Crystalline Glycylglycine Bolaamphiphile Tubules and Their pH-Sensitive Structural Transformation

Abstract

The assembled structure of the heptane bolaamphiphile, bis(N-α-amido-glycylglycine)-1,7-heptane dicarboxylate, displays a sensitivity to the acidity of a solution. At pH 4, the heptane bolaamphiphile grows to a crystalline tubule in two weeks. At pH 8, a helical ribbon structure is formed in one week. The degree of carboxylic acid protonation was used to control the final assembled structures since the structures are determined by the strengths of the amide−amide and carboxylic acid dimer hydrogen bonds. Direct structural transformation between tubules and helical ribbons was also confirmed as a function of pH using optical microscopy and Raman microscopy. Conversion from the helical ribbons to the tubules occurs within one day, while the reverse conversion, from the tubules to the helical ribbons, is ten times slower.