Stereochemical determination and bioactivity assessment of (S)-(+)-curcuphenol dimers isolated from the marine sponge Didiscus aceratus and synthesized through laccase biocatalysis

Therapeutic Potential of Polyphenols and Other Micronutrients of Marine Origin

Polyphenols are compounds found in various plants and foods, known for their antioxidant and anti-inflammatory properties. Recently, researchers have been exploring the therapeutic potential of marine polyphenols and other minor nutrients that are found in algae, fish and crustaceans. These compounds have unique chemical structures and exhibit diverse biological properties, including anti-inflammatory, antioxidant, antimicrobial and antitumor action. Due to these properties, marine polyphenols are being investigated as possible therapeutic agents for the treatment of a wide variety of conditions, such as cardiovascular disease, diabetes, neurodegenerative diseases and cancer. This review focuses on the therapeutic potential of marine polyphenols and their applications in human health, and also, in marine phenolic classes, the extraction methods, purification techniques and future applications of marine phenolic compounds.

Anticancer Activities of Marine-Derived Phenolic Compounds and Their Derivatives

Since the middle of the last century, marine organisms have been identified as producers of chemically and biologically diverse secondary metabolites which have exerted various biological activities including anticancer, anti-inflammatory, antioxidant, antimicrobial, antifouling and others. This review primarily focuses on the marine phenolic compounds and their derivatives with potent anticancer activity, isolated and/or modified in the last decade. Reports on the elucidation of their structures as well as biosynthetic studies and total synthesis are also covered. Presented phenolic compounds inhibited cancer cells proliferation or migration, at sub-micromolar or nanomolar concentrations (lamellarins D (37), M (38), K (39), aspergiolide B (41), fradimycin B (62), makulavamine J (66), mayamycin (69), N-acetyl-N-demethylmayamycin (70) or norhierridin B (75)). In addition, they exhibited anticancer properties by a diverse biological mechanism including induction of apoptosis or inhibition of cell migration and invasive potential. Finally, phlorotannins 1–7 and bromophenols 12–29 represent the most researched phenolic compounds, of which the former are recognized as protective agents against UVB or gamma radiation-induced skin damages. Finally, phenolic metabolites were assorted into six main classes: phlorotannins, bromophenols, flavonoids, coumarins, terpenophenolics, quinones and hydroquinones. The derivatives that could not be attributed to any of the above-mentioned classes were grouped in a separate class named miscellaneous compounds.

Bioactive Properties of Marine Phenolics

Phenolic compounds from marine organisms are far less studied than those from terrestrial sources since their structural diversity and variability require powerful analytical tools. However, both their biological relevance and potential properties make them an attractive group deserving increasing scientific interest. The use of efficient extraction and, in some cases, purification techniques can provide novel bioactives useful for food, nutraceutical, cosmeceutical and pharmaceutical applications. The bioactivity of marine phenolics is the consequence of their enzyme inhibitory effect and antimicrobial, antiviral, anticancer, antidiabetic, antioxidant, or anti-inflammatory activities. This review presents a survey of the major types of phenolic compounds found in marine sources, as well as their reputed effect in relation to the occurrence of dietary and lifestyle-related diseases, notably type 2 diabetes mellitus, obesity, metabolic syndrome, cancer and Alzheimer's disease. In addition, the influence of marine phenolics on gut microbiota and other pathologies is also addressed.

Optimization of the Number of Considered Conformers for the Absolute Configuration Determination of Catechin and Epicatechin Peracetates by VCD

Epimeric catechin and epicatechin peracetates offer the possibility to explore the number of conformers needed for the absolute configuration determination without affecting the reliability of the assignment. Comparisons between experimental and DFT B3PW91/DGDZVP calculated curves showed that, in molecules where the conformational flexibility generates a large number of conformers, it is possible to significantly reduce the number of conformers needed to generate VCD results that provide secure absolute configuration assignment.

Recent advances in the use of vibrational chiroptical spectroscopic methods for stereochemical characterization of natural products

A comprehensive look into application of vibrational optical activity methods for conformational and configurational assignments in natural product molecules over the last 15 years is provided.

Crowdsourcing natural products discovery to access uncharted dimensions of fungal metabolite diversity

A fundamental component for success in drug discovery is the ability to assemble and screen compounds that encompass a broad swath of biologically relevant chemical-diversity space. Achieving this goal in a natural-products-based setting requires access to a wide range of biologically diverse specimens. For this reason, we introduced a crowdsourcing program in which citizen scientists furnish soil samples from which new microbial isolates are procured. Illustrating the strength of this approach, we obtained a unique fungal metabolite, maximiscin, from a crowdsourced Alaskan soil sample. Maximiscin, which exhibits a putative combination of polyketide synthase (PKS), non-ribosomal peptide synthetase (NRPS), and shikimate pathway components, was identified as an inhibitor of UACC-62 melanoma cells (LC50=0.93 μM). The metabolite also exhibited efficacy in a xenograft mouse model. These results underscore the value of building cooperative relationships between research teams and citizen scientists to enrich drug discovery efforts.

Conformational analysis of perezone and dihydroperezone using vibrational circular dichroism

The vibrational circular dichroism (VCD) spectra of perezone and dihydroperezone measured from CDCl(3) solutions were quite similar, suggesting analogous conformations for both molecules. Their absolute configurations were confirmed by comparison of the experimental VCD spectrum of each compound with curves generated from theoretical calculations using density functional theory (DFT) at the B3LYP/DGDZVP level of theory taking into account their conformational mobility. Conformational analysis of the 8-(R) enantiomer showed 19 low energy conformers in a 2.4 kcal/mol energy range, while for 8-(R), with the saturated side alkyl chain, 34 conformers were considered in the first 2 kcal/mol. Initial analyses were carried out using a Monte Carlo searching with the MMFF94 molecular mechanics force field, all MMFF94 conformers were geometrically optimized using DFT at the B3LYP/6-31G(d) level of theory, followed by reoptimization and calculations of their vibrational frequencies at the B3LYP/DGDZVP level. Good agreement between the theoretical 8-(R) enantiomers and experimental VCD curves were observed for both.

New bisabolane sesquiterpenoids from a marine-derived fungus Aspergillus sp. isolated from the sponge Xestospongia testudinaria

Three new phenolic bisabolane sesquiterpenoid dimers, disydonols A-C (1-3), and one known compound (S)-(+)-sydonol (4) were isolated from the fermentation broth of a marine-derived fungus Aspergillus sp., which was isolated from the sponge Xestospongia testudinaria collected from the South China Sea. Their structures were elucidated on the basis of comprehensive spectral analysis including 1D and 2D NMR spectra and HR-ESI-MS. These compounds were evaluated for cytotoxic activity against HepG-2 and Caski human tumour cell lines. Among them, compounds 1 and 3 exhibited cytotoxicity against the two cell lines.

Functional metagenomic strategies for the discovery of novel enzymes and biosurfactants with biotechnological applications from marine ecosystems

Marine ecosystems are home to bacteria which are exposed to a wide variety of environmental conditions, such as extremes in temperature, salinity, nutrient availability and pressure. Survival under these conditions must have necessitated the adaptation and the development of unique cellular biochemistry and metabolism by these microbes. Thus, enzymes isolated from these microbes have the potential to possess quite unique physiological and biochemical properties. This review outlines a number of function-based metagenomic approaches which are available to screen metagenomic libraries constructed from marine ecosystems to facilitate the exploitation of some of these potentially novel biocatalysts. Functional screens to isolate novel cellulases, lipases and esterases, proteases, laccases, oxidoreductases and biosurfactants are described, together with approaches which can be employed to help overcome some of the typical problems encountered with functional metagenomic-based screens.

Stereochemical study of a novel tautomeric furanone, homofuraneol

A mixture of tautomers with unique keto-enol structures, 5-ethyl-4-hydroxy-2-methylfuran-3(2H)-one and 2-ethyl-4-hydroxy-5-methylfuran-3(2H)-one (EHMF, homofuraneol, 1a and 1b), comprises four structural isomers including their enantiomers. The four isomers were successfully separated by chromatographic optical resolution, and their odor evaluation was performed. Determination of the absolute chemistry of 1a and 1b were accomplished for the first time by direct measurement of the VCD spectra of their methyl ether derivatives 4a and 4b compared with the calculated ones as well as chemical relay reaction. The relationship between odor characteristics and stereochemistry was also examined.

Finding new enzymes from bacterial physiology: a successful approach illustrated by the detection of novel oxidases in Marinomonas mediterranea

The identification and study of marine microorganisms with unique physiological traits can be a very powerful tool discovering novel enzymes of possible biotechnological interest. This approach can complement the enormous amount of data concerning gene diversity in marine environments offered by metagenomic analysis, and can help to place the activities associated with those sequences in the context of microbial cellular metabolism and physiology. Accordingly, the detection and isolation of microorganisms that may be a good source of enzymes is of great importance. Marinomonas mediterranea, for example, has proven to be one such useful microorganism. This Gram-negative marine bacterium was first selected because of the unusually high amounts of melanins synthesized in media containing the amino acid L-tyrosine. The study of its molecular biology has allowed the cloning of several genes encoding oxidases of biotechnological interest, particularly in white and red biotechnology. Characterization of the operon encoding the tyrosinase responsible for melanin synthesis revealed that a second gene in that operon encodes a protein, PpoB2, which is involved in copper transfer to tyrosinase. This finding made PpoB2 the first protein in the COG5486 group to which a physiological role has been assigned. Another enzyme of interest described in M. mediterranea is a multicopper oxidase encoding a membrane-associated enzyme that shows oxidative activity on a wide range of substrates typical of both laccases and tyrosinases. Finally, an enzyme very specific for L-lysine, which oxidises this amino acid in epsilon position and that has received a new EC number (1.4.3.20), has also been described for M. mediterranea. Overall, the studies carried out on this bacterium illustrate the power of exploring the physiology of selected microorganisms to discover novel enzymes of biotechnological relevance.

Determination of the absolute configurations of synthetic daunorubicin analogues using vibrational circular dichroism spectroscopy and density functional theory

The absolute configurations of three synthesized anthracycline analogues have been determined using vibrational circular dichroism (VCD) spectroscopy and the density functional theory (DFT) calculations. The experimental VCD spectra of the three compounds have been measured for the first time in the film state, prepared from their CDCl(3) solutions. Conformational searches for the monomers and some dimers of the three compounds have been performed at the DFT level using the B3LYP functional and the 6-311G** and 6-311++G** basis sets. The corresponding vibrational absorption and VCD spectra have been calculated. The good agreement between the experimental and the calculated spectra allows one to assign the absolute configurations of the three compounds with high confidence. In addition, the dominant conformers of the three compounds have also been identified.

Vibrational circular dichroism spectroscopy of chiral molecules

In this chapter, new developments and main applications of vibrational circular dichroism (VCD) spectroscopy reported in the last 5 years are described. This includes the determinations of absolute configurations of chiral molecules, understanding solvent effects and modeling solvent-solute explicit hydrogen bonding networks using induced solvent chirality, studies of transition metal complexes and their peculiar and enormous intensity enhancements in VCD spectra, investigations of conformational preference of chiral ligands bound to gold nano particles, and two new advances in applying matrix isolation VCD spectroscopy to flexible, multi-conformational chiral molecules and complexes, and in development of femtosecond laser based VCD instruments for transient VCD monitoring. A brief review of the experimental techniques and theoretical methods is also given. The purpose of this chapter is to provide an up-to-date perspective on the capability of VCD to solve significant problems about chiral molecules in solution, in thin film states, or on surfaces.

A selective account of effective paradigms and significant outcomes in the discovery of inspirational marine natural products

Marine natural products continue to be a source of significant molecular structures that serve as a stimulus to seed further significant research. This account reviews some of the major advances in the study of marine biomolecules made at UC Santa Cruz over more than three decades. The continuing challenge of discovery and characterization of what we term "inspirational molecular structures" will be presented in a comprehensive fashion. Examples of privileged molecular structures and their impact on biomedicinal research will be an important theme. The three major groups of organisms explored include seaweeds, sponges, and marine-derived fungi, and the study of their active principles has greatly benefited from synergistic collaborations with both academic and biopharmaceutical groups. The concluding sections of this chronicle will touch on prospects for future outcomes involving new sources and strategies.

Bioactive Semisynthetic Derivatives of (S)-(+)-Curcuphenol

New semisynthetic sesquiterpene derivatives (3–7, 10), together with two known ones (8–9) were prepared from ( S)-(+)-curcuphenol (1), isolated from the Caribbean sponge Didiscus oxeata. The natural sesquiterpenes 1 and ( S)-(+)-curcudiol (2), as well as semisynthetic derivatives 3–10, were evaluated against the herbivorous insect species Spodoptera littoralis, Leptinotarsa decemlineata and Rhopalosiphum padi. All compounds had an antifeedant effect on L. decemlineata. With R. padi, a structure-dependent antifeedant response was observed, with 1 being the most active, followed by 4 and 5. Furthermore, compounds 1, 4, 5, 7, 8 and 10 reduced S. littoralis larval weight, with 10 also reducing ingestion by S. littoralis larvae. Of the derivatives studied here, none showed stronger effects (either antifeedant or postingestive) than the parent compound 1. It can, therefore, be concluded that the structure of the side chain and the presence of the phenol ring of curcuphenol are important structural requirements for its insect antifeedant and growth regulation effects.

Absolute configuration of eremophilanoids by vibrational circular dichroism

The absolute configurations (AC) of natural occurring 6-hydroxyeuryopsin (1), of its acetyl derivative 2, and of eremophilanolide 8 were confirmed by comparison of the experimental vibrational circular dichroism (VCD) spectra with theoretical curves generated from density functional theory (DFT) calculations. Initial analyses were carried out using a Monte Carlo searching with the MMFF94 molecular mechanics force field. All MMFF94 conformers were further optimized using DFT at the B3LYP/6-31G(d) level of theory, followed by calculations of their vibrational frequencies at the B3LYP/6-31G(d,p); the VCD spectra of 2 and 8 were also calculated at the B3PW91/DGDZVP level of theory. Good agreement between theoretical and experimental VCD curves unambiguously verified the 4S,5R,6S absolute configuration for 1 and 2, and the 1S,4S,5R,6S,8S,10S configuration for 8.

Vibrational circular dichroism of Africanane and lippifoliane sesquiterpenes from Lippia integrifolia

The agreement between theoretical and experimental vibrational circular dichroism curves of (4R,9R,10R)-(+)-african-1(5)-ene-2,6-dione (1) and (4R,9S,10R)-lippifoli-1(6)-en-5-one (2), isolated from the widely used plant Lippia integrifolia, allowed the determination of the conformation and absolute configuration of 1 and confirmed both structural features for 2. Molecular modeling of 1 and 2 was carried out by means of a systematic and a Monte Carlo search protocol followed by geometry optimization employing density functional theory calculations with the B3LYP/6-31G(d), B3LYP/DGDZVP, and/or B3PW91/DGDZVP2 functionals/basis sets. Validation of the minimum energy conformations for both tricyclic substances was achieved by comparison of the experimental and theoretical vicinal (1)H-(1)H NMR coupling constants obtained by DFT-GIAO calculations.

Vibrational Circular Dichroism: A New Tool for the Solution-State Determination of the Structure and Absolute Configuration of Chiral Natural Product Molecules

Vibrational circular dichroism (VCD) has been used in recent years to determine the absolute configuration of a number of natural product chiral molecules. In this brief review, these applications will be described and the methodology of VCD determination of absolute configuration (AC) will be explained. The principal advantages of VCD versus X-ray crystallography for absolute configuration determination are: 1) only solution-state samples are needed and therefore single crystals are not required, 2) high enantiomeric sample purity is not required, 3) high chemical purity is not required as long as impurities are not chiral and 4) solution-state conformations are obtained as an extra feature of the AC determination.

Discovery of platelet-type 12-human lipoxygenase selective inhibitors by high-throughput screening of structurally diverse libraries

Human lipoxygenases (hLO) have been implicated in a variety of diseases and cancers and each hLO isozyme appears to have distinct roles in cellular biology. This fact emphasizes the need for discovering selective hLO inhibitors for both understanding the role of specific lipoxygenases in the cell and developing pharmaceutical therapeutics. To this end, we have modified a known lipoxygenase assay for high-throughput (HTP) screening of both the National Cancer Institute (NCI) and the UC Santa Cruz marine extract library (UCSC-MEL) in search of platelet-type 12-hLO (12-hLO) selective inhibitors. The HTP screen led to the characterization of five novel 12-hLO inhibitors from the NCI repository. One is the potent but non-selective michellamine B, a natural product, anti-viral agent. The other four compounds were selective inhibitors against 12-hLO, with three being synthetic compounds and one being alpha-mangostin, a natural product, caspase-3 pathway inhibitor. In addition, a selective inhibitor was isolated from the UCSC-MEL (neodysidenin), which has a unique chemical scaffold for a hLO inhibitor. Due to the unique structure of neodysidenin, steady-state inhibition kinetics were performed and its mode of inhibition against 12-hLO was determined to be competitive (K(i)=17microM) and selective over reticulocyte 15-hLO-1 (K(i) 15-hLO-1/12-hLO>30).

Absolute configuration of verticillane diterpenoids by vibrational circular dichroism

Good agreement between theoretical and experimental vibrational circular dichroism curves of (1S,11S,12S)-(+)-verticilla-3E,7E-dien-12-ol (1) established the absolute configuration of this natural diterpene isolated from Bursera suntui. Molecular modeling of 1 was carried out using the Monte Carlo protocol followed by geometry optimization at the B3LYP 6-31G(d,p) level of theory. The 12-membered ring of 1 was found in a single preferred chair-chair-chair-chair conformation. In the six-membered ring a chair prevails over a distorted boat, and the C-OH bond rotation generates three predominant rotamers. Validation of the minimum energy conformation for 1 was achieved by comparison of theoretical and experimental infrared frequencies, vicinal 1H NMR coupling constants, and X-ray diffraction data. This study confirms that (+)-verticillol 1 isolated from Bursera species has the 1S,11S,12S absolute configuration that corresponds to the same enantiomeric series as verticillanes from Sciadopitys and Taxus, while verticillanes from Jackiella and Jungermannia have antipodal structures.

Marine natural products

This review covers the literature published in 2005 for marine natural products, with 704 citations (493 for the period January to December 2005) referring to compounds isolated from marine microorganisms and phytoplankton, green algae, brown algae, red algae, sponges, coelenterates, bryozoans, molluscs, tunicates and echinoderms. The emphasis is on new compounds (812 for 2005), together with their relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Application of chiral technology in a pharmaceutical company. Enantiomeric separation and spectroscopic studies of key asymmetric intermediates using a combination of techniques. Phenylglycidols

Phenylglycidols substituted in the 2-, 3-, and 4- positions with fluorine, chlorine, and trifluoromethyl, and with methoxy in the 3- position, were synthesized from the corresponding E-cinnamic acids and separated into their (R,R)- and (S,S)- enantiomers using subcritical fluid chromatography with mixtures of MeOH in CO(2), on either a Chiralpak AD or AS chiral stationary phase. These compounds and commercially-available (R,R)- and (S,S)-phenylglycidol were analyzed for their vibrational circular dichroism (VCD), electronic circular dichroism (ECD), and optical rotation (OR) properties to exemplify a strategy whereby the absolute stereochemistry of common and key chiral intermediates is established early in the structure-activity and structure-property relationship phase of a drug discovery program in a pharmaceutical company. From this study, substituents in the phenyl group of the synthesized molecules were found not to grossly alter spectroscopic features, and therefore, diagnostic absorption bands in the respective VCD spectra, and the sign and shape of the measured ECD curves could be used to determine and track the absolute stereochemistry of analogs without necessarily requiring time-consuming ab initio calculations of all low energy conformers for all compounds. VCD, OR, and ECD calculations for the determination of absolute configuration carried out at the DFT level with the hybrid B3PW91 functional and the TZVP basis set were found to be especially useful in this study.

An investigation of the absolute configuration of the potent histamine H3 receptor antagonist GT-2331 using vibrational circular dichroism

GT-2331 [(+)-1] is one of the most potent members of a class of chiral drug substances used to regulate the synthesis and release of histamine by the histamine H3 receptor, and as such, is an important biomarker for pharmaceutical companies conducting research in this field. In addition to overall structural features, the bioactivity of this molecule has also been found to be highly dependent on absolute stereochemistry, making the reliable assignment of this property a necessity. X-ray diffraction studies have provided conflicting data, leaving its three-dimensional structure uncertain. In view of this, its absolute configuration was investigated by vibrational circular dichroism. Results from this study provided independent assignment of this important molecule as the (1S,2S)-enantiomer.

Enantiomeric separation and determination of absolute stereochemistry of asymmetric molecules in drug discovery—Building chiral technology toolboxes

The application of Chiral Technology, or the (extensive) use of techniques or tools for the determination of absolute stereochemistry and the enantiomeric or chiral separation of racemic small molecule potential lead compounds, has been critical to successfully discovering and developing chiral drugs in the pharmaceutical industry. This has been due to the rapid increase over the past 10-15 years in potential drug candidates containing one or more asymmetric centers. Based on the experiences of one pharmaceutical company, a summary of the establishment of a Chiral Technology toolbox, including the implementation of known tools as well as the design, development, and implementation of new Chiral Technology tools, is provided.

Absolute configuration of natural diastereoisomers of 6beta-hydroxyhyoscyamine by vibrational circular dichroism

The absolute configuration of the two natural diastereoisomers of 6beta-hydroxyhyoscyamine has been determined using vibrational circular dichroism (VCD) spectroscopy. The predicted VCD and IR spectra of (3R,6R,2'S)-6beta-hydroxyhyoscyamine (1) and (3S,6S,2'S)-6beta-hydroxyhyoscyamine (2) were calculated using density functional theory (DFT) with the B3LYP functional and 6-31G(d) basis set and considering the eight lower energy conformations of each diastereoisomer. In both cases, the first four conformers showed the N-Me group in the syn orientation, permitting the formation of a hydrogen bond between the hydroxy group at the tropane ring and the tertiary nitrogen atom. In addition the eight conformers showed an intramolecular hydrogen bond between the hydroxy and carbonyl groups of the tropic ester moiety. The calculated IR spectra of both molecules showed good agreement with the experimental spectra, while comparison of the experimental and calculated VCD spectra showed that the absolute configuration of dextrorotatory 6beta-hydroxyhyoscyamine is (3R,6R,2'S), while the levorotatory isomer is (3S,6S,2'S).

Absolute configurations of brominated sesquiterpenes determined by vibrational circular dichroism

Two brominated sesquiterpenes, majapolene B (1) and acetylmajapolene B (2), isolated from the red algal genus Laurencia were investigated using vibrational circular dichroism (VCD). The ab initio theoretical VCD and IR calculations of 1 and 2 were performed by density functional theory (DFT) using the B3PW91/6-31G(d,p) basis set. The experimental VCD spectra and corresponding population-weighted theoretical VCD spectra were found to be in excellent agreement in CCl(4) solution in the 1800-850 cm(-1) region, which allowed unambiguous determination of the absolute configurations of (-)-1 and (-)-2 as 7S,10S and 7S,10S, respectively.