A new general solid-phase method for the synthesis of backbone-to-backbone cyclized peptides

A model peptide with the sequences Ala-Pro-Lys(2ClZ)-Tyr(2BrZ) was synthesized on a 4-methylbenzhydryl amine (MBHA) polystyrene resin using conventional Boc/benzyl protective group strategy. The amino acid aldehyde Boc-valinal was coupled by reductive alkylation with NaCNBH3 in acidified DMF for 1 h. The secondary amine in the peptide-resin Boc-Val psi[CH2NH]Ala-Pro-Lys(2ClZ)-Tyr(2BrZ)-MBHA was reductively alkylated by 3(4-methylbenzylthio)-propanal at 40 degrees C for 6 h, resulting the peptide-resin Boc-Val psi[CH2N(CH2CH2CH2-S-pMeBzl)]Ala-Pro-Lys(2ClZ)-T yr(2BrZ)-MBHA. After the removal of the Boc group the synthesis was continued employing the above-mentioned methods, which led to the resin-bound peptide Leu psi[CH2N(CH2CH2CH2S-pMeBzl)]Ser-Pro-Gly-Lys(2ClZ )-Val psi [CH2N(CH2CH2CH2-S-pMeBzl)]Ala-Pro-Lys(2ClZ)-Tyr( 2BrZ)-MBHA. The peptide was cleaved from the resin with hydrogen fluoride. Reversed-phase HPLC and plasma desorbtion mass spectrometry analysis showed that the expected peptide Leu psi[CH2N(CH2CH2CH2SH)]Ser-Pro-Gly-Lys-Val psi[CH2N(CH2CH2CH2-SH)] Ala-Pro-Lys-Tyr-NH2 was obtained as the major product with low levels of side products. Intramolecular oxidation of the thiols gave the backbone to backbone cyclized peptide Leu psi[CH2N(CH2CH2CH2S)]Ser-Pro-Gly-Lys- Val psi[CH2N(CH2CH2CH2-S)]Ala-Pro-Lys-Tyr-NH2.