Unraveling the Structure and Function of Melanin through Synthesis.

Melanin is ubiquitous in living organisms across different biological kingdoms of life, making it an important, natural biomaterial. Its presence in nature from microorganisms to higher animals and plants is attributed to the many functions of melanin, including pigmentation, radical scavenging, radiation protection, and thermal regulation. Generally, melanin is classified into five types-eumelanin, pheomelanin, neuromelanin, allomelanin, and pyomelanin-based on the various chemical precursors used in their biosynthesis. Despite its long history of study, the exact chemical makeup of melanin remains unclear, and it moreover has an inherent diversity and complexity of chemical structure, likely including many functions and properties that remain to be identified. Synthetic mimics have begun to play a broader role in unraveling structure and function relationships of natural melanins. In the past decade, polydopamine, which has served as the conventional form of synthetic eumelanin, has dominated the literature on melanin-based materials, while the synthetic analogues of other melanins have received far less attention. In this perspective, we will discuss the synthesis of melanin materials with a special focus beyond polydopamine. We will emphasize efforts to elucidate biosynthetic pathways and structural characterization approaches that can be harnessed to interrogate specific structure-function relationships, including electron paramagnetic resonance (EPR) and solid-state nuclear magnetic resonance (ssNMR) spectroscopy. We believe that this timely Perspective will introduce this class of biopolymer to the broader chemistry community, where we hope to stimulate new opportunities in novel, melanin-based poly-functional synthetic materials.

Tuning electronic structure through halide modulation of mesoionic carbene cobalt complexes.

The first examples of Co(ii) mesoionic carbene complexes (CoX2DippMIC2; X = Cl-, Br-, I-) demonstrate a new electronic perturbation on tetrahedral Co(ii) complexes. Using absorption spectroscopy and magnetometry, the consequences of the MIC's strong σ-donating/minimal π-accepting nature are analyzed and shown to be further tunable by the nature of the coordinated halide.

Activating Ru nanoparticles on oxide supports for ring-opening metathesis polymerization.

Nano-sized particles mounted on heterogeneous oxide supports such as silica have altered reactivity when compared to their homogeneous analogs. In particular, catalyzed olefin metathesis using supported Ru nanoparticles has shown great promise and various methods have been employed to develop functional heterogeneous Ru catalysts. This article reports a method for synthesizing Ru nanoparticles supported on silica and titania. The nanoparticles were characterized using XPS showing Ru(0) dominates when not exposed to air. TEM showed Ru nanoparticle sizes of 3.1 ± 0.8 nm for Ru supported on SiO2 and 5.6 ± 1.3 nm for Ru supported on TiO2. The materials demonstrated modest activity in ring-opening metathesis reactions via diazo activation and nanoparticle-embedded polymers of norbornene and norbornadiene were synthesized using dry box and Schlenk line techniques. The Ru/support/polymer composite materials were characterized using proton NMR, XPS, and SEM/EDS.