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As an important research tool, peptide have received prominence in molecular biology, clinical research, instrumentation, study of protein structure and functioning, smart biomaterials etc. This has encouraged to develop synthetic peptides (by replacing enzyme sensitive amide linkage) i.e. Peptidomimetics having enhanced stability and attractive properties. 1,2,3-triazole motif is widely accepted isosteric group to surrogate such linkages present in biopolymers. Instead of using classical approach to peptide synthesis leading to poor yield, bad control over the stereo-and regioselectivity, solubility issues, slow reaction rate, use of toxic catalysts, difficult purification etc., we have used Topochemical Azide-Alkyne Cycloaddition (TAAC, lattice controlled) reaction. Utilizing the principle of crystal engineering, we have synthesized a structurally modified dipeptide (using alanine and valine) which undergoes heat induced topochemical 1,3-dipolar cycloaddition reaction to give triazole-linked pseudopolypeptide. Also, the crystals underwent cracking due to the packing strain generated at higher temperature due to thermal topochemical polymerization. Systematic characterization is carried out to study the following TAAC reaction. [1] Biradha, K. et al. (2013) Chem. Rev. 42, 950-967.
As an important research tool, peptide have received prominence in molecular biology, clinical research, instrumentation, study of protein structure and functioning, smart biomaterials etc. This has encouraged to develop synthetic peptides (by replacing enzyme sensitive amide linkage) i.e. Peptidomimetics having enhanced stability and attractive properties. 1,2,3-triazole motif is widely accepted isosteric group to surrogate such linkages present in biopolymers. Instead of using classical approach to peptide synthesis leading to poor yield, bad control over the stereo-and regioselectivity, solubility issues, slow reaction rate, use of toxic catalysts, difficult purification etc., we have used Topochemical Azide-Alkyne Cycloaddition (TAAC, lattice controlled) reaction. Utilizing the principle of crystal engineering, we have synthesized a structurally modified dipeptide (using alanine and valine) which undergoes heat induced topochemical 1,3-dipolar cycloaddition reaction to give triazole-linked pseudopolypeptide. Also, the crystals underwent cracking due to the packing strain generated at higher temperature due to thermal topochemical polymerization. Systematic characterization is carried out to study the following TAAC reaction. [1] Biradha, K. et al. (2013) Chem. Rev. 42, 950-967.
Though topochemical reactions are attractive, the difficulty associated with crystallization such as low yield, unsuitability for large-scale synthesis, etc. warranted the exploitation of other self-assembled media for topochemical reactions. We synthesized a dipeptide gelator decorated with azide and alkyne at its termini, N-Ala-Val-NHCH-C≡CH, which is designed to self-assemble through intermolecular hydrogen bonds to β-sheets thereby placing the azide and alkyne motifs in proximity. As anticipated, this peptide forms gels in organic solvents and water via hydrogen-bonded β-sheet assembly as evidenced from IR spectroscopy and PXRD profiling. The microscopic fibers present in organogel and hydrogel have different morphology as was evident from scanning electron microscopy (SEM) imaging of their xerogels, XG (xerogel made from hydrogel) and XG (xerogel made from organogel). Heating of xerogels at 80 °C resulted in the topochemical azide-alkyne cycloaddition (TAAC) polymerization to 1,4-triazole-linked oligopeptides. Under identical conditions, XG produced larger oligopeptides, and XG produced smaller peptides, as evidenced from MALDI-TOF spectrometry. We have also shown that degree of TAAC polymerization can be controlled by changing gel fiber thickness, which in turn can be controlled by concentration. SEM studies suggested the morphological intactness of the fibers even after the reaction, and their PXRD profiles revealed that both XG and XG undergo fiber-to-fiber oligomerization without losing their crystallinity. In contrast to crystals, the xerogels undergo TAAC polymerization in two distinct stages as shown by DSC analyses. Interestingly, XG and XG undergo spontaneous TAAC polymerization at room temperature; the latter shows faster kinetics. This is not only the first demonstration of the use of xerogels for thermally induced topochemical polymerization but also the first report on a spontaneous topochemical reaction in xerogels.
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