Study on Non-covalent Interaction of Polypeptides by Mass Spectrometry

In order to study the non-covalent interaction between peptides, the peptides were mixed in the molar ratio of 1∶1, and the interaction between peptides were analyzed by electrospray ionization mass spectrometry after equilibrium. The results showed that some of the pentapeptides did not interact with each other to form non-covalent complexes, and another part of the pentapeptides could easily form non-covalent complexes through interaction. After the amino acid sequence of peptides were changed and the peptide chain were lengthened, the changes of peptides interaction were analyzed by electrospray ionization mass spectrometry, and the effects of sulfhydryl group, hydrogen bond and hydrophobic action on the non-covalent action of peptides were studied. The pentapeptide Ⅱ-5(Ile-Leu-Gly-Tyr-Ile) and IP-5(Ile-Phe-Thr-Thr-Pro) interacted with each other and strong non-covalent complex peaks were obtained. The hydrophobic branching amino acid phenylalanine Phe in IP-5 was replaced by glycine Gly to become pentapeptide IP′-5(Ile-Gly-Thr-Thr-Pro), and the hydroxyside chain threonine Thr was replaced by glycine Gly to become pentapeptide IP″-5(Ile-Phe-Gly-Thr-Pro), the pentapeptide Ⅱ-5 interacted with IP′-5 and IP′-5, respectively, the spectral signals of non-covalent bond composites were weakened. The results showed that hydrogen bonding and hydrophobic interaction could enhance the non-covalent binding between peptides. The pentapeptide AK-5(Ala-Val-Ile-Phe-Lys) and ED-5(Glu-Ile-Cys-Ala-Asp) did not interact with each other. The semipatropine Cys containing sulfhydryl side chain in ED-5 was replaced by Ile without sulfhydryl side chain, and the amino acid sequence was changed to ED′-5(Glu-Ile-Ile-Ala-Asp). When AK-5 was mixed with ED′-5，the non-covalent effect of AK-5 and ED′-5 was stronger than that of AK-5 and ED-5. Increased the hydroxyl group and hydrophobic group in the polypeptide chain, and the influences of the peptide chain′s length on the polypeptide interaction were studied. By lengthening the peptide chain, the pentapeptide ED-5 was changed into nine-peptide ED-9(Ala-Glu-Ala-Ile-Cys-Ala-Asp-Pro-Lys), and AK-5 was changed into AK-9(Lys-Glu-Ala-Val-Ile-Phe-Lys-Thr-Ile). Then AK-5 was mixed with ED-9, AK-9 was mixed with ED-5, there was not non-covalent binding. The results showed that hydrogen bonding and hydrophobicity can enhance the non-covalent binding between peptides, and the thiol group is easy to form disulfide bonds between peptides, which hinders the non-covalent binding between different peptides. Even enhanced the hydrogen bonding and hydrophobicity were enhanced between peptides, the thiol group still plays a dominant role.