Quartz Crystal Microbalance Measurement of Self-Assembled Micellar Tubules of the Amphiphilic Decyl Ester of D-Tyrosine and Their Enzymatic Polymerization

Amphiphilic decyl derivatives of D-tyrosine self-assemble into long rodlike or tubular aggregate structures in aqueous buffered solution. In this report we demonstrate the novel use of the quartz crystal microbalance (QCM) to measure the presence in solution, and subequent enzymatic polymerization, of long rodlike monomer aggregates of the decyl ester of D-tyrosine (DEDT) as a function of their formation and increasing surface binding level as pH values increase from 3 to 7. From these data, using the Sauerbray equation to calculate the effective elastic mass surface binding of deprotonated DEDT aggregates, a pK(app) of 8.3 is obtained for the DEDT a-NHg group protonationdeprotonation and subsequent aggregation equilibrium. Furthermore, once aggregates are bound to the QCM surface, we initiate and subsequently monitor enzymatic polymerization of the DEDT monomers by horseradish peroxidase through the measurement of significant changes in the quartz crystal frequency and motional resistance. Following the onset of polymerization, the viscoelastic properties of the bound monomer aggregates change. A final polymerized state is achieved in which the altered physical properties of the polymerized rodlike aggregates make the solution immediately above the QCM surface-solution interface behave as a Newtonian fluid, producing a nearly pure viscosity-density energy dissipative effect on the measured crystal frequency and motional resistance values.