Does thermotropic liquid crystalline self-assembly control biological activity in amphiphilic amino acids? – tyrosine ILCs as a case study

Abstract

Amphiphilic amino acids represent promising scaffolds for biologically active soft matter. In order to understand the bulk self-assembly of amphiphilic amino acids into thermotropic liquid crystalline phases and their biological properties a series of tyrosine ionic liquid crystals (ILCs) was synthesized, carrying a benzoate unit with 0–3 alkoxy chains at the tyrosine unit and a cationic guanidinium head group. Investigation of the mesomorphic properties by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (WAXS, SAXS) revealed smectic A bilayers (SmA_d) for ILCs with 4-alkoxy- and 3,4-dialkoxybenzoates, whereas ILCs with 3,4,5-trisalkoxybenzoates showed hexagonal columnar mesophases (Col_h), while different counterions had only a minor influence. Dielectric measurements revealed a slightly higher dipole moment of non-mesomorphic tyrosine-benzoates as compared to their mesomorphic counterparts. The absence of lipophilic side chains on the benzoate unit was important for the biological activity. Thus, non-mesomorphic tyrosine benzoates and crown ether benzoates devoid of additional side chains at the benzoate unit displayed the highest cytotoxicities (against L929 mouse fibroblast cell line) and antimicrobial activity (against Escherichia coli ΔTolC and Staphylococcus aureus) and promising selectivity ratio in favour of antimicrobial activity.