New Perspectives in the Chemistry and Biochemistry of the Tunichromes and Related Compounds

Bioactive Molecules from the Innate Immunity of Ascidians and Innovative Methods of Drug Discovery: A Computational Approach Based on Artificial Intelligence

The study of bioactive molecules of marine origin has created an important bridge between biological knowledge and its applications in biotechnology and biomedicine. Current studies in different research fields, such as biomedicine, aim to discover marine molecules characterized by biological activities that can be used to produce potential drugs for human use. In recent decades, increasing attention has been paid to a particular group of marine invertebrates, the Ascidians, as they are a source of bioactive products. We describe omics data and computational methods relevant to identifying the mechanisms and processes of innate immunity underlying the biosynthesis of bioactive molecules, focusing on innovative computational approaches based on Artificial Intelligence. Since there is increasing attention on finding new solutions for a sustainable supply of bioactive compounds, we propose that a possible improvement in the biodiscovery pipeline might also come from the study and utilization of marine invertebrates' innate immunity.

Sea squirt-inspired bio-derived tissue sealants

Sea squirts' or tunicates' bodies are composed of cellulose nanofibers and gallol- functionalized proteins. These sea creatures are known to heal their injuries under seawater by forming crosslinks between gallols and functional groups from other proteins in their bodies. Inspired by their wound healing mechanism, herein, we have developed a tissue sealant using zein (a plant-based protein) and tannic acid (gallol-containing polyphenol). Except for fibrin- based sealants, most commercial surgical adhesives, and sealants available today are derived from petroleum products that compromise their biodegradability. They often have complicated and multi-step synthesis processes that ultimately affect their affordability. To overcome this challenge, we ensured that these sea squirt-inspired tissue sealants are bio-based, easily synthesized, and low-cost. The sealants were studied on their own and with a food-grade enzyme transglutaminase. The adhesion performances of the sealants were found to be higher than physiological pressures in seven out of nine different tissue substrates studied here. Their performance was also better than or on par with the FDA-approved fibrin sealant Tisseel. Ex vivo models demonstrate instant sealing of leaking wounds in less than a minute. The sealants were not only cytocompatible but also showed complete wound healing on par with sutures and Tisseel when applied in vivo on skin incisions in rats. Overall, these sea squirt-inspired bio-based sealants show great potential to replace currently available wound closure methods.

Catecholamine Derivatives as Novel Crosslinkers for the Synthesis of Versatile Biopolymers

Catecholamine metabolites are not only involved in primary metabolism, but also in secondary metabolism, serving a diverse array of physiologically and biochemically important functions. Melanin, which originates from dopa and dopamine, found in the hair, eye, and skin of all animals, is an important biopolymeric pigment. It provides protection against damaging solar radiation to animals. N-Acetyldopamine and N-β-alanyldopamine play a crucial role in the hardening of the exoskeletons of all insects. In addition, insects and other arthropods utilize the melanogenic process as a key component of their defense systems. Many marine organisms utilize dopyl peptides and proteins as bonding materials to adhere to various substrata. Moreover, the complex dopa derivatives that are precursors to the formation of the exoskeletons of numerous marine organisms also exhibit antibiotic properties. The biochemistry and mechanistic transformations of different catecholamine derivatives to produce various biomaterials with antioxidant, antibiotic, crosslinking, and gluing capabilities are highlighted. These reactivities are exhibited through the transient and highly reactive quinones, quinone methides, and quinone methide imine amide intermediates, as well as chelation to metal ions. A careful consideration of the reactivities summarized in this review will inspire numerous strategies for synthesizing novel biomaterials for future medical and industrial use.

Adhesion and Cohesion Differences between Catechol- and Pyrogallol-Functionalized Chitosan

Marine-inspired phenolic compounds that exhibit underwater adhesion are used as biomedical adhesives under wet conditions. While these applications mainly use catechol and pyrogallol moieties that contain different numbers of hydroxyl groups on their benzene rings, how this difference affects adhesion and cohesion is not well understood. Herein, the chitosan backbone is functionalized with catechol and pyrogallol at similar modification rates (to give chitosan-catechol (CS-CA) and chitosan-pyrogallol (CS-GA), respectively) and their interaction energies are compared by using a surface forces apparatus (SFA). The phenolic moieties decrease the rigidity of the chitosan chain and increase solubility; consequently, CS-CA and CS-GA are more cohesive and adhesive than chitosan at pH 7.4. Moreover, the additional hydroxyl group of GA provides a further interacting chance; hence, CS-GA is more cohesive and adhesive than CS-CA. This study provides in-depth insight into interactions involving chitosan derivatives bearing introduced phenolic moieties that will help to develop biomedical adhesives.

Biomaterials and Bioactive Natural Products from Marine Invertebrates: From Basic Research to Innovative Applications

Aquatic invertebrates are a major source of biomaterials and bioactive natural products that can find applications as pharmaceutics, nutraceutics, cosmetics, antibiotics, antifouling products and biomaterials. Symbiotic microorganisms are often the real producers of many secondary metabolites initially isolated from marine invertebrates; however, a certain number of them are actually synthesized by the macro-organisms. In this review, we analysed the literature of the years 2010–2019 on natural products (bioactive molecules and biomaterials) from the main phyla of marine invertebrates explored so far, including sponges, cnidarians, molluscs, echinoderms and ascidians, and present relevant examples of natural products of interest to public and private stakeholders. We also describe omics tools that have been more relevant in identifying and understanding mechanisms and processes underlying the biosynthesis of secondary metabolites in marine invertebrates. Since there is increasing attention on finding new solutions for a sustainable large-scale supply of bioactive compounds, we propose that a possible improvement in the biodiscovery pipeline might also come from the study and utilization of aquatic invertebrate stem cells.

Systematic Approach to Mimic Phenolic Natural Polymers for Biofabrication

In nature, phenolic biopolymers are utilized as functional tools and molecular crosslinkers to control the mechanical properties of biomaterials. Of particular interest are phenolic proteins/polysaccharides from living organisms, which are rich in catechol and/or gallol groups. Their strong underwater adhesion is attributed to the representative phenolic molecule, catechol, which stimulates intermolecular and intramolecular crosslinking induced by oxidative polymerization. Significant efforts have been made to understand the underlying chemistries, and researchers have developed functional biomaterials by mimicking the systems. Owing to their unique biocompatibility and ability to transform their mechanical properties, phenolic polymers have revolutionized biotechnologies. In this review, we highlight the bottom-up approaches for mimicking polyphenolic materials in nature and recent advances in related biomedical applications. We expect that this review will contribute to the rational design and synthesis of polyphenolic functional biomaterials and facilitate the production of related applications.

Tunicate-Inspired Photoactivatable Proteinic Nanobombs for Tumor-Adhesive Multimodal Therapy

Near-IR (NIR) light-responsive multimodal nanotherapeutics have been proposed to achieve improved therapeutic efficacy and high specificity in cancer therapy. However, their clinical application is still elusive due to poor biometabolization and short retention at the target site. Here, innovative photoactivatable vanadium-doped adhesive proteinic nanoparticles (NPs) capable of allowing biological photoabsorption and NIR-responsive anticancer therapeutic effects to realize trimodal photothermal-gas-chemo-therapy treatments in a highly biocompatible, site-specific manner are proposed. The photoactivatable tumor-adhesive proteinic NPs can enable efficient photothermal conversion via tunicate-inspired catechol-vanadium complexes as well as prolonged tumor retention by virtue of mussel protein-driven distinctive adhesiveness. The incorporation of a thermo-sensitive nitric oxide donor and doxorubicin into the photoactivatable adhesive proteinic NPs leads to synergistic anticancer therapeutic effects as a result of photothermal-triggered "bomb-like" multimodal actions. Thus, this protein-based phototherapeutic tumor-adhesive NPs have great potential as a spatiotemporally controllable therapeutic system to accomplish effective therapeutic implications for the complete ablation of cancer.

Shape retaining self-healing metal-coordinated hydrogels

Metal-coordinated hydrogels are physical hydrogels entirely crosslinked by complexes between ligand decorated polymers and metal ions. The mechanical properties of these hydrogels strongly depend on the density and dynamics of metal-coordinated interactions. Most commonly, telechelic metal-coordinated hydrogels contain catechol or histidine ligands, although hydrogels containing a stronger complexation agent, nitrocatechol, have been reported. Here, we introduce a pyrogallol end-functionalized polymer that can be crosslinked with di- and trivalent ions, in contrast to previously reported metal-coordinated hydrogels. We can tune the mechanical properties of the hydrogels with the types of ions used and the density of crosslinking sites. Ions form nm-sized precipitates that bind to pyrogallols and impart distinct properties to the hydrogels: strong ion-pyrogallol interactions that form in the presence of Al3+, V3+, Mn2+, Fe3+, Co2+, Ni2+ and Cu2+ result in long relaxation times. The resulting hydrogels display solid-like yet reversible mechanical properties, such that they can be processed into macroscopic 3D structures that retain their shapes. Weak ion-pyrogallol interactions that form in the presence of Ca2+ or Zn2+ result in short relaxation times. The resulting hydrogels display a fast self-healing behavior, suited for underwater glues, for example. The flexibility of tuning the mechanical properties of hydrogels simply by selecting the adequate ion-pyrogallol pair broadens the mechanical properties of metal-coordinated hydrogels to suit a wide range of applications that require them to retain their shape for a given time to act as dampers.

Marine invertebrates are a source of bioadhesives with biomimetic interest

Protein-based bioadhesives are found in diverse marine invertebrates that developed attachment devices to adhere to various substrates. These adhesives are of interest to materials science to create bioinspired-adhesives that can perform in water or wet conditions and can be applied in a broad variety of biotechnological and industrial fields. Due to the high variety of invertebrates that inhabit the marine environment, an enormous diversity of structures and principles used in biological adhesives remains unexplored and a very limited number of model systems have inspired novel biomimetic adhesives, the most notable being the mussel byssus adhesive. In this review we give an overview of other marine invertebrates studied for their bioadhesive properties in view of their interest for the development of new biomimetic adhesives for application in the biomedical field but also for antifouling coatings. The molecular features are described, highlighting relevant structures, and examples of biomimetic materials are discussed and explored, opening an avenue for a new set of medical products.

Identifying adhesive components in a model tunicate

Tunicates populate a great variety of marine underwater substrates worldwide and represent a significant concern in marine shipping and aquaculture. Adhesives are secreted from the anterior papillae of their swimming larvae, which attach and metamorphose into permanently adhering, filter-feeding adults. We recently described the cellular composition of the sensory adhesive organ of the model tunicate Ciona intestinalis in great detail. Notably, the adhesive secretions of collocytes accumulate at the tip of the organ and contain glycoproteins. Here, we further explore the components of adhesive secretions and have screened for additional specificities that may influence adhesion or cohesion of the Ciona glue, including other carbohydrate moieties, catechols and substrate properties. We found a distinct set of sugar residues in the glue recognized by specific lectins with little overlap to other known marine adhesives. Surprisingly, we also detect catechol residues that likely originate from an adjacent cellular reservoir, the test cells. Furthermore, we provide information on substrate preferences where hydrophobicity outperforms charge in the attachment. Finally, we can influence the settlement process by the addition of hydrophilic heparin. The further analysis of tunicate adhesive strategies should provide a valuable knowledge source in designing physiological adhesives or green antifoulants. This article is part of the theme issue 'Transdisciplinary approaches to the study of adhesion and adhesives in biological systems'.

Direct Observation of the Interplay of Catechol Binding and Polymer Hydrophobicity in a Mussel-Inspired Elastomeric Adhesive

Marine organisms such as mussels have mastered the challenges in underwater adhesion by incorporating post-translationally modified amino acids like l-3,4-dihydroxyphenylalanine (DOPA) in adhesive proteins. Here we designed a catechol containing elastomer adhesive to identify the role of catechol in interfacial adhesion in both dry and wet conditions. To decouple the adhesive contribution of catechol to the overall adhesion, the elastomer was designed to be cross-linked through [2 + 2] photo-cycloaddition of coumarin. The elastomer with catechol moieties displayed a higher adhesion strength than the catechol-protected elastomer. The contact interface was probed using interface-sensitive sum frequency generation spectroscopy to explore the question of whether catechol can displace water and bond with hydrophilic surfaces. The spectroscopy measurements reveal that the maximum binding energy of the catechol and protected-catechol elastomers to sapphire substrate is 7.0 ± 0.1 kJ/(mole of surface O-H), which is equivalent to 0.10 J/m2. The higher dry and wet adhesion observed in the macroscopic adhesion measurements for the catechol containing elastomer originates from multiple hydrogen bonds of the catechol dihydroxy groups to the surface. In addition, our results show that catechol by itself does not remove the confined interstitial water. In these elastomers, it is the hydrophobic groups that help in partially removing interstitial water. The observation of the synergy between catechol binding and hydrophobicity in enabling the mussel-inspired soft adhesive elastomer to stick underwater provides a framework for designing materials for applications in tissue adhesion and moist-skin wearable electronics.

Tunicate-Inspired Gallol Polymers for Underwater Adhesive: A Comparative Study of Catechol and Gallol

Man-made glues often fail to stick in wet environments because of hydration-induced softening and dissolution. The wound healing process of a tunicate inspired the synthesis of gallol-functionalized copolymers as underwater adhesive. Copolymers bearing three types of phenolic groups, namely, phenol, catechol, and gallol, were synthesized via the methoxymethyl protection/deprotection route. Surprisingly, the newly synthesized copolymers bearing gallol groups exhibited stronger adhesive performances (typically 7× stronger in water) than the widely used catechol-functionalized copolymers under all tested conditions (in air, water, seawater, or phosphate-buffered saline solution). The higher binding strength was ascribed to the tridentate-related interfacial interaction and chemical cross-linking. Moreover, gallol-functionalized copolymers adhered to all tested surfaces including plastic, glass, metal, and biological material. In seawater, the performance of gallol-functionalized copolymer even exceeds the commercially available isocyanate-based glue. The insights from this study are expected to help in the design of biomimetic materials containing gallol groups that may be utilized as potential bioadhesives and for other applications. The results from such a kind of comparable study among phenol, catechol, and gallol suggests that tridentate structure should be better than bidentate structure for bonding to the surface.

Tunichrome-Inspired Gold-Enrichment Dispersion Matrix and Its Application in Water Treatment: A Proof-of-Concept Investigation

Tunicate, a filter-feeder in seawater, is able to accumulate high amount of metals using intracellular polymer matrices. The woven pyrogallol structures of tunichrome, a small peptide contained in tunicate's blood cells, is believed to be responsible for selective metal sequestration in tunicates from seawater. However, the intriguing tunichrome matrix is difficult both to harvest from the tunicate and to synthesize massively due to the extreme oxidation sensitivity of the pyrogallol moiety which limits the study scope. Here, we succeeded to mimic tunichrome by conjugating two cheap and naturally occurring components: pyrogallol-5-carboxylic acid (gallic acid) and chitin nanofiber. A tunicate-mimetic infiltration matrix of surface-tailored chitin nanofibers with pyrogallol moieties (CGa) demonstrated the versatility of this strategy in generation of ingenious filtration material, especially for unprecedented fine and clean gold recovery inside of the tunicate-mimetic infiltration matrix (>99%, 533 mg gold per gram weight), which exceeds that of the presently most popular materials. Complexation between pyrogallol on the nanofiber and gold was similar to that of a tunichrome's metal sequestration. Extended X-ray absorption fine structure (EXAFS) spectroscopy and data-fitting elucidated the decreased coordination numbers for Au-Au nearest neighbors, demonstrating that gold coordinated to pyrogallol units, followed by an intramolecular association of Au⁰. A catalytic reduction of 4-nitrophenol mediated by the tunicate-mimetic matrix with harvested gold revealed excellent recyclability up to 30 cycles (∼95% reduction), which together with methylene blue reduction and antimicrobial performances indicates the versatile characteristics of sustainable processes by the tunichrome mimetics. This strategy opens the door for fast-developing new biomimetic alternatives for precious metal recovery, which is not restricted to gold and can offer a tool for multifaceted soft/hard nanomaterials.

Mussel adhesion - essential footwork

Robust adhesion to wet, salt-encrusted, corroded and slimy surfaces has been an essential adaptation in the life histories of sessile marine organisms for hundreds of millions of years, but it remains a major impasse for technology. Mussel adhesion has served as one of many model systems providing a fundamental understanding of what is required for attachment to wet surfaces. Most polymer engineers have focused on the use of 3,4-dihydroxyphenyl-l-alanine (Dopa), a peculiar but abundant catecholic amino acid in mussel adhesive proteins. The premise of this Review is that although Dopa does have the potential for diverse cohesive and adhesive interactions, these will be difficult to achieve in synthetic homologs without a deeper knowledge of mussel biology; that is, how, at different length and time scales, mussels regulate the reactivity of their adhesive proteins. To deposit adhesive proteins onto target surfaces, the mussel foot creates an insulated reaction chamber with extreme reaction conditions such as low pH, low ionic strength and high reducing poise. These conditions enable adhesive proteins to undergo controlled fluid-fluid phase separation, surface adsorption and spreading, microstructure formation and, finally, solidification.

Contemplating a role for titanium in organisms

Titanium is the ninth most abundant element in the Earth's crust and some organisms sequester it avidly, though no essential biological role has yet been recognized. This Minireview addresses how the properties of titanium, especially in an oxic aqueous environment, might make a biological role difficult to recognize. It further considers how new -omic technologies might overcome the limitations of the past and help to reveal a specific role for this metal. While studies with well established model organisms have their rightful place, organisms that are known avid binders or sequesterers of titanium should be promising places to investigate a biological role.

Tunicate-Inspired Gallic Acid/Metal Ion Complex for Instant and Efficient Treatment of Dentin Hypersensitivity

Dentin hypersensitivity is sharp and unpleasant pains caused by exposed dentinal tubules when enamel outside of the tooth wears away. The occlusion of dentinal tubules via in situ remineralization of hydroxyapatite is the best method to alleviate the symptoms caused by dentin hypersensitivity. Commercially available dental desensitizers are generally effective only on a specific area and are relatively toxic, and their performance usually depends on the skill of the clinician. Here, a facile and efficient dentin hypersensitivity treatment with remarkable aesthetic improvement inspired by the tunicate-self-healing process is reported. As pyrogallol groups in tunicate proteins conjugate with metal ions to heal the torn body armor of a tunicate, the ingenious mechanism by introducing gallic acid (GA) as a cheap, abundant, and edible alternative to the pyrogallol groups of the tunicate combined with a varied daily intake of metal ion sources is mimicked. In particular, the GA/Fe(3+) complex exhibits the most promising results, to the instant ≈52% blockage in tubules within 4 min and ≈87% after 7 d of immersion in artificial saliva. Overall, the GA/metal ion complex-mediated coating is facile, instant, and effective, and is suggested as an aesthetic solution for treating dentin hypersensitivity.

Novel post-translational oligomerization of peptidyl dehydrodopa model compound, 1,2-dehydro-N-acetyldopa methyl ester

Post-translational modification of peptidyl tyrosine to peptidyl dopa is widely observed in different marine organisms. While peptidyl dopas are oxidatively converted to dehydrodopa derivatives, nothing is known about the further fate of dehydrodopyl compounds. To fill this void, we studied the oxidation chemistry of a peptidyl dehydrodopa mimic, 1,2-dehydro-N-acetyldopa methyl ester with mushroom tyrosinase. We employed both routine biochemical studies and reversed phase liquid chromatography mass spectrometry to investigate the course of the reaction. Tyrosinase catalyzed the oxidation of 1,2-dehydro-N-acetyldopa methyl ester readily generating its typical o-quinone as the transient two-electron oxidation product. This quinone was extremely unstable and rapidly reacted with the parent compound forming benzodioxan type oligomeric products. Reaction mixture containing chemically made o-benzoquinone and 1,2-dehydro-N-acetyldopa methyl ester generated a mixed adduct of benzoquinone and 1,2-dehydro-N-acetyldopa methyl ester. Based on this finding, we propose that peptidyl dehydrodopa also exhibits a similar transformation accounting partially for the adhesive and cementing properties of dopyl proteins in nature.

Tunicate-mimetic nanofibrous hydrogel adhesive with improved wet adhesion

The main impediment to medical application of biomaterial-based adhesives is their poor wet adhesion strength due to hydration-induced softening and dissolution. To solve this problem, we mimicked the wound healing process found in tunicates, which use a nanofiber structure and pyrogallol group to heal any damage on its tunic under sea water. We fabricated a tunicate-mimetic hydrogel adhesive based on a chitin nanofiber/gallic acid (a pyrogallol acid) composite. The pyrogallol group-mediated cross-linking and the nanofibrous structures improved the dissolution resistance and cohesion strength of the hydrogel compared to the amorphous polymeric hydrogels in wet condition. The tunicate-mimetic adhesives showed higher adhesion strength between fully hydrated skin tissues than did fibrin glue and mussel-mimetic adhesives. The tunicate mimetic hydrogels were produced at low cost from recyclable and abundant raw materials. This tunicate-mimetic adhesive system is an example of how natural materials can be engineered for biomedical applications.

A rapid, efficient, and facile solution for dental hypersensitivity: The tannin-iron complex

Dental hypersensitivity due to exposure of dentinal tubules under the enamel layer to saliva is a very popular and highly elusive technology priority in dentistry. Blocking water flow within exposed dentinal tubules is a key principle for curing dental hypersensitivity. Some salts used in "at home" solutions remineralize the tubules inside by concentrating saliva ingredients. An "in-office" option of applying dense resin sealants on the tubule entrance has only localized effects on well-defined sore spots. We report a self-assembled film that was formed by facile, rapid (4 min), and efficient (approximately 0.5 g/L concentration) dip-coating of teeth in an aqueous solution containing a tannic acid-iron(III) complex. It quickly and effectively occluded the dentinal tubules of human teeth. It withstood intense tooth brushing and induced hydroxyapatite remineralisation within the dentinal tubules. This strategy holds great promise for future applications as an effective and user-friendly desensitizer for managing dental hypersensitivity.

Thirty years through vanadium chemistry

The relevance of vanadium in biological systems is known for many years and vanadium-based catalysts have important industrial applications, however, till the beginning of the 80s research on vanadium chemistry and biochemistry did not receive much attention from the scientific community. The understanding of the broad bioinorganic implications resulting from the similarities between phosphate and vanadate(V) and the discovery of vanadium dependent enzymes gave rise to an enormous increase in interest in the chemistry and biological relevance of vanadium. Thereupon the last 30years corresponded to a period of enormous research effort in these fields, as well as in medicinal applications of vanadium and in the development of catalysts for use in fine-chemical synthesis, some of these inspired by enzymatic active sites. Since the 80s my group in collaboration with others made contributions, described throughout this text, namely in the understanding of the speciation of vanadium compounds in aqueous solution and in biological fluids, and to the transport of vanadium compounds in blood plasma and their uptake by cells. Several new types of vanadium compounds were also synthesized and characterized, with applications either as prospective therapeutic drugs or as homogeneous or heterogenized catalysts for the production of fine chemicals. The developments made are described also considering the international context of the evolution of the knowledge in the chemistry and bioinorganic chemistry of vanadium compounds during the last 30years. This article was compiled based on the Vanadis Award presentation at the 9th International Vanadium Symposium.

Jack of all trades: versatile catechol crosslinking mechanisms

Catechols play an important role in many natural systems. They are known to readily interact with both organic (e.g., amino acids) and inorganic (e.g., metal ions, metal oxides) compounds, thereby providing a powerful system for protein curing. Catechol crosslinked protein networks, such as sclerotized cuticle and byssal threads of the mussel, have been shown to exhibit excellent mechanical properties. A lot of effort has been devoted to mimicking the natural proteins using synthetic catechol-functionalized polymers. Despite the success in developing catechol-functionalized materials, the crosslinking chemistry of catechols is still a subject of debate. To develop materials with controlled and superior properties, a clear understanding of the crosslinking mechanism of catechols is of vital importance. This review describes the crosslinking pathways of catechol and derivatives in both natural and synthetic systems. We discuss existing pathways of catechol crosslinking and parameters that affect the catechol chemistry in detail. This overview will point towards a rational direction for further investigation of the complicated catechol chemistry.

Metal-coordination: Using one of nature's tricks to control soft material mechanics

Growing evidence supports a critical role of dynamic metal-coordination crosslinking in soft biological material properties such as self-healing and underwater adhesion1. Using bio-inspired metal-coordinating polymers, initial efforts to mimic these properties have shown promise2. Here we demonstrate how bio-inspired aqueous polymer network mechanics can be easily controlled via metal-coordination crosslink dynamics; metal ion-based crosslink stability control allows aqueous polymer network relaxation times to be finely tuned over several orders of magnitude. In addition to further biological material insights, our demonstration of this compositional scaling mechanism should provide inspiration for new polymer material property-control designs.

Structure, biosynthesis and possible function of tunichromes and related compounds

Several species of ascidians (phylum Chordata, subphylum Urochordata) contain a group of oligopeptides called "tunichromes" in their blood cells. These peptides have been implicated in (a) metal chelation and accumulation/sequestration of vanadium or iron; (b) crosslinking of structural fibers in tunic formation, (c) wound healing and (d) defense reactions. However, their biosynthesis, metabolism, and biological function remain largely un-elucidated due to their extreme instability and high reactivity. Tunichromes and related compounds uniquely possess dehydrodopamine moieties, all originating from post-translational modification of peptidyl tyrosine. It is conceivable that the presence of such novel post-translationally modified groups provide attributes that are crucial for their biological roles. Therefore, we examined the chemistry and reactivity of tunichromes in light of the available knowledge of the biochemistry of simple monomeric dehydro-N-acyldopamine units. Based on the reactivity of such simple compounds, the potential biological activities of tunichromes are predicted. Their possible biosynthetic route from peptidyl tyrosine is critically evaluated to provide a better basis for unraveling their biological functions. Prevalence of dehydro-N-acyldopamine units in different tunichromes, some marine antibiotic compounds, insect cuticular sclerotizing precursors and some bioadhesive marine proteins may aid in the de novo design of unique biomaterials with potential antibiotic/adhesive properties.

o-Iminobenzosemiquinonate and o-imino-p-methylbenzosemiquinonate anion radicals coupled VO2+ stabilization

The diamagnetic VO(2+)-iminobenzosemiquinonate anion radical (L(R)(IS)(•-), R = H, Me) complexes, (L(-))(VO(2+))(L(R)(IS)(•-)): (L(1)(-))(VO(2+))(L(H)(IS)(•-))•3/2MeOH (1•3/2MeOH), (L(2)(-))(VO(2+))(L(H)(IS)(•-)) (2), and (L(2)(-))(VO(2+))(L(Me)(IS)(•-))•1/2 L(Me)(AP) (3•1/2 L(Me)(AP)), incorporating tridentate monoanionic NNO-donor ligands {L = L(1)(-) or L(2)(-), L(1)H = (2-[(phenylpyridin-2-yl-methylene)amino]phenol; L(2)H = 1-(2-pyridylazo)-2-naphthol; L(H)(IS)(•-) = o-iminobenzosemiquinonate anion radical; L(Me)(IS)(•-) = o-imino-p-methylbenzosemiquinonate anion radical; and L(Me)(AP) = o-amino-p-methylphenol} have been isolated and characterized by elemental analyses, IR, mass, NMR, and UV-vis spectra, including the single-crystal X-ray structure determinations of 1•3/2MeOH and 3•1/2 L(Me)(AP). Complexes 1•3/2MeOH, 2, and 3•1/2 L(Me)(AP) absorb strongly in the visible region because of intraligand (IL) and ligand-to-metal charge transfers (LMCT). 1•3/2MeOH is luminescent (λ(ext), 333 nm; λ(em), 522, 553 nm) in frozen dichloromethane-toluene glass at 77 K due to π(diimine→)π(diimine)* transition. The V-O(phenolato) (cis to the V═O) lengths, 1.940(2) and 1.984(2) Å, respectively, in 1•3/2MeOH and 3•1/2 L(Me)(AP) are consistent with the VO(2+) description. The V-O(iminosemiquinonate) (trans to the V═O) lengths, 2.1324(19) in 1•3/2MeOH and 2.083(2) Å in 3•1/2 L(Me)(AP), are expectedly ∼0.20 Å longer due to the trans influence of the V═O bond. Because of the stronger affinity of the paramagnetic VO(2+) ion to the L(H)(IS)(•-) or L(Me)(IS)(•-), the V-N(iminosemiquinonate) lengths, 1.908(2) and 1.921(2) Å, respectively, in 1•3/2MeOH and 3•1/2 L(Me)(AP), are unexpectedly shorter. Density functional theory (DFT) calculations using B3LYP, B3PW91, and PBE1PBE functionals on 1 and 2 have established that the closed shell singlet (CSS) solutions (VO(3+)-amidophenolato (L(R)(AP)(2-)) coordination) of these complexes are unstable with respect to triplet perturbations. But BS (1,1) M(s) = 0 (VO(2+)-iminobenzosemiquinonate anion radical (L(R)(IS)(•-)) coordination) solutions of these species are stable and reproduce the experimental bond parameters well. Spin density distributions of one electron oxidized cations are consistent with the [(L(-))(VO(2+))(L(R)(IQ))](+) descriptions [VO(2+)-o-iminobenzoquinone (L(R)(IQ)) coordination], and one electron reduced anions are consistent with the [(L(•2-))(VO(3+))(L(R)(AP)(2-))](-) descriptions [VO(3+)-amidophenolato (L(R)(AP)(2-)) coordination], incorporating the diimine anion radical (L(1)(•2-)) or azo anion radical (L(2)(3-)). Although, cations and anions are not isolable, but electro-and spectro-electrochemical experiments have shown that 3(+) and 3(-) ions are more stable than 1(+), 2(+) and 1(-), 2(-) ions. In all cases, the reductions occur with simultaneous two electron transfer, may be due to formation of coupled diimine/azo anion radical-VO(2+) species as in [(L(•2-))(VO(2+))(L(R)(AP)(2-))](2-).

Bioactive dehydrotyrosyl and dehydrodopyl compounds of marine origin

The amino acid, tyrosine, and its hydroxylated product, 3,4-dihydroxyphenylalanine (dopa), plays an important role in the biogenesis of a number of potentially important bioactive molecules in marine organisms. Interestingly, several of these tyrosyl and dopa-containing compounds possess dehydro groups in their side chains. Examples span the range from simple dehydrotyrosine and dehydrodopamines to complex metabolic products, including peptides and polycyclic alkaloids. Based on structural information, these compounds can be subdivided into five categories: (a) Simple dehydrotyrosine and dehydrotyramine containing molecules; (b) simple dehydrodopa derivatives; (c) peptidyl dehydrotyrosine and dehydrodopa derivatives; (d) multiple dehydrodopa containing compounds; and (e) polycyclic condensed dehydrodopa derivatives. These molecules possess a wide range of biological activities that include (but are not limited to) antitumor activity, antibiotic activity, cytotoxicity, antioxidant activity, multidrug resistance reversal, cell division inhibition, immunomodulatory activity, HIV-integrase inhibition, anti-viral, and anti-feeding (or feeding deterrent) activity. This review summarizes the structure, distribution, possible biosynthetic origin, and biological activity, of the five categories of dehydrotyrosine and dehydrodopa containing compounds.

Synthesis, spectroscopic characterization and reactivity studies of oxovanadium(IV) complexes with bulky N,N'-polymethylenebis(3,5-(t)Bu(2)salicylaldimine) ligands

A series of new sterically hindered N,N'-polymethylenebis(3,5-(t)Bu(2)salicylaldimine) ligands (H(2)L(x)) VO(IV) complexes, [VO{(2-O-3,5-(t)Bu(2)C(6)H(2))CHN-R-NCH-(3,5-(t)Bu(2)-C(6)H(2)O-2)] (X), where R=-(CH(2))(3)- (3), -(CH(2))(4)- (4), -(CH(2))(5)- (5), -(CH(2))(6)- (6) and -CH(2)C(CH(3))(2)CH(2)- (7) and early reported -(CH(2))(2)- (1) and -CH(2)CH(CH(3))- (2), has been synthesized and characterized by spectroscopic (IR, UV/vis, (1)H NMR, EPR), electrochemical and magnetic susceptibility measurements. Complexes 1-7 are described a trigonal distorted pyramids. All seven compounds give nearly the same parallel hyperfine coupling constant (A(z)) regardless that the geometry of VO(IV) changes from square pyramidal to trigonal distorted pyramids. Chemical oxidation of 1-7 by one equiv Ce(IV) leads to the formation of stable [VO(V)L(x)](+) complexes. Cyclic voltammograms of 2-6 in DMSO along with a quasi-reversible VO(IV)/VO(V) redox couple also showed irreversible phenolate/phenoxyl responses. Each 1 and 7 shows only one reversible VO(IV) centered oxidation waves. Chemical oxidation of H(2)L(x) forms the stable [H(2)L(x)](+) radical species.