Fate of L-DOPA in the Presence of Inorganic Matrices: Vectorization or Composite Material Formation?

Novel bionanocomposite materials were successfully prepared using 2:1 phyllosilicates and an amino acid, L-DOPA. A systematic study of DOPA interactions with these smectites and with a nonporous amorphous silica (Aerosil) used for comparison was undertaken by combining macroscopic information, including XRD, thermogravimetric analysis, and proton relaxation times in the synthesis supensions, with microscopic observations and several spectroscopic techniques: vibrational (transmission mode IR and phatoacoustic spectroscopies), solid-state C-13 NMR, and XPS. A significant variety of behaviors is revealed. DOPA interacts very weakly with Aerosil, although it contributes to the structuring of the solution with the appearance of a population of bound water molecules. In contrast, the weakly basic conditions in smectite suspensions catalyze oxidation by dissolved O-2 to indolic species and polymerization to pseudomelanin. With saponite, structural characterization reveals that L-DOPA zwitterions are accommodated vertically in the interlayer region as a monolayer of partially superimposed species. In addition, TEM indicates that lamellar deposits of pseudomelanin grow epitaxially on the saponite layers. With Laponite, the oxidation reactions seem to be somewhat modified by the specific catalytic effect of edge groups, and the organic species cause a strong structuring of the dispersion with gelification. The interesting chemistry taking place in these systems has implications for both DOPA vectorization and nanocomposite materials synthesis.