Journal of the American Chemical Society, Vol.138, No.45, 14824-14827, 2016
Crystal-to-Crystal Synthesis of Triazole-Linked Pseudo-proteins via Topochemical Azide-Alkyne Cycloaddition Reaction
Isosteric replacement of amide bond(s) of peptides with surrogate groups is an important strategy for the synthesis of peptidomimetics (pseudo-peptides). Triazole is a well-recognized bio-isostere for peptide bonds, and peptides with one or more triazole units are of great interest for different applications. We have used a catalyst-free and solvent-free method, viz., topochemical azide-alkyne cycloaddition (TAAC) reaction, to synthesize pseudo-proteins with repeating sequences. A designed beta-sheet-forming L-Ala-L-Val dipeptide containing azide and alkyne at its termini (N-3-Ala-Val-NHCH2C CH, 1) was synthesized. Single-crystal XRD analysis of the dipeptide 1 showed parallel beta-sheet arrangement along the b-direction and head-to-tail arrangement of such beta-sheets along the c-direction. This head-to-tail arrangement along the c-direction places the complementary reacting motifs, viz., azide and alkyne, of adjacent molecules in proximity. The crystals of dipeptide 1, upon heating at 85 degrees C, underwent crystal-to-crystal polymerization, giving 1,4-triazole-linked pseudo-proteins. This TAAC polymerization was investigated by various time-dependent techniques, such as NMR, IR, DSC, and PXRD. The crystal-to-crystal nature of this transformation was revealed from polarizing microscopy and PXRD experiments, and the regiospecificity of triazole formation was evidenced from various NMR techniques. The MALDI-TOF spectrum showed the presence of pseudo-proteins >7 kDa.