CARRIER DESIGN - CONFORMATIONAL STUDIES OF AMINO ACID(X) AND OLIGOPEPTIDE (X-DL-ALA(M)) SUBSTITUTED POLY(L-LYSINE)

The present study was undertaken to examine the influence of the rever sal of the side-chain sequential order on the conformation of branched polypeptides. At the same time, the influence of the optically active amino acid joined directly to the poly (L-Lys) backbone and the DL-Al a oligomer grafted as chain-terminating fragment were separately analy zed. Therefore two sets of polypeptides were synthesized corresponding to the general formula poly [Lys-(X(i))] (XK) and poly [Lys-(DL-Ala(m )-X(i))] (AXK) when X = Ala, D-Ala, Leu, D-Leu, Phe, D-Phe, Ile, Pro, Glu, D-Glu, or His. For coupling amino acid X to polylysine, three typ es of active ester methods were compared: the use of pentafluorophenyl or pentachlorophenyl ester, and the effect of the addition of an equi molar amount of 1-hydroxybenzotriazole. After cleavage of protecting g roups, AXK polypeptides were synthesized by grafting short oligo (DL-A la) chains to XK by using N-carboxy-DL-Ala anhydride. The CD measureme nts performed in water solutions of various pH values and ionic streng ths were used for classification of the polypeptide conformations as e ither ordered (helical) or unordered. Different from what was observed with the unsubstituted poly(L-Lys), poly[Lys-(X(i))] type polypeptide s can adopt ordered structure even under nearly physiological conditio ns (pH 7.3, 0.2M NaCl). These data suggest that the introduction of am ino acid residue with either (ar)alkyl side chain (Ala, Leu, Phe) or n egatively charged side chain (Glu) promotes markedly the formation of ordered structure. Comparison of chiroptical properties of poly [Lys-( DL-Ala(m)-X(i))] and of poly [Lys-(X(i))] reveals that side-chain inte ractions play an important role in the stabilization of ordered soluti on conformation of AXK type branched polypeptides. The results give ra ther conclusive evidence that not only hydrophobic interactions, but a lso ionic attraction, can be involved in the formation and stabilizati on of helical conformation of branched polypeptides.