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Dedicated to Professor Duilio Arigoni on the occasion of his 75th birthdayThe synthesis of some 3-aryl-3-(trifluoromethyl)3H-diazirine and benzophenone-based photoaffinity labels is reported. The photolabile group is bound to a scaffold that also accommodates functional groups to which an indicator unit (biotin) and the bioactive ligand can be attached orthogonally. To three of the labels, moenomycin was conjugated with the aim to provide tools for the identification of the moenomycin binding site within the transglycosylase domain of the enzyme PBP 1b. Some preliminary photoaffinity-labeling experiments were carried out.Introduction. ± Peptidoglycan is an essential cell-wall constituent of almost all eubacteria. It is a heteropolymer consisting of glycan strands cross-linked via short peptide chains. The final events of peptidoglycan biosynthesis at the outside of the cytoplasmic membrane are the formation of the macromolecular architecture by two major types of reaction, i) formation of the polysaccharide strands (transglycosylation) and ii) formation of peptide cross-linkages between the glycan chains (transpeptidation) (for review, see [1]). A representation of the transition state of the transglycosylation reaction is shown in Fig. 1, a [2]. Both the glycosyl donor (the growing peptidoglycan chain) and the glycosyl acceptor (the disaccharide-derived lipid II) (see [1]) are attached to the outer face of the cytoplasmic membrane via a C 55 anchor. The transglycosylation reaction is catalyzed by a number of multimodular bifunctional polymerases (that catalyze also the transpeptidation reaction) designated as class-A high-molecular-mass penicillin-binding proteins (PBPs). Of these, PBP 1b from Escherichia coli has been studied in great detail [3]. It comprises 844 amino acid residues and contains a short cytosolic N-terminus, a membrane span, the D198-G435 glycosyl-transferase module, and the Q447-N844 acyl-transferase module. Within the glycosyl-transferase module, Glu233 has been shown to be central to the transglycosylation, and, in the active site of the acyl transferase, Ser510 is essential, the acylation of which forms the basis of the antibiotic properties of the b-lactams. Recently, the soluble extracellular region of PBP 1b from Streptococcus pneumoniae has been expressed and characterized biochemically [4]. From the most-recent kinetic characterization of the E. coli PBP 1b, it was deduced that a doubly charged metal ion bound to the diphosphate group assists departure of the leaving group, whereas the essential Glu233 acts as a base, removing the proton from the 4-OH group of the glycosyl acceptor (see Fig. 1, a) [5].

Photoaffinity labeling has been demonstrated to be a remarkably efficient method for studying the interactions of biologically significant compounds (ligands) with their target macromolecules. [1] The method allows the identification of the targets (for example, binding proteins) and, also the binding domain within the target protein. An appropriate photoaffinity-labeling compound should contain three structural elements: a) a ligand which directs the label to the binding site on the protein, b) a photolabile group for attaching to the protein, c) an indicator that allows the identification of the labeled peptides after enzymatic digestion of the labeled protein.favorable cell-penetration properties. In the whole-blood assay, IC 50 values (mm) for the most active derivatives 4 b (TNF-a: 5.6 AE 0.95, IL-1b: 1.5 AE 0.7), 4 c (TNF-a: 0.51 AE 0.24, IL-1b: 0.11 AE 0.03), and 4 d (TNF-a: 5.1 AE 0.4, IL-1b: 1.1 AE 0.7) were lower than those of lead compound ML 3163 (TNF-a: 20.3 AE 4.8, IL-1b: 2.78 AE 0.13), and close to the nanomolar range. Finally, the most promising results came from the toxicity screen, in which 4 b ± d (Table 1) only moderately interacted with those P450 isoforms most important for drug metabolism. [9] This profile gives 4 b ± d a clear advantage over both SB 203 580 and ML 3163, and makes them strong candidates for further development as anti-inflammatory drugs.Table 1. Inhibition of p38 MAP kinase, cytokine release, and cytochrome P450 isoforms by selected compounds. IC 50 AE SEM [mm] [a] Inhibition [%] of P450 isoforms [b] Compound p38 TNF-a IL-1b 2D6 3A4 SB 203580 0.29 AE 0.03 (7) 0.59 AE 0.09 (21) 0.037 AE 0.006 (20) 73.1 76.6 ML 3163 4.0 AE 1.0 1.1 AE 0.4 (4) 0.38 AE 0.13 (4) 71.8 87.1 4 b 2.2 (1) 2.2 AE 0.9 0.45 AE 0.03 7.8 28.3 4 c 0.50 (1) 0.51 AE 0.24 (4) 0.11 AE 0.03 (4) 13.4 16.5 4 d 2.2 (1) 1.1 AE 0.3 0.38 AE 0.04 0.7 28.8 [a] Tests were carried out in duplicate, except where the number in brackets denotes otherwise. SEM Standard error of measurement. [b] Results are from one experiment each, carried out at a test-compound concentration of 10 mm in phosphate buffer (pH 7.4) with DMSO (0.1%).

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