Highly potent side-chain to side-chain cyclized enkephalin analogues containing a carbonyl bridge: synthesis, biology and conformation.

Six novel cyclic enkephalin analogues have been synthesized. Cyclization of the linear peptides containing basic amino acid residues in position 2 and 5 was achieved by treatment with bis(4-nitrophenyl)carbonate. It was found that some of the compounds exibit unusually high mu-opioid activity in the guinea pig ileum (GPI) assay. The 18-membered analogue cyclo(N(epsilon),N(beta)-carbonyl-D-Lys2,Dap5)-enkephalinamide turned out to be one of the most potent mu-agonists reported so far. NMR spectra of the peptides were recorded and structural parameters were determined. The conformational space was exhaustively examined for each of them using the electrostatically driven Monte Carlo method. Each peptide was finally described as an ensemble of conformations. A model of the bioactive conformation of this class of opioid peptides was proposed.

N-cyclohexyl-N'-isopropylcarbodiimide: a hybrid that combines the structural features of DCC and DIC.

N-Cyclohexyl-N'-isopropylcarbodiimide was prepared and used for peptide couplings by various procedures. Reaction between Z-Val and Gly-OEt was used for yield studies and determination of the extent of the side reaction in solution. The value of this reagent for solid peptide synthesis was demonstrated by the synthesis of sequences comprising the 65-74 residues of acyl carrier protein. The efficiency of activation was determined by competition experiments. From all these studies the conclusion can be drawn that N-cyclohexyl-N'-isopropylcarbodiimide is comparable to or even better than the commonly used DCC for mediation of peptide bond formation in solid phase synthesis. By virtue of the solubility of N-cyclohexyl-N'-isopropylurea in dichloromethane, the carbodiimide seems to be a good candidate for practical execution of peptide bond formation by the standard carbodiimide procedure.