SELF-ASSEMBLY OF MELANIN STUDIED BY LASER-LIGHT SCATTERING

The unknown molecular weight and chemical structure of melanin place t he study of these pigments outside the range of the classical biochemi cal techniques; thus in this paper the problem of characterizing these heterogeneous biopolymers was approached by means of light scattering techniques, static and dynamic. The static technique allowed us to id entify the macromolecular properties ([MW] and [R-g(2)](1/2)) of melan in extracted from sepia inksac and of two synthetic analogues: L-Dopa melanin obtained by autooxidation and by enzymatic oxidation by Tyrosi nase. By dynamic light scattering (DLS), the hydrodynamic radius Rh wa s measured to monitor the temporal behaviour of the polymerization and aggregation processes and R-h variation by changing the chemical cons traints of the polymerization medium, such as pH and ionic strength. T he fractal dimension d of the aggregates of melanin, both natural and synthetic, in the past only recognized during the aggregation of the s ynthetic one by lowering the pH of the medium, was a useful parameter to further investigate and compare the structure of melanin granules o f differing origins, revealing for the natural sample, a structure wit h clusters that are spherical, not largely hydrated and self-assembled , following a reaction limited aggregation kinetics (d = 2.38) (C) 199 8 Elsevier Science B.V. All rights reserved.