UV-absorption spectra of melanosomes containing varying 5,6-dihydroxyindole and 5,6-dihydroxyindole-2-carboxylic acid content

Transcriptomic and Metabolomic Analyses Reveal Molecular Regulatory Networks for Pigmentation Deposition in Sheep

Domestic animals have multiple phenotypes of skin and coat color, which arise from different genes and their products, such as proteins and metabolites responsible with melanin deposition. However, the complex regulatory network of melanin synthesis remains to be fully unraveled. Here, the skin and tongue tissues of Liangshan black sheep (black group) and Liangshan semi-fine-wool sheep (pink group) were collected, stained with hematoxylin-eosin (HE) and Masson-Fontana, and the transcriptomic and metabolomic data were further analyzed. We found a large deposit of melanin granules in the epidermis of the black skin and tongue. Transcriptome and metabolome analysis identified 744 differentially expressed genes (DEGs) and 443 differentially expressed metabolites (DEMs) between the pink and black groups. Gene ontology (GO) and Kyoto encyclopedia of genes and genomes (KEGG) enrichment analyses revealed the DEGs and DEMs were mainly enriched in the pathways of secondary metabolic processes, melanin biosynthesis processes, melanin metabolism processes, melanosome membranes, pigment granule membranes, melanosome, tyrosine metabolism, and melanogenesis. Notably, we revealed the gene ENSARG00020006042 may be a family member of YWHAs and involved in regulating melanin deposition. Furthermore, several essential genes (TYR, TYRP1, DCT, PMEL, MLANA, SLC45A2) were significantly associated with metabolite prostaglandins and compounds involved in sheep pigmentation. These findings provide new evidence of the strong correlation between prostaglandins and related compounds and key genes that regulate sheep melanin synthesis, furthering our understanding of the regulatory mechanisms and molecular breeding of pigmentation in sheep.

Quantum Mechanics/Molecular Mechanics Studies on the Photophysical Mechanism of Methyl Salicylate

Methyl salicylate (MS) as a subunit of larger salicylates found in commercial sunscreens has been shown to exhibit keto-enol tautomerization and dual fluorescence emission via excited-state intramolecular proton transfer (ESIPT) after the absorption of ultraviolet (UV) radiation. However, its excited-state relaxation mechanism is unclear. Herein, we have employed the quantum mechanics(CASPT2//CASSCF)/molecular mechanics method to explore the ESIPT and excited-state relaxation mechanism of MS in the lowest three electronic states, that is, S₀, S₁, and T₁ states, in a methanol solution. Based on the optimized geometric and electronic structures, conical intersections and crossing points, and minimum-energy paths combined with the computed linearly interpolated Cartesian coordinate paths, the photophysical mechanism of MS has been proposed. The S₁ state is a spectroscopically bright ¹ππ* state in the Franck-Condon region. From the initially populated S₁ state, there exist three nonradiative relaxation paths to repopulate the S₀ state. In the first one, the S₁ system (i.e., ketoB form) first undergoes an ESIPT path to generate an S₁ tautomer (i.e., enol form) that exhibits a large Stokes shift in experiments. The generated S₁ enol tautomer further evolves toward the nearby S₁/S₀ conical intersection and then hops to the S₀ state, followed by the backward ground-state intramolecular proton transfer (GSIPT) to the initial ketoB form S₀ state. In the second one, the S₁ system first hops through the S₁ → T₁ intersystem crossing (ISC) to the T₁ state, which then further decays to the S₀ state via T₁ → S₀ ISC at the T₁/S₀ crossing point. In the third path, the T₁ system that stems from the S₁ → T₁ ISC process via the S₁/T₁ crossing point first takes place a T₁ ESIPT to generate a T₁ enol tautomer, which can further decay to the S₀ state via T₁-to-S₀ ISC. Finally, the GSIPT occurs to back the system to the initial ketoB form S₀ state. Our present work could contribute to understanding the photophysics of MS and its derivatives.

QM/MM Studies on the Photophysical Mechanism of a Truncated Octocrylene Model

Methyl 2-cyano-3,3-diphenylacrylate (MCDPA) shares the same molecular skeleton with octocrylene (OCR) that is one of the most common molecules used in commercially available sunscreens. However, its excited-state relaxation mechanism is unclear. Herein, we have used the QM(CASPT2//CASSCF)/MM method to explore spectroscopic properties, geometric and electronic structures, relevant conical intersections and crossing points, and excited-state relaxation paths of MCDPA in methanol solution. We found that in the Franck-Condon (FC) region, the V(¹ππ*) state is energetically lower than the V'(¹ππ*) state only by 2.8 kcal/mol and is assigned to experimentally observed maximum absorption band. From these two initially populated singlet states, there exist three nonradiative relaxation paths to repopulate the S₀ state. In the first one, when the V(¹ππ*) state is populated in the FC region, the system diabatically evolves along the V(¹ππ*) state into its minimum where the internal conversion to S₀ occurs. In the second one, the V'(¹ππ*) state is populated in the FC region and the system adiabatically overcomes a barrier of ca. 3.0 kcal/mol to approach the V(¹ππ*) minimum eventually leading to a V(¹ππ*)-to-S₀ internal conversion. In the third one, the V'(¹ππ*) state first hops via the intersystem crossing to the T₂ state, which then decays through the internal conversion to the T₁ state. The T₁ state is finally converted to the S₀ state via the T₁/S₀ crossing point. Our present work contributes to understanding the photophysics of OCR and its variants.

Photophysics of a UV-B Filter 4-Methylbenzylidene Camphor: Intersystem Crossing Plays an Important Role

4-Methylbenzylidene camphor (4MBC) is a frequently used ultraviolet (UV) filter in commercial sunscreens, which is experimentally found to undergo efficient intersystem crossing to triplet manifolds followed by predominant radiationless decay to the ground state. However, its photophysical mechanism is unclear. Herein, we have employed combined CASPT2 and CASSCF methods to study the spectroscopic properties, geometric and electronic structures, conical intersections and crossing points, and excited-state deactivation channels of 4MBC. We have found that the V(¹ ππ*) state is populated with large probability in the Franck-Condon region. Starting from this state, there are two efficient nonradiative relaxation processes to populate the ³ ππ* state. In the first one, the V(¹ ππ*) state decays to the V'(¹ ππ*) state. The resultant V'(¹ ππ*) state further jumps to the ¹ nπ* state by internal conversion at the ¹ ππ*/¹ nπ* conical intersection. Then, the ¹ nπ* state hops to the ³ ππ* state through an efficient ¹ nπ*→³ ππ* intersystem crossing process. In the second one, the V(¹ ππ*) state can diabatically relax along the photoisomerization reaction coordinate. In this process, a ¹ ππ*/³ nπ* crossing point helps the ¹ ππ* system decay to the ³ nπ* state, which further decays to the ³ ππ* state through internal conversion at the ³ nπ*/³ ππ* conical intersection. Once the ³ ππ* state is formed, a nearly barrierless relaxation path drives the ³ ππ* system to hop to the S₀ state via the ³ ππ*/S₀ crossing point. Our current work not only rationalizes recent experimental observations but also enriches our photophysical knowledge of UV filters.

Anomalous evolution of broadband optical absorption reveals dynamic solid state reorganization during eumelanin build-up in thin films

The origin of eumelanin optical properties remains a formidable conundrum preventing a detailed understanding of the complex photo-protective role of these widespread natural pigments and the rational design of innovative bioinspired materials for optoelectronic applications. Here we report the unusual kinetic and thickness-dependent evolution of the optical properties of black eumelanin polymers generated by spontaneous aerial polymerization of 5,6-dihydroxyindole (DHI) thin films (0.1-1 μm), consistent with peculiar solid state reorganization mechanisms governing broadband absorption. The complete reversal of eumelanin UV-visible transmittance spectrum curvature on passing from 0.2 to 0.5 μm thick films, the marked increase in visible extinction coefficients with increasing film thickness and the higher UV extinction coefficients in slowly vs. rapidly generated polymers concur to support distinct dynamic regimes of solid-state molecular reorganization at the nanoscale level and to do affect the development of broadband visible absorption. Solid state control of molecular reorganization disclosed herein may delineate new rational strategies for tuning optical properties in eumelanin thin films for optoelectronic applications.

A Perspective on the Ultrafast Photochemistry of Solution-Phase Sunscreen Molecules

Sunscreens are one of the most common ways of providing on-demand additional photoprotection to the skin. Ultrafast transient absorption spectroscopy has recently proven to be an invaluable tool in understanding how the components of commercial sunscreen products display efficient photoprotection. Important examples of how this technique has unravelled the photodynamics of common components are given in this Perspective, and some of the remaining unanswered questions are discussed.

Isolation of Intact and Functional Melanosomes from the Retinal Pigment Epithelium

Melanosomes of retinal pigment epithelium (RPE) have many vision supporting functions. Melanosome research would benefit from a method to isolate pure and characterized melanosomes. Sucrose gradient centrifugation is the most commonly used method for isolation of RPE melanosomes, but the isolated products are insufficiently characterized and their quality is unclear. Here we introduce a new gentle method for fractionation of porcine RPE that produces intact functional melanosomes with minimal cross-contamination from other cell organelles. The characterization of isolated organelles was conducted with several methods confirming the purity of the isolated melanosomal fraction (transmission electron microscopy, immunoblotting) and presence of the melanosomal membrane (fluorescence staining of melanosomal membrane, zeta potential measurement). We demonstrate that our isolation method produces RPE melanosomes with the ability to generate free phosphate (Pi) from ATP thereby proving that many membrane proteins remain functional after isolation. The isolated porcine RPE melanosomes represented V-type H+ATPase activity that was demonstrated with bafilomycin A1, a specific V-ATPase inhibitor. We anticipate that the isolation method described here can easily be optimized for the isolation of stage IV melanosomes from other pigmented cell types and tissues.

Onset of the Electronic Absorption Spectra of Isolated and π-Stacked Oligomers of 5,6-Dihydroxyindole: An Ab Initio Study of the Building Blocks of Eumelanin

Eumelanin is a naturally occurring skin pigment which is responsible for developing a suntan. The complex structure of eumelanin consists of π-stacked oligomers of various indole derivatives, such as the monomeric building block 5,6-dihydroxyindole (DHI). In this work, we present an ab initio wave-function study of the absorption behavior of DHI oligomers and of doubly and triply π-stacked species of these oligomers. We have simulated the onset of the electronic absorption spectra by employing the MP2 and the linear-response CC2 methods. Our results demonstrate the effect of an increasing degree of oligomerization of DHI and of an increasing degree of π-stacking of DHI oligomers on the onset of the absorption spectra and on the degree of red-shift toward the visible region of the spectrum. We find that π-stacking of DHI and its oligomers substantially red-shifts the onset of the absorption spectra. Our results also suggest that the optical properties of biological eumelanin cannot be simulated by considering the DHI building blocks alone, but instead the building blocks indole-semiquinone and indole-quinone have to be considered as well. This study contributes to advancing the understanding of the complex photophysics of the eumelanin biopolymer.