Synthesis and conformation of aromatic cyclic dipeptides. Cyclo(phenylalanyl)2, cyclo(1-naphthylalanyl)2, and cyclo(2-naphthylalanyl)2.

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Cyclic dipeptides of aromatic amino acids, cyclo(L-phenylalanyl)2, cyclo(L-1-naphthylalanyl)2, and cyclo(L-2-naphthylalanyl)2 were synthesized and subjected to spectroscopic analyses using 1H NMR, absorption, circular dichroism (CD), fluorescence, and fluorescence-detected circular dichroism (FDCD). The 1H NMR data suggested that the 2,5-piperazinedione rings of the three cyclic dipeptides are in planar or nearly planar bowsplit boat-type conformation. The aromatic side groups of cyclo(1- and 2-naphthylalanyl)2's were found to take asymmetric configurations, one naphthyl group being folded onto the 2,5-piperazinedione ring, the other being unfolded. Strong exciton couplet was observed in CD spectra of the three compounds. The signs of the exciton splitting were opposite in the two naphthyl cyclic dipeptides. Fluorescence spectra of the two naphthyl cyclic dipeptides showed no excimer emission. The absence of strong interchromophoric interaction in the lowest excited state was also suggested by the virtual coincidence of CD spectrum with FDCD spectrum. From the above spectroscopic data, probable conformations were proposed for the naphthyl cyclic dipeptides.
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