PROTEIN-LOOP MIMETICS - A DIKETOPIPERAZINE-BASED TEMPLATE TO STABILIZE LOOP CONFORMATIONS IN CYCLIC-PEPTIDES CONTAINING THE NPNA AND RGD MOTIFS

We explore here an approach to mimic the structures and biological fun ctions of protein loops in small synthetic molecules, by grafting the loop of interest onto an organic template comprising a bicyclic diketo piperazine, prepared by the formal coupling of (2S,4S)-4-aminoproline (Pro(NH2)) and aspartic acid (Asp). The Fmoc-protected template 4 is u sed to prepare (-Ala(1)-Asn(2)-Pro(3)-Asn(4)-Ala(5)-Ala(6)-Temp-) (5) and cyclo(-Ala(1)- Arg(2)-Gly(3)-Asp(4)-Temp-) (6) (where Temp = templ ate derived from 4), containing the Asn-Pro-Asn-Ala (NPNA) and Arg-Gly -Asp (RGD) motifs. The conformational properties of these molecules ar e studied in aqueous solution by NMR and simulated-annealing methods. The NPNA motif, an immunodominant epitope on the circumsporozoite surf ace protein of the malaria parasite Plasmodium falciparum, is shown to adopt a stable type-I beta-turn in 5. The template in 5 adopts a pref erred conformation with Pro(NH2) chi(1) approximate to -35 degrees and the Asp moiety chi(1) approximate to 70 degrees. A different template conformation is inferred for 6, with Pro(NH2)chi(1) approximate to 0 degrees, but the ARGD loop appears by NMR to undergo rapid conformatio nal averaging. Solid-phase binding assays reveal that 6 displays modes t antagonist activity towards both the integrin alpha(IIb)beta(3) and alpha(v)/beta(3) receptors.