Conformationally constrained analogues of diacylglycerol (DAG). 23. Hydrophobic ligand-protein interactions versus ligand-lipid interactions of DAG-lactones with protein kinase C (PK-C).

The constrained glycerol backbone of DAG-lactones, when combined with highly branched alkyl chains, has engendered a series of DAG-lactone ligands capable of binding protein kinase C (PK-C) with affinities that approximate those of phorbol esters. These branched chains not only appear to be involved in making important hydrophobic contacts with the protein (specific interactions) but also provide adequate lipophilicity to facilitate partitioning into the lipid-rich membrane environment (nonspecific interactions). With the idea of minimizing the nonspecific interactions without reducing lipophilicity, the present work explores the strategy of relocating lipophilicity from the side chain to the lactone "core". Such a transfer of lipophilicity, exemplified by compounds 1 and 3, was conceived to allow the new hydrophobic groups on the lactone to engage in specific hydrophobic contacts inside the binding pocket without any expectation of interfering with the hydrogen-bonding network of the DAG-lactone pharmacophore. Surprisingly, both (E)-3 and (Z)-3 showed a significant decrease in binding affinity. From the molecular docking studies performed with the new ligands, we conclude that the binding pocket of the C1 domain of PK-C is sterically restricted and prevents the methyl groups at the C-3 position of the lactone from engaging in productive hydrophobic contacts with the receptor.