Modification Natural Cyclic Peptides Based On Photoinduced Electron Transfer Reaction And Bioactivity Study

In the past few decades,more than 40 cyclic peptide drugs have been used clinically,most of them are natural products.Compared with linear peptides,cyclic peptides have high affinity and selectivity,higher lipophilicity and membrane permeability make them have good biological activity.Due to the low content from natural products,difficulty extract,so reserchers are committed to artificial synthesis and modification of natural cyclic peptides.In addition,the conformational structure-activity relationship of cyclic peptides is also the important subject of our research.Galaxamide,a cyclic pentapeptide,isolated from the seaweed Galaxaura filamentous algae.It has good in vitro biological activity against GRC-1 and human liver cancer HepG-2 cell lines.Therefore,this paper used chiral L-leucine and D-leucine to synthesis two Galaxamide cyclic peptide analogs by photo-induced single-electron transfer reaction.The stereochemistry was studied by CD and theoretical calculations.It was found that the cyclic peptide synthesized from L-leucine as a starting material was a C-3-S isomer,and the D-leucine synthesized from a C-3-R isomer,which was a conformationally restricted cyclic peptide.Synthesis provides important ideas.In addition,human liver cancer HepG-2 cells were selected and their antitumor activity was studied by MTT method.Compound 1 was showed strong cytotoxicity.Moreover,unnatural amino acid substitutions tend to improve the biological activity of cyclic peptides.Based on previous studies,thioproline-substituted peptides have significantly better activity than proline-substituted peptides because highly electronegative sulfur atoms increase the affinity between the drug and the target.Therefore,a natural cyclic peptide,Fenestin A,was prepared in this paper as thioproline substituted analog.Cell activity studies have found that Fenestin A analogues after thioproline replacement have better inhibitory ability on HepG-2 and HeLa cells and less toxicity on normal cells（mouse fibroblast L929 cells）.The lactate dehydrogenase activity experiment and scanning electron microscope analysis proved that the prepared cyclic peptide can damage the cell membrane of tumor cells.At the same time,the in-depth absolute configuration structure of the obtained cyclic peptide product was studied using CD,theoretical calculations,and combined thermomagnetic NMR.The results show that compound 9 has intramolecular hydrogen bonds between C-3-OH and C=O of Ile₅,and compound 10 may have very weak intramolecular hydrogen bonds between NH of Ile₅ and C=O of Leu₂（2.093?）.Temperature-varying NMR experiments show that the temperature coefficients of Leu₄ and Ile₅ of compound 9 are greater than 4×10^-3ppm/K,indicating that the hydrogen bond formed between NH and water is easily cleaved,further indicating that Leu₄ and Ile₅ of compound 9 and the internal groups in the framework No stable intramolecular hydrogen bonds are formed.In the end,several photo-induced single-electron transfer（SET）reactions in the molecule were used to prepare several phagolistatin cyclic peptide analogs with special restricted stereostructures.We performed in vitro biological activity assays,lactate dehydrogenase activity and scanning electron microscope analysis revealed its mechanism on tumor cells.These results indicate that the prepared cyclic peptide analogs have excellent antitumor activity and are widely used Compared to the anticancer drug paclitaxel,most of the prepared analogs have stronger growth inhibitory effects.Our research shows the necessity of studying the steric structure of cyclic peptides,and provides a reference for the steric structure of other excellent active cyclic peptides.In short,most of the cyclic peptide analogs synthesized in this research have strong tumor growth inhibitory effects,and their stereostructure has an important effect on their activity.The established photo-inducted single electron transfer methods and stereostructure research programs provide important information for related drug research.