X-ray diffraction studies on melanins in lyophylized melanosomes

Influence of Melanin Characteristics on Drug Binding Properties

Melanins are biopolymers encompassing a high degree of chemical heterogeneity. Binding of small-molecule drugs to ocular melanin significantly affects the ocular pharmacokinetics, and could serve as a strategy for prolonged drug retention in the eye. The influence of the structural and physical characteristics of melanins originating from different sources on their drug binding properties has not yet been methodically investigated. We performed physical characterization of Sepia officinalis, synthetic and porcine melanin. The particle size distribution was analyzed by laser diffractometry. A dynamic vapor sorption method, requiring small amounts of the material, was developed to analyze the differences in the specific surface area of the melanins. The extent of melanin binding at equilibrium was determined for a set of 34 small-molecule drugs and compared across different melanin types. Despite systematic shifts in the extent of binding within a twofold range, binding data were highly correlated across the melanins. These moderate differences in binding could not be directly explained by the substantial differences in particle size and were more in line with the relatively similar specific surface area of these different melanin materials. Overall, these results suggest that the specific surface area reflects the actual accessibility of a small molecule in the melanin structure and could serve as a surrogate to explain the binding differences observed for the respective melanin materials.

Isolation and biophysical studies of natural eumelanins: applications of imaging technologies and ultrafast spectroscopy

The major pigments found in the skin, hair, and eyes of humans and other animals are melanins. Despite significant research efforts, the current understanding of the molecular structure of melanins, the assembly of the pigment within its organelle, and the structural consequences of the association of melanins with protein and metal cations is limited. Likewise, a detailed understanding of the photochemical and photophysical properties of melanins has remained elusive. Many types of melanins have been studied to date, including natural and synthetic model pigments. Such studies are often contradictory and to some extent the diversity of systems studied may have detracted from the development of a basic understanding of the structure and function of the natural pigment. Advances in the understanding of the structure and function of melanins require careful characterization of the pigments examined so as to assure the data obtained may be relevant to the properties of the pigment in vivo. To address this issue, herein the influence of isolation procedures on the resulting structure of the pigment is examined. Sections describing the applications of new technologies to the study of melanins follow this. Advanced imaging technologies such as scanning probe microscopies are providing new insights into the morphology of the pigment assembly. Recent photochemical studies on photoreduction of cytochrome c by different mass fraction of sonicated natural melanins reveal that the photogeneration of reactive oxygen species (ROS) depends upon aggregation of melanin. Specifically, aggregation mitigates ROS photoproduction by UV-excitation, suggesting the integrity of melanosomes in tissue may play an important role in the balance between the photoprotective and photodamaging behaviors attributed to melanins. Ultrafast laser spectroscopy studies of melanins are providing insights into the time scales and mechanisms by which melanin dissipates absorbed light energy.

Fluorescence properties of melanins from opioid peptides

Recently our group synthesized a new class of melanins obtained by the tyrosinase-catalyzed oxidation of opioid peptides (opiomelanins). Owing to the presence of the peptide moiety such pigments exhibit high solubility in hydrophilic solvents, which allows spectroscopic investigations. In particular, the absence of solid-state quenching effects enables the study of melanin fluorescence properties, till now poorly investigated due to the complete insolubility of melanins produced from tyrosine or Dopa. Opiomelanins dissolved in aqueous medium show a characteristic emission peaked at 440 and 520 nm when excited around 330 nm, where a maximum is observed in the absorption spectrum. Kinetic measurements performed on the tyrosinase-catalyzed oxidation of opioid peptides show that the 440-nm fluorescence band arises in the early stages of peptide oxidation, whereas the 520-nm band appears in later stages of oxidation, i.e., during the polymerization of indole-quinone units. Moreover, molecular sieve fractionation shows that in the opiomelanin fraction with a molecular weight lower than 10 kDa the 440-nm band is dominant in the fluorescence spectrum. The breakdown of the polymer induced by hydrogen peroxide and light (i.e., the photobleaching of melanin pigments) produces a marked enhancement of the 440-nm fluorescence band while the 520-nm band disappears. Hence, our findings suggest that the observed fluorescence contains contributions from both oligomeric units (440-nm band) and high-molecular-weight polymers (520-nm band).

Melanins from tetrahydroisoquinolines: spectroscopic characteristics, scavenging activity and redox transfer properties

Tetrahydroisoquinolines (TIQs) are endogenous compounds deriving from the nonenzymatic Pictet-Spengler condensation of catecholamines (CA) with aldehydes. TIQs have been extensively studied in the last years not only because they have been found in the brain of postmortem specimens of Parkinson's patients, but also because they are able to induce parkinsonian symptoms if injected in animals. In the present article we demonstrate that TIQs bearing a catecholic moiety (tetrahydropapaveroline, salsolinol, laudanosoline, and apomorphine) are easily oxidized in the presence of hydrogen peroxide by various enzymes--i.e., peroxidase (POD), lipoxygenase (LOX), and xanthine oxidase (XO)--into the corresponding TIQ-melanins. The kinetic parameters of the above-mentioned reactions and some spectroscopic characteristics of the synthetized pigments are reported. In particular, UV-VIS and EPR spectra emerge as very similar to those exhibited by dopa-melanin. Furthermore, TIQ-melanins appear to be similar to dopa-melanin regarding some specific physico-chemical properties: NADH-oxidizing properties, oxy-radicals scavenging activity, and ability to form soluble mixed polymers with melanins from opioid peptides.

X-ray characterization of melanins--II

Structural modeling of amorphous eumelanin has been carried out by comparing calculated data, S(q) and RDF(r), in reciprocal and real space, respectively, for limited random network models with the experimental X-ray scattering data of tyrosine melanin (Cheng et al., 1994). A basic picture of the atomic arrangements in amorphous eumelanin, which accounts for the short and intermediate range order, has been formulated. This reveals domains of a fundamental "particle" dimension of R approximately 15 A, consisting of a paracrystalline array of disordered planar networks polymerized by 4-8 DHI monomers with a graphite-like stacking spacing of approximately 3.45 A, 4-5 layers thick.

X-ray characterization of melanins--I

The intrinsic local structure characterization of natural sepia melanin and L-dopa and tyrosine synthetic melanin powder has been carried out by X-ray diffraction using synchrotron radiation. The derived structure factor, S(q), shows six significant diffuse peaks within the q-range from 0.3 A-1 to 16 A-1 in the reciprocal space (q = (4 pi sin theta)/lambda, 2 theta is the scattering angle). The Fourier transform of S(q), which yields the radial distribution function (RDF), gives us information in real space of a 1.42 A distance averaged over the C-C, C-O and C-N bond lengths as well as peaks at 2.40-2.41 A, 3.67-3.71 A and 4.67-4.70 A discrete neighbor distances. There is a great similarity in the scattering intensity profiles of the natural and synthetic melanins indicating that the synthetically prepared material may be essentially similar to "real" melanin in its local atomic arrangements. An evidence of a prepeak at q congruent to 0.45 A-1 has been confirmed which indicates a preferred length scale of approximately 13-20 A that corresponds to the initial particle size in colloidal melanin solutions.

The fundamental unit of synthetic melanin: a verification by tunneling microscopy of X-ray scattering results

A characteristic dimension of a melanin protomolecule synthesized from tyrosine has been investigated by scanning tunneling microscopy (STM). Identification of a melanin protomolecule of approximately approximately 20 A lateral extent and approximately 10 A height has been established. This size is in good agreement with models constructed to fit wide angle X-ray diffraction experiments on melanin. These protomolecules are believed to consist of Van der Waals interacting stacks of a basic random polymer of 5.6 indolequinone units. There is extensive pi-delocalization within the individual polymeric sheets. Structure minimization and molecular orbital techniques were employed to verify the X-ray and STM results.

Porphyrin-melanin interaction: effect on fluorescence and non-radiative relaxations

Optical techniques and pulsed-laser, time-resolved photoacoustics (PA) were employed to obtain information on the mechanism of interaction between cationic zinc tetrabenzilpyridilporphyrin (ZnTBzPyP) and synthetic L-Dopa melanins. Synthetic eumelanin and pheomelanin strongly quench the fluorescence of ZnTBzPyP, but Stern-Volmer plots suggest a mechanism of interaction quite different for the two pigments. This diversity was confirmed by PA: for eumelanin no thermal relaxation was observed other than prompt heat, whereas for the complexed form of ZnTBzPyP with pheomelanin we were able to detect a heat-emitting species with a non-radiative lifetime in the microsecond range. The involvement of oxygen in the photophysics of the complexes formed between the cationic porphyrin and the two pigments was demonstrated, but its role has yet to be described.