Formation of specific hydrophobic sites for incorporation of methylene blue by laterally arranged L-glutamate residues in anionic, crystalline bilayer aggregates

Abstract

Bilayer membranes, formed from dioctadecyl L-glutamate-derived anionic amphiphiles, incorporated the cationic dye, methylene blue (MB), into their crystalline, hydrophobic region. The incorporated, monomeric MB was extraordinarily converted into a dimeric species during the gel-to-liquid crystalline phase transition by binding to carboxylates. A further increase in temperature upon the phase transition temperature led to the ordinary dimer-to-monomer transition of the bound, aggregated MB as well as the conventional various dye–polyion systems including micellar systems. However, such extraordinary behaviour strongly depends on the chemical structure of the bilayer-forming component amphiphiles. The molecular structure requirements of anionic amphiphiles for such incorporation has been investigated by varying the alkyl chain length, the average degree of polymerization (x), and the kind of amino acid residue in the connector moiety. Specific incorporation of MB into the crystalline, hydrophobic region is peculiar to molecular assemblies formed from L-glutamate amphiphiles with long alkyl chains and low x values. Corresponding DL-glutamate derivatives showed ambiguous transitions and L-aspartate derivatives did not show such specific incorporation, indicating that the specific behaviour is peculiar to the L-glutamate derivatives. The same conclusion that the monomeric dye species in the crystalline bilayer state is incorporated in the hydrophobic region of the bilayer aggregates could be confirmed by using the solvatochromic dye, 4-[4-(dimethylamino)styryl]-N-methylpyridinium iodide (St-4C₁), as a micro-environmental probe instead of MB.