(5) and 4.0 x 10(6) M-1, respectively. Lysozyme, similarly, indicated a 6-fold preference for PMA-D-Phe with binding constants of 3.3 x 10(5) and 2.0 x 10(6) M-1 for the L- and D-isomers, respectively. Such strong chiral discrimination illustrates the key role of the chiral center of the probe (Phe) in the binding interactions. The enantiomers were tested to examine how the chiral discrimination for their binding influences reactivity toward protein photocleavage. Irradiation of the probe-protein complexes, at 342 nm in the presence of hexammine cobalt(III) chloride, resulted in the cleavage of the protein backbone. Photocleavage did not proceed in the dark or in the absence of the pyrenyl probes. Both enantiomers indicated low reactivity with BSA (<5% yield), while large photocleavage yields (similar to 57%) have been noted with lysozyme. This lysozyme photocleavage yield is a significant improvement over previous reports. However, both enantiomers cleaved lysozyme at the same location. between Trp108-Va1109, despite the strong chiral selectivity for binding. H-atom abstraction from Trp 108, accessible from the active site cleft, could initiate the observed peptide bond cleavage.