Cyclopeptide Nanotubes Self-Assembling And Microstructure Characterization

Linear multipeptide was firstly synthesized by Fmoc solid phase synthesis method using L- and D-amino acid as raw materials in this paper. Based on cyclization and purification of linear multipeptide, cyclic peptide nanotubes were self-assembled. The molecular structures of linear multipeptide and cyclopeptides were characterized by mass spectrometry(MS), and nanotube structures were investigated by means of scanning electron microscopy (SEM), optical microscopy, fourier-transform infrared spectroscopy(FT-IR), X-ray diffraction (XRD),atom force microscopy(AFM), and transmission electron microscopy (TEM).Six kinds of linear multipeptides were synthesized successfully by standard solid phase synthesis under Fmoc protection. The sequence of linear multipeptides are as follows: homo-amino acid linear multipeptides hexapeptide (-[L-Phe-D-Phe]3-) and octapeptide (-[L-Ala-D-Ala-]4-); hetero-amino acid linear multipeptides octapeptide( -[L-Phe-D-Ala-]4-, -[L-Ala-D-Phe-]4-, -[L-Phe-D-Ala- L-Ala-D-Phe-]2- and -[L-Phe-D-Phe-L-Ala-D-Ala-]2-). Gaseous nitrogen blow the reagent from the bottom to the top of the reactor in order to give protection of product and stirring action of reagent. The reaction is at the temperature of 20-50℃and reaction time is 3h-5h. The cleaved reagents are TFA:TIS: Thioanisole=95:2.5:2.5(V/V) . The yield of products is about 80% and the harvest yield can reach to 96.17%.Cyclization is in the dilute solution of CH2Cl2 using TBTU and DIEA as peptide activation, the solution composing rate was linear multipeptides: TBTU:DIEA=1:1:4. The quantity of CH2Cl2 was 100 times of linear multi- peptides. Cyclic peptide is purified by recrystalization in TFA and dd H20. Purified cyclic peptides were self-assembled at the two phases of TFA and dd H20, FT-IR anlysis indicated that singularity cyclic peptides all shew the characteristic peak of cyclic nanotubes after self-assembling, that is, peptide self- assembly has Nanotube structures. Nanotube and nanotube could congregate by intertube hydrogen-bonding interactions. SEM show that cyclo-[L-Ala-D-Ala-]4- could assembled groups of sicks,whose diameter was 5μm and length was about 10μm. The other nanotubes were assembled to reticulation structure. Using AFM and TEM observe the microstructure of cyclo-[L-Phe-D-Phe-L-Ala-D-Ala-]2-, the results indicated that it could formed nanotubes, the tube diameter was 5-20 nm Heterokinds of cyclic peptide self-assembling indicated that: when the diameters of two cyclic peptides were different, it could form nanotubes; the same diameters coud not assemble to nanotubes, it is due to groups N-H and C=O of the backbone and side chains are dismatchable.The investigation on cyclo-[L-Phe-D-Phe-L- Ala-D-Ala-]2- self-assembling in the CH2Cl2 and equilibrating via vapor-phase diffusion against hexanes indicated that cyclic peptide were prone to assembled and crystallized in organic solvent.