Topochemical Synthesis of a Heterochiral Peptide Polymer in Different Polymorphic Forms from Crystals and Aerogels

We have designed a heterochiral dipeptide monomer (N₃ -D-Ala-L-Val-NHCH₂ C≡CH) modified with an azide group and an alkyne at its termini for topochemical azide-alkyne cycloaddition (TAAC) polymerization. We obtained two different polymorphs: PI (P2₁ ) from a DCM-toluene mixture, and PII (P-1) from a hexane-ethyl acetate mixture. PI adopts parallel β-sheet packing, and PII adopts antiparallel β-sheet packing. In both the cases, molecules from adjacent β-stacks are arranged in a head-to-tail manner. On heating, the polymorphs underwent TAAC reaction to form linear polymers having parallel β-sheet-like ordering (PI) and an antiparallel β-sheet structure (PII). PI reacted spontaneously at room temperature and the crystals showed cracking after the reaction, but PII was intact even after complete reaction. The dipeptide congealed various solvents, and the aerogels were identical to PI. These aerogels also underwent spontaneous TAAC reaction at room temperature to yield a pseudoprotein with alternate D- and L-amino acids.