Melanin from a physicochemical point of view

In Situ Synthesis of Keratin and Melanin Chromophoric Submicron Particles

In humans, melanin plays an esthetic role, dictating hair and skin color and traits, while keratin is the protein that comprises most of the epidermis layer. Eumelanin and pheomelanin are types of melanin synthesized from the same building blocks via enzymatic oxidation. Pheomelanin has an additional building block, cysteine amino acid, which affects its final structure. Keratin contains high cysteine content, and by exploiting free thiols in hydrolyzed keratin, we demonstrate the formation of keratin-melanin (KerMel) chromophoric submicron particles. Cryo-TEM analyses found KerMel particle sizes to be 100-300 nm and arranged in the form of a central keratin particle with polymerized l-dopa chains. Attenuated total reflection (ATR)-FTIR, UV-vis, and fluorescence measurements identified new chemical bonds, indicating the formation of KerMel particles. Finally, KerMel replicated natural skin tones and proved cytocompatibility for human epidermal keratinocytes at concentrations below 0.1 mg/mL. Taken together, KerMel is a novel, tunable material that has the potential to integrate into the cosmetic industry.

Melanin Nanoparticles Obtained from Preformed Recombinant Melanin by Bottom-Up and Top-Down Approaches

Melanin is an insoluble, amorphous polymer that forms planar sheets that aggregate naturally to create colloidal particles with several biological functions. Based on this, here, a preformed recombinant melanin (PRM) was utilized as the polymeric raw material to generate recombinant melanin nanoparticles (RMNPs). These nanoparticles were prepared using bottom-up (nanocrystallization-NC, and double emulsion-solvent evaporation-DE) and top-down (high-pressure homogenization-HP) manufacturing approaches. The particle size, Z-potential, identity, stability, morphology, and solid-state properties were evaluated. RMNP biocompatibility was determined in human embryogenic kidney (HEK293) and human epidermal keratinocyte (HEKn) cell lines. RMNPs prepared by NC reached a particle size of 245.9 ± 31.5 nm and a Z-potential of -20.2 ± 1.56 mV; 253.1 ± 30.6 nm and -39.2 ± 0.56 mV compared to that obtained by DE, as well as RMNPs of 302.2 ± 69.9 nm and -38.6 ± 2.25 mV using HP. Spherical and solid nanostructures in the bottom-up approaches were observed; however, they were an irregular shape with a wide size distribution when the HP method was applied. Infrared (IR) spectra showed no changes in the chemical structure of the melanin after the manufacturing process but did exhibit an amorphous crystal rearrangement according to calorimetric and PXRD analysis. All RMNPs presented long stability in an aqueous suspension and resistance to being sterilized by wet steam and ultraviolet (UV) radiation. Finally, cytotoxicity assays showed that RMNPs are safe up to 100 μg/mL. These findings open new possibilities for obtaining melanin nanoparticles with potential applications in drug delivery, tissue engineering, diagnosis, and sun protection, among others.

Improving the solubility of melanin nanoparticles from apricot kernels is a potent drug delivery system

Background:

Melanin can be used in biomedical nanomaterials, but its solubility in water and bioavailability are low.

Aim:

Melanin nanoparticles were prepared and then PEG-natural melanin nanoparticles (NMNP-PEG) were obtained with good performance and optimize their (water solubility, dispersion stability, chelating metal ions, photothermal stability, drug delivery, and biocompatibility), therefore improve the water solubility of melanin and broaden its application scope in biology, medicine, food, and other fields.

Methods:

MFAK (melanin from apricot kernels) and NMNP-PEG were prepared and characterized using ultraviolet-visible spectrophotometry (UV-Vis), high-performance liquid chromatography (HPLC), Fourier-transform infrared spectroscopy (FTIR), 1H nuclear magnetic resonance (NMR), and electron microscopy. The chelation rate of metal ions, photothermal effect, doxorubicin loading, and cytotoxicity (MCF-7 cells) were examined.

Results:

UV-Vis, HPLC, FTIR, and NMR indicated that NMNPs contained melanin. NMNPs could be successfully modified using PEG. Under physiological pH conditions (pH 7.4), the metal ion chelation rate of NMNP-PEG increased with time and peaked at 12 h. The photothermal assay showed a temperature enhancement of 26.3°C with 1 mg/mL NMNP-PEG, compared with 1.9℃ with water. The NMNP-PEGs had a typical peak for doxorubicin in the FTIR spectrum, and the peak intensity was proportional to the drug loading. The release of doxorubicin in an acidic buffer was 40.8% at 24 h, almost threefold that in a neutral buffer (11.9%). There was no obvious cytotoxicity from NMNP-PEG.

Conclusion:

NMNP-PEG displays good stability, high metal ion chelation ability, efficient photothermal conversion potential, drug-retaining capability, sustained controlled drug release, and biocompatibility. This study provides a theoretical basis for NMNP-PEG applications in medicine (targeting specific sites to diagnose and treat diseases), food (extending the shelf life of food), and biology (as metal ion chelating agents to remove heavy metals from wastewater).

New insight into melanin for food packaging and biotechnology applications

Melanin is a dark brown to black biomacromolecule with biologically active multifunctional properties that do not have a precise chemical structure, but its structure mainly depends on the polymerization conditions during the synthesis process. Natural melanin can be isolated from various animal, plant, and microbial sources, while synthetic melanin-like compounds can be synthesized by simple polymerization of dopamine. Melanin is widely used in various areas due to its functional properties such as photosensitivity, light barrier property, free radical scavenging ability, antioxidant activity, etc. It also has an excellent ability to act as a reducing agent and capping agent to synthesize various metal nanoparticles. Melanin nanoparticles (MNP) or melanin-like nanoparticles (MLNP) have the unique potential to act as functional materials to improve nanocomposite films' physical and functional properties. Various food packaging and biomedical applications have been made alone or by mixing melanin or MLNP. In this review, the general aspects of melanin that highlight biological activity, along with a description of MNP and the use as nanofillers in packaging films as well as reducing and capping agents and biomedical applications, were comprehensively reviewed.

Melanins as Sustainable Resources for Advanced Biotechnological Applications

Melanins are a class of biopolymers that are widespread in nature and have diverse origins, chemical compositions, and functions. Their chemical, electrical, optical, and paramagnetic properties offer opportunities for applications in materials science, particularly for medical and technical uses. This review focuses on the application of analytical techniques to study melanins in multidisciplinary contexts with a view to their use as sustainable resources for advanced biotechnological applications, and how these may facilitate the achievement of the United Nations Sustainable Development Goals.

Kinetics of dopamine release from poly(aspartamide)-based prodrugs

Preparation of novel biocompatible and biodegradable polymer-based prodrugs that can be applied in complex drug delivery systems is one of the most researched fields in pharmaceutics. The kinetics of the drug release strongly depends on the physicochemical parameters of prodrugs as well as environmental properties, therefore precise kinetical description is crucial to design the appropriate polymer prodrug formula. The aim of the present study was to investigate the dopamine release from different poly(aspartamide) based dopamine drug conjugates in different environments and to work out a kinetic description which can be extended to describe drug release in similar systems. Poly(aspartamide) was conjugated with different amounts of dopamine. In order to alter the solubility of the conjugates, 2-aminoethanol was also grafted to the main chain. Chemical structure as well as physical properties such as solubility, lipophilicity measurements and thermogravimetric analysis has been carried out. Kinetics of dopamine release from the macromolecular prodrugs which has good water solubility has been studied and compared in different environments (phosphate buffer, Bromelain and α-Chymotrypsin). It was found that the kinetics of release in those solutions can be satisfactorily described by first order reaction rate. For poorly-soluble conjugates, the release of dopamine was considered as a result of coupling of diffusion and chemical reaction. Besides the time dependence of dopamine cleavage, a practical quantity, the half-life of the release of loading capacity has been introduced and evaluated. It was found, that dopamine containing macromolecular prodrugs exhibit prolonged release kinetics and the quantitative description of the kinetics, including the most important physical parameters provides a solid base for future pharmaceutical and medical studies.

STATEMENT OF SIGNIFICANCE

Poly(aspartamide) based polymer-drug conjugates are promising for controlled and prolonged drug delivery due to their biocompatibility and biodegradability. In this study different poly(aspartamide) based dopamine conjugates were synthesized which can protect dopamine from deactivation in the human body. Since there is no satisfying kinetics description for drug release from covalent polymer-drug conjugates in the literature, dopamine release was investigated in different environments and a complete kinetical description was worked out. This study demonstrates that poly(aspartamide) is able to protect conjugated dopamine from deactivation and provide prolonged release in alkaline pH as well as in the presence of different enzymes. Furthermore, detailed kinetical descriptions were demonstrated which can be used in case of other covalent polymer-drug conjugates.