Synthesis of a cyclic fibrin-like peptide and its analysis by fast atom bombardment mass spectrometry.

For immunochemical purposes, a cyclic 12-peptide was synthesized to model the gamma-gamma-chain cross-link site in human fibrin. The model was based upon the structure proposed by Chen & Doolittle (Biochemistry (1971) 10, 4486-4491) which is characterized by two reciprocating epsilon-(gamma-Glu)Lys bonds between adjacent fibrin gamma-chains oriented in an antiparallel manner. To achieve the antiparallel orientation of the peptide backbone, Pro and Gly were inserted at positions 6 and 7 of the linear 12-peptide: acetyl-Gly-Glu-Gln-His-His-Pro-Gly-Gly-Gly-Ala-Lys-Gly-amide. The insertions were made to facilitate a reverse turn of the peptide during the last synthetic step, which was formation of the epsilon-(gamma-Glu)Lys bond between Glu at position 2 and Lys at position 11 with diphenylphosphorylazide. The resulting cyclic peptide represented half of the symmetrical cross-linked region in clotted fibrin. Following purification by HPLC, both linear and cyclic 12-peptides were analyzed by fast atom bombardment mass spectrometry. Abundant molecular protonated ions were observed for both peptides. In addition, the amino acid sequence of the linear peptide and the location of the epsilon-(gamma-Glu)Lys bond in the cyclized peptide could be verified.