Biological Activity of Recently Discovered Halogenated Marine Natural Products

Macroalgae Extracts From Antarctica Have Antimicrobial and Anticancer Potential

Background: Macroalgae are sources of bioactive compounds due to the large number of secondary metabolites they synthesize. The Antarctica region is characterized by extreme weather conditions and abundant aggregations of macroalgae. However, current knowledge on their biodiversity and their potential for bio-prospecting is still fledging. This study evaluates the antimicrobial and cytotoxic activity of different extracts of four macroalgae (Cystosphaera jacquinotii, Iridaea cordata, Himantothallus grandifolius, and Pyropia endiviifolia) from the Antarctic region against cancer and non-cancer cell lines.

Methods: The antimicrobial activity of macroalgae was evaluated by the broth microdilution method. Extracts were assessed against Staphylococcus aureus ATCC 19095, Enterococcus faecalis ATCC 4083, Escherichia coli ATCC29214, Pseudomonas aeruginosa ATCC 9027, Candida albicans ATCC 62342, and the clinical isolates from the human oral cavity, namely, C. albicans (3), C. parapsilosis, C. glabrata, C. lipolytica, and C. famata. Cytotoxicity against human epidermoid carcinoma (A-431) and mouse embryonic fibroblast (NIH/3T3) cell lines was evaluated with MTT colorimetric assay.

Results: An ethyl acetate extract of H. grandifolius showed noticeable antifungal activity against all fungal strains tested, including fluconazole-resistant samples. Cytotoxicity investigation with a cancer cell line revealed that the ethyl acetate extract of I. cordata was highly cytotoxic against A-431 cancer cell line, increasing the inhibitory ratio to 91.1 and 95.6% after 24 and 48 h exposure, respectively, for a concentration of 500 μg mL⁻¹. Most of the algal extracts tested showed little or no cytotoxicity against fibroblasts.

Conclusion: Data suggest that macroalgae extracts from Antarctica may represent a source of therapeutic agents.

HIGHLIGHTS

Different macroalgae samples from Antarctica were collected and the lyophilized biomass of each macroalgae was extracted sequentially with different solvents

The antimicrobial and anticancer potential of macroalgae extracts were evaluated

Ethyl acetate extract of H. grandifolius showed noticeable antifungal activity against all the fungal strains tested, including fluconazole-resistant samples

Ethyl acetate extract of I. cordata was highly cytotoxic against the A-431 cancer cell line

Most of the algal extracts tested showed little or no cytotoxicity against normal cell lines

Comprehensive Analysis of the Great Lakes Top Predator Fish for Novel Halogenated Organic Contaminants by GC×GC-HR-ToF Mass Spectrometry

The U.S. Environmental Protection Agency's Great Lakes Fish Monitoring and Surveillance Program (GLFMSP) has traced the fate and transport of anthropogenic chemicals in the Great Lakes region for decades. Isolating and identifying halogenated species in fish is a major challenge due to the complexity of the biological matrix. A nontargeted screening methodology was developed and applied to lake trout using a 2-dimensional gas chromatograph coupled to a high resolution time-of-flight mass spectrometer (GC×GC-HR-ToF MS). Halogenated chemicals were identified using a combination of authentic standards and library spectral matching, with molecular formula estimations provided by exact mass spectral interpretation. In addition to the halogenated chemicals currently being targeted by the GLFMSP, more than 60 nontargeted halogenated species were identified. Most appear to be metabolites or breakdown products of larger halogenated organics. The most abundant compound class was halomethoxyphenols accounting for more than 60% of the total concentration of halogenated compounds in top predator fish from all five Great Lakes illustrating the need and utility of nontargeted halogenated screening of aquatic systems using this platform.

Expanding beyond canonical metabolism: Interfacing alternative elements, synthetic biology, and metabolic engineering

Metabolic engineering offers an exquisite capacity to produce new molecules in a renewable manner. However, most industrial applications have focused on only a small subset of elements from the periodic table, centered around carbon biochemistry. This review aims to illustrate the expanse of chemical elements that can currently (and potentially) be integrated into useful products using cellular systems. Specifically, we describe recent advances in expanding the cellular scope to include the halogens, selenium and the metalloids, and a variety of metal incorporations. These examples range from small molecules, heteroatom-linked uncommon elements, and natural products to biomining and nanotechnology applications. Collectively, this review covers the promise of an expanded range of elemental incorporations and the future impacts it may have on biotechnology.

DeltaMS: a tool to track isotopologues in GC- and LC-MS data

INTRODUCTION

Stable isotopic labeling experiments are powerful tools to study metabolic pathways, to follow tracers and fluxes in biotic and abiotic transformations and to elucidate molecules involved in metal complexing.

OBJECTIVE

To introduce a software tool for the identification of isotopologues from mass spectrometry data.

METHODS

DeltaMS relies on XCMS peak detection and X¹³CMS isotopologue grouping and then analyses data for specific isotope ratios and the relative error of these ratios. It provides pipelines for recognition of isotope patterns in three experiment types commonly used in isotopic labeling studies: (1) search for isotope signatures with a specific mass shift and intensity ratio in one sample set, (2) analyze two sample sets for a specific mass shift and, optionally, the isotope ratio, whereby one sample set is isotope-labeled, and one is not, (3) analyze isotope-guided perturbation experiments with a setup described in X¹³CMS.

RESULTS

To illustrate the versatility of DeltaMS, we analyze data sets from case-studies that commonly pose challenges in evaluation of natural isotopes or isotopic signatures in labeling experiment. In these examples, the untargeted detection of sulfur, bromine and artificial metal isotopic patterns is enabled by the automated search for specific isotopes or isotope signatures.

CONCLUSION

DeltaMS provides a platform for the identification of (pre-defined) isotopologues in MS data from single samples or comparative metabolomics data sets.

NXS, Morpholine, and HFIP: The Ideal Combination for Biomimetic Haliranium-Induced Polyene Cyclizations

In contrast to Nature that accomplishes polyene cyclizations seemingly with ease, such transformations are difficult to conduct in the lab. In our program dealing with the development of selective halogenations of alkenes, we now asserted that standard X⁺ reagents are perfectly suited for the biomimetic cation-π cyclization of both electron rich and poor linear polyenes in the presence of the Lewis base morpholine and the Lewis acid HFIP. The method stands out due to its broad substrate scope and practicability together with high chemical yields and excellent selectivities, even for highly challenging chloriranium-induced polyene cyclizations.

(Some) current concepts in antibacterial drug discovery

The rise of multidrug resistance in bacteria rendering pathogens unresponsive to many clinical drugs is widely acknowledged and considered a critical global healthcare issue. There is broad consensus that novel antibacterial chemotherapeutic options are extremely urgently needed. However, the development pipeline of new antibacterial drug lead structures is poorly filled and not commensurate with the scale of the problem since the pharmaceutical industry has shown reduced interest in antibiotic development in the past decades due to high economic risks and low profit expectations. Therefore, academic research institutions have a special responsibility in finding novel treatment options for the future. In this mini review, we want to provide a broad overview of the different approaches and concepts that are currently pursued in this research field.

Enantioselective total synthesis and structural assignment of callyspongiolide

We have elucidated the complete absolute configuration of callyspongiolide and unambiguously assigned its stereochemistry at the C-21 center through synthesis. Four stereoisomers of callyspongiolide were synthesized in a convergent and enantioselective manner. A late-stage Sonogashira coupling forges the diene-ynic side chain. Other notable reactions are Yonemitsu's variation of Yamaguchi macrolactonization to cyclize an alkynic seco acid, highly trans-selective Julia-Kocienski olefination, CBS reduction to set the C-21 stereocenter, and methyl cuprate addition to an unsaturated pyranone to install the C-5 methyl center.

Organocatalyzed Decarboxylative Trichloromethylation of Morita-Baylis-Hillman Adducts in Batch and Continuous Flow

Two protocols for the organocatalyzed decarboxylative trichloromethylation of Morita-Baylis-Hillman (MBH) substrates have been developed. Applying sodium trichloroacetate, as the trichloromethyl anion precursor, in combination with an organocatalyst and acetylated MBH-alcohols, the desired trichloromethylated products were obtained in good yields at room temperature in batch. The method was next extrapolated into a two-step continuous flow protocol, starting directly from the MBH alcohols, in combination with tributylamine acting both as base and catalyst. The flow process proved superior to the batch approach, reducing the reaction time from 16 hours to only 20 minutes, with increased yields for all investigated entries. Two examples were also taken to scale-up in flow producing more than 10 grams of both trichloromethylated targets. Finally, substitution of the organocatalyst to (DHQ)₂ PHAL or (DHQD)₂ PHAL induced chiral transfer to the generated stereocenter in the reaction attaining selectivities with nearly 90 % ee.

Natural products as inspiration for the development of new synthetic methods

Natural products have played an important role in shaping modern synthetic organic chemistry. In particular, their complex molecular skeletons have stimulated the development of many new synthetic methods. We highlight in this article some recent examples of synthetic design inspired by the biosynthesis of natural products.

Gold(I)-Catalyzed Synthesis of Indenes and Cyclopentadienes: Access to (±)-Laurokamurene B and the Skeletons of the Cycloaurenones and Dysiherbols

The formal (3+2) cycloaddition between terminal allenes and aryl or styryl gold(I) carbenes generated by a retro-Buchner reaction of 7-substituted 1,3,5-cycloheptatrienes led to indenes and cyclopentadienes, respectively. These cycloaddition processes have been applied to the construction of the carbon skeleton of the cycloaurenones and the dysiherbols as well as to the total synthesis of (±)-laurokamurene B.

Strategies to diversify natural products for drug discovery

Natural product libraries contain specialized metabolites derived from plants, animals, and microorganisms that play a pivotal role in drug discovery due to their immense structural diversity and wide variety of biological activities. The strategies to greatly extend natural product scaffolds through available biological and chemical approaches offer unique opportunities to access a new series of natural product analogues, enabling the construction of diverse natural product-like libraries. The affordability of these structurally diverse molecules has been a crucial step in accelerating drug discovery. This review provides an overview of various approaches to exploit the diversity of compounds for natural product-based drug development, drawing upon a series of examples to illustrate each strategy.

Ketone-catalyzed photochemical C(sp3)-H chlorination

Photoexcited arylketones catalyze the direct chlorination of C(sp3)-H groups by N-chlorosuccinimide. Acetophenone is the most effective catalyst for functionalization of unactivated C-H groups while benzophenone provides better yields for benzylic C-H functionalization. Activation of both acetophenone and benzophenone can be achieved by irradiation with a household compact fluorescent lamp. This light-dependent reaction provides a better control of the reaction as compared to the traditional chlorination methods that proceed through a free radical chain propagation mechanism.

Marine natural products

Covering: 2015. Previous review: Nat. Prod. Rep., 2016, 33, 382-431This review covers the literature published in 2015 for marine natural products (MNPs), with 1220 citations (792 for the period January to December 2015) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1340 in 429 papers for 2015), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Ultra-high resolution band-selective HSQC for nanomole-scale identification of chlorine-substituted 13 C in natural products drug discovery

Ultra-high resolution band-selective HSQC (bsHSQC) has been employed for detection of 35 Cl-37 Cl isotope shifted 13 C NMR signals for assignment of regioisomerism in bromo-chloro natural products. Optimum pulse sequence and instrumental parameters for maximization of detection of the isotope shifts were explored. The chlorine isotope shifts (Δδ) were detected within crosspeaks and were shown to vary with hybridization of 13 C, substitution of 13 C, presence of β-chloro substituents, and their relative configuration. Deconvolution of Cl-substituted CH bsHSQC crosspeaks may provide other useful information, including a potentially MS-independent method for quantitating 37 Cl/35 C isotopic fractionation during the biosynthesis of halogenated natural products. Copyright © 2016 John Wiley & Sons, Ltd.

Enantioselective Addition of Alkynes to α,α-Dichlorinated Aldehydes

Enantioselective addition of terminal alkynes to α,α-dichlorinated aldehydes employing Zn(OTf)₂/NME is disclosed. The propargylic alcohols obtained are accessed in good yields and high enantioselectivity from easily accessible α,α-dichloroaldehydes. The method opens new strategic opportunities for the synthesis of chlorinated natural products, such as the chlorosulfolipids.

Zunyimycins B and C, New Chloroanthrabenzoxocinones Antibiotics against Methicillin-Resistant Staphylococcus aureus and Enterococci from Streptomyces sp. FJS31-2

This study performed an optimization of the fermentation conditions to activate the expression of the zunyimycin family biosynthesis genes of the zunyimycin-producing streptomycetes strain Streptomyces sp. FJS31-2. Bioassay-guided isolation and purification by varied chromatographic methods yielded two new compounds of the zunyimycin derivatives, namely, 31-2-7 and 31-2-8, accompanied with three known anthrabenzoxocinones family members of zunyimycin A, BE24566B, and chloroanthrabenzoxocinone. Their structures were elucidated by NMR, HRESIMS, IR, UV, and CD. Results showed that these two compounds were structurally similar to the previously reported compound zunyimycin A but differed in positions and number of chlorine atom substitution. The two novel compounds were called zunyimycins B and C. Antibacterial activity assay indicated that zunyimycin C showed a good inhibitory effect on the methicillin-resistant Staphylococcus aureus and Enterococci.

Total syntheses of smenothiazoles A and B

Stereocontrolled installation of vinyl chloride and the 2,5-diene system via silastannation, Stille reaction and desilylchlorination, and the final peptide coupling reactions led to the concise total synthesis of smenothiazoles A (1) and B (2).

Chiral Alkyl Halides: Underexplored Motifs in Medicine

While alkyl halides are valuable intermediates in synthetic organic chemistry, their use as bioactive motifs in drug discovery and medicinal chemistry is rare in comparison. This is likely attributable to the common misconception that these compounds are merely non-specific alkylators in biological systems. A number of chlorinated compounds in the pharmaceutical and food industries, as well as a growing number of halogenated marine natural products showing unique bioactivity, illustrate the role that chiral alkyl halides can play in drug discovery. Through a series of case studies, we demonstrate in this review that these motifs can indeed be stable under physiological conditions, and that halogenation can enhance bioactivity through both steric and electronic effects. Our hope is that, by placing such compounds in the minds of the chemical community, they may gain more traction in drug discovery and inspire more synthetic chemists to develop methods for selective halogenation.

Stereoselective Halogenation in Natural Product Synthesis

At last count, nearly 5000 halogenated natural products have been discovered. In approximately half of these compounds, the carbon atom to which the halogen is bound is sp(3) -hybridized; therefore, there are an enormous number of natural products for which stereocontrolled halogenation must be a critical component of any synthesis strategy. In this Review, we critically discuss the methods and strategies used for stereoselective introduction of halogen atoms in the context of natural product synthesis. Using the successes of the past, we also attempt to identify gaps in our synthesis technology that would aid the synthesis of halogenated natural products, as well as existing methods that have not yet seen application in complex molecule synthesis. The chemistry described herein demonstrates yet again how natural products continue to provide the inspiration for critical advances in chemical synthesis.

Biological fluorination from the sea: discovery of a SAM-dependent nucleophilic fluorinating enzyme from the marine-derived bacterium Streptomyces xinghaiensis NRRL B24674

The first ever marine originated fluorinating enzyme, which promises to be useful in biotransformation and synthetic biology, is described.

Targeted solid phase fermentation of the soil dwelling fungus Gymnascella dankaliensis yields new brominated tyrosine-derived alkaloids

Seven new brominated alkaloids, gymnastatins T–Y (1–6) and dankastatin D (7), together with three known analogues were isolated from the soil fungus Gymnascella dankaliensis through fermentation on solid rice mediumcontaining NaBr.

Terpenoids from the Marine-Derived Fungus Aspergillus fumigatus YK-7

Two new β-bergamotane sesquiterpenoids, E-β-trans-5,8,11-trihydroxybergamot-9-ene (1) and β-trans-2β,5,15-trihydroxybergamot-10-ene (2), were isolated from the marine-derived fungus Aspergillusfumigatus YK-7, along with three known terpenoids 3–5. Their structures were determined by spectroscopic methods (1D and 2D NMR, HR-ESI-MS). Antiproliferative effects on human leukemic monocyte lymphoma U937 and human prostate cancer PC-3 cell lines were measured in vitro. Compound 4 exhibited potent activity against the U937 cell line with an IC₅₀ value of 4.2 μM.

Mollenynes B-E from the marine sponge Spirastrella mollis. Band-selective heteronuclear single quantum coherence for discrimination of bromo-chloro regioisomerism in natural products

Four new chlorobromohydrins, mollenynes B-E, were isolated from the marine sponge Spirastrella mollis collected from Hogsty Reef, Bahamas. Their structures were elucidated by integrated analysis of NMR, MS, and computational methods. A high-resolution band-selective HSQC experiment was developed to identify (13)C NMR signals in samples at the nanomole-scale that arise from Cl-substituted (13)C by exploiting the (35)Cl/(37)Cl isotope shift.

Mycemycins A-E, New Dibenzoxazepinones Isolated from Two Different Streptomycetes

Five new dibenzoxazepinone derivatives, mycemycins A–E (1–5), were isolated from the ethanol extracts of mycelia of two different streptomycetes. 1 and 2 were isolated from an acidic red soil-derived strain, Streptomyces sp. FXJ1.235, and 3–5 from a gntR gene-disrupted deep-sea strain named Streptomyces olivaceus FXJ8.012Δ1741. The structures of mycemycins were elucidated by a combination of spectroscopic analyses, including 1D- and 2D-NMR techniques.