Self-assembled micron-scale fibre structures are formed by amphiphilic decyl ester derivatives of the d- and l-tyrosine amino acids prior to and following enzymatic ring polymerization

In aqueous solution, amphiphilic decyl esters of the amino acids d- and l-tyrosine are capable of self-assembly into aggregates. We present light scattering measurements that determined the critical micelle concentrations (c.m.c.) of the d- and l-isomers of this compound at pH values of 5.5 (c.m.c.=0.23 mM) and 6.0 (c.m.c.=0.17 mM). These data support the notion of an increasing pH dependent self-assembly process involving the deprotonation of the α-NH3+. The self-assembled amphiphilic monomers formed rod or plate-like fibres that possess widths of a few microns and lengths ranging from tens to hundreds of microns when measured by scanning electron microscopy (SEM). Horseradish peroxidase (HRP) was used to polymerize the monomers in these self-assembled structures. The kinetics of enzymatic polymerization were not second order above the c.m.c., supporting the model of self-assembled aggregates being the existing solution structure. SEM provided evidence that enzymatic polymerization left the gross shape and diameter dimensions of the fibres unaltered. However, polymerization allowed the existence of much longer fibres, hundreds of microns in length, in contrast to the shorter length fibres observed before polymerization. These data suggest that the polymerized fibres are more robust mechanical structures than unpolymerized fibres. Also, these data suggest that there is little difference in macro-scale self-assembly structure due to isomer differences or to polymerization. However, polymerized fibres possessed a smooth outer surface, in contrast to the rougher often fibrillar surface of the unpolymerized self-assembled fibre structures. This suggests a difference in the micro-scale molecular organization of the fibre monomer units prior to and following polymerization.