Dendritic physical gels: Structural parameters for gelation with peptide-core dendrimers

Eleven different peptide-core dendritic macromolecules, each having a [G2] poly(benzyl ether) dendritic wedge (Den) at the peptide side chain, N-terminal, or C-terminal, were synthesized, and their potentials as macromolecular organic gelators were investigated in MeCN. Among these candidates, side-chain-dendronized Boe-Tyr(Den)-Ala and cyclo-(Tyr(Den)-Ala) and N-dendronized Den-Phe-Ala and Den-Phe-Ala-OMe (Figure 1) were found to form physical gels with critical concentrations for gelation of 1.0-2.2 mM, whereas C-dendronized Boc-Phe-Ala-ODen and Phe-Ala-ODen were unable to induce gelation. FE-SEM micrographs of the dry gels showed the presence of fibrous or ribbonlike self-assembled structures, which melted at 112-214 degreesC (T-m), depending on the modes of hydrogen bonding and van der Waals interactions. Infrared spectroscopy and X-ray diffraction analysis of the dry gels indicated columnar molecular orderings with a rectangular packing geometry for Boe-Tyr(Den)-Ala and Den-Phe-Ala-OMe, while a lamellar geometry for Den-Phe-Ala. Circular dichroism analysis of the transparent gel with Boc-Tyr(Den)-Ala indicated a helically twisted geometry of hydrogen-bonded dipeptide arrays.