Edge-to-Face CH/.PI. Interaction between Ligand Ph-Phenyl and Receptor Aromatic Group in the Thrombin Receptor Activation.

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In the ligand/receptor interaction, the side chain phenyl group of phenylalanine (Phe) is involved in a so-called hydrophobic interaction, in which the Phe-phenyl group func-tions as a π element or merely as a hydrophobic element. The thrombin receptor-teth-ered ligand SFLLRNP consists of the Phe-2 residue essential for receptor activation. In order to explore the molecular characteristics of this Phe-2-phenyl group, a complete set of S/Phe/LLRNP peptides comprising six different difluorophenylalanine isomers [(F2)Phe] was newly synthesized and assayed to evaluate their ability to induce the aggregation of human platelets. The assay results clarified several important structural elements to conclude that Phe-2-phenyl of S/Phe/LLRNP is in the edge-to-face CH/π interaction with the receptor aromatic group, utilizing the Phe-phenyl edge along with adjacent benzene hydrogens at positions (2-3) or (5-6). It was also found that the fluo-rine atom at position 4 increases the acidity of the hydrogen mainly at its ortho position, resulting in a reinforcement of the CH/π interaction and thus in an enhancement of biological activity. The H→F replacement in the benzene ring was found to provide an effective structural examination to the Phe residue; i.e., to identify the hydrogens in the CH/π interaction, and to strengthen the CH/π interaction.
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