Unveiling an indole alkaloid diketopiperazine biosynthetic pathway that features a unique stereoisomerase and multifunctional methyltransferase

The 2,5-diketopiperazines are a prominent class of bioactive molecules. The nocardioazines are actinomycete natural products that feature a pyrroloindoline diketopiperazine scaffold composed of two D-tryptophan residues functionalized by N- and C-methylation, prenylation, and diannulation. Here we identify and characterize the nocardioazine B biosynthetic pathway from marine Nocardiopsis sp. CMB-M0232 by using heterologous biotransformations, in vitro biochemical assays, and macromolecular modeling. Assembly of the cyclo-L-Trp-L-Trp diketopiperazine precursor is catalyzed by a cyclodipeptide synthase. A separate genomic locus encodes tailoring of this precursor and includes an aspartate/glutamate racemase homolog as an unusual D/L isomerase acting upon diketopiperazine substrates, a phytoene synthase-like prenyltransferase as the catalyst of indole alkaloid diketopiperazine prenylation, and a rare dual function methyltransferase as the catalyst of both N- and C-methylation as the final steps of nocardioazine B biosynthesis. The biosynthetic paradigms revealed herein showcase Nature's molecular ingenuity and lay the foundation for diketopiperazine diversification via biocatalytic approaches.

Biomimetic Total Synthesis of (+)-Nocardioazine B and Analogs

Nocardioazines A and B are prenylated, bioactive pyrroloindoline natural products, isolated from Nocardiopsis, with a desymmetrized cyclo-d-Trp-d-Trp DKP core. Based on our deeper biosynthetic understanding, a biomimetic total synthesis of (+)-nocardioazine B is accomplished in merely seven steps and 23.2% overall yield. This pathway accesses regio- and stereoselectively C3-isoprenylated analogs of (+)-nocardioazine B, using the same number of steps and in similar efficiency. The successful strategy mandated that the biomimetic C3-prenylation step be executed early. The use of an unprotected carboxylic acid of Trp led to high diastereoselectivity toward formation of key intermediates exo-12a, exo-12b, and exo-12c (>19:1). Evidence shows that N1-methylation causes the prenylation reaction to bifurcate away to result in a C2-normal-prenylated isomer. Nocardioazine A, possessing an isoprenoidal-epoxide bridge, inhibits P-glycoprotein (P-gp)-mediated membrane efflux, in multidrug-resistant mammalian colon cancer cells. As several P-gp inhibitors have failed due to their toxicity effects, endogenous amino-acid-derived noncytotoxic inhibitors (from the nocardioazine core) are worthy leads toward a rejuvenated strategy against resistant carcinomas. This total synthesis provides direct access to Trp-derived isoprenylated DKP natural products and their derivatives.

Bioinspired Brønsted Acid-Promoted Regioselective Tryptophan Isoprenylations

Tryptophan-containing isoprenoid indole alkaloid natural products are well known for their intricate structural architectures and significant biological activities. Nature employs dimethylallyl tryptophan synthases (DMATSs) or aromatic indole prenyltransferases (iPTs) to catalyze regio- and stereoselective prenylation of l-Trp. Regioselective synthetic routes that isoprenylate cyclo-Trp-Trp in a 2,5-diketopiperazine (DKP) core, in a desymmetrizing manner, are nonexistent and are highly desirable. Herein, we present an elaborate report on Brønsted acid-promoted regioselective tryptophan isoprenylation strategy, applicable to both the monomeric amino acid and its dimeric l-Trp DKP. This report outlines a method that regio- and stereoselectively increases sp3 centers of a privileged bioactive core. We report on conditions involving screening of Brønsted acids, their conjugate base as salt, solvent, temperature, and various substrates with diverse side chains. Furthermore, we extensively delineate effects on regio- and stereoselection of isoprenylation and their stereochemical confirmation via NMR experiments. Regioselectively, the C3-position undergoes normal-isoprenylation or benzylation and forms exo-ring-fused pyrroloindolines selectively. Through appropriate prenyl group migrations, we report access to the bioactive tryprostatin alkaloids, and by C3-normal-farnesylation, we access anticancer drimentines as direct targets of this method. The optimized strategy affords iso-tryprostatin B-type products and predrimentine C with 58 and 55% yields, respectively. The current work has several similarities to biosynthesis, such as-reactions can be performed on unprotected substrates, conditions that enable Brønsted acid promotion, and they are easy to perform under ambient conditions, without the need for stoichiometric levels of any transition metal or expensive ligands.

A Virtual Screening Platform Identifies Chloroethylagelastatin A as a Potential Ribosomal Inhibitor

Chloroethylagelastatin A (CEAA) is an analogue of agelastatin A (AA), a natural alkaloid derived from a marine sponge. It is under development for therapeutic use against brain tumors as it has excellent central nervous system (CNS) penetration and pre-clinical therapeutic activity against brain tumors. Recently, AA was shown to inhibit protein synthesis by binding to the ribosomal A-site. In this study, we developed a novel virtual screening platform to perform a comprehensive screening of various AA analogues showing that AA analogues with proven therapeutic activity including CEAA have significant ribosomal binding capacity whereas therapeutically inactive analogues show poor ribosomal binding and revealing structural fingerprint features essential for drug-ribosome interactions. In particular, CEAA was found to have greater ribosomal binding capacity than AA. Biological tests showed that CEAA binds the ribosome and contributes to protein synthesis inhibition. Our findings suggest that CEAA may possess ribosomal inhibitor activity and that our virtual screening platform may be a useful tool in discovery and development of novel ribosomal inhibitors.

Qualitative Ampule and Multitest for Beta-Lactam Residues in Fluid Milk Products: Collaborative Study

A collaborative study was performed on a rapid Bacillus stearothermophilus agar diffusion ampule method to detect low levels of penicillin G in 7 types of fluid milk products. A multitest technique for processing a large number of samples simultaneously was also studied. Slight modifications were made in the original method to establish more uniformity and to eliminate doubtful responses by specifying a confirmation procedure. Twenty samples spiked with penicillin G (0.000 to 0.008 IU/mL) and tetracycline hydrochloride were frozen and sent to 20 laboratories in the ampule test, and 16 laboratories in the multitest. Each analyst was asked to do a screening run and a confirmation run. Results were reported by color reaction and also as positive or negative for β-lactam inhibitors. The concentrations (penicillin G) where percent positive results equal 100 or not significantly less than 100 (α = 0.05) ranged from 0.005 to 0.007 IU/mi in the ampule test and from 0.004 to 0.007 IU/mL in the multitest. Both techniques have been adopted official first action.

The expanding spectrum of diketopiperazine natural product biosynthetic pathways containing cyclodipeptide synthases

Microorganisms are remarkable chemists, with enzymes as their tools for executing multi-step syntheses to yield myriad natural products. Microbial synthetic aptitudes are illustrated by the structurally diverse 2,5-diketopiperazine (DKP) family of bioactive nonribosomal peptide natural products. Nonribosomal peptide synthetases (NRPSs) have long been recognized as catalysts for formation of DKP scaffolds from two amino acid substrates. Cyclodipeptide synthases (CDPSs) are more recently recognized catalysts of DKP assembly, employing two aminoacyl-tRNAs (aa-tRNAs) as substrates. CDPS-encoding genes are typically found in genomic neighbourhoods with genes encoding additional biosynthetic enzymes. These include oxidoreductases, cytochrome P450s, prenyltransferases, methyltransferases, and cyclases, which equip the DKP scaffold with groups that diversify chemical structures and confer biological activity. These tailoring enzymes have been characterized from nine CDPS-containing biosynthetic pathways to date, including four during the last year. In this review, we highlight these nine DKP pathways, emphasizing recently characterized tailoring reactions and connecting new developments to earlier findings. Featured pathways encompass a broad spectrum of chemistry, including the formation of challenging C-C and C-O bonds, regioselective methylation, a unique indole alkaloid DKP prenylation strategy, and unprecedented peptide-nucleobase bond formation. These CDPS-containing pathways also provide intriguing models of metabolic pathway evolution across related and divergent microorganisms, and open doors to synthetic biology approaches for generation of DKP combinatorial libraries. Further, bioinformatics analyses support that much unique genetically encoded DKP tailoring potential remains unexplored, suggesting opportunities for further expansion of Nature's biosynthetic spectrum. Together, recent studies of DKP pathways demonstrate the chemical ingenuity of microorganisms, highlight the wealth of unique enzymology provided by bacterial biosynthetic pathways, and suggest an abundance of untapped biosynthetic potential for future exploration.

Correction: The expanding spectrum of diketopiperazine natural product biosynthetic pathways containing cyclodipeptide synthases

Correction for 'The expanding spectrum of diketopiperazine natural product biosynthetic pathways containing cyclodipeptide synthases' by Paul Borgman et al., Org. Biomol. Chem., 2019, DOI: 10.1039/c8ob03063d.

Accelerated bottom-up drug design platform enables the discovery of novel stearoyl-CoA desaturase 1 inhibitors for cancer therapy

Here we present an innovative computational-based drug discovery strategy, coupled with machine-based learning and functional assessment, for the rational design of novel small molecule inhibitors of the lipogenic enzyme stearoyl-CoA desaturase 1 (SCD1). Our methods resulted in the discovery of several unique molecules, of which our lead compound SSI-4 demonstrates potent anti-tumor activity, with an excellent pharmacokinetic and toxicology profile. We improve upon key characteristics, including chemoinformatics and absorption/distribution/metabolism/excretion (ADME) toxicity, while driving the IC50 to 0.6 nM in some instances. This approach to drug design can be executed in smaller research settings, applied to a wealth of other targets, and paves a path forward for bringing small-batch based drug programs into the Clinic.

Two Distinct Cyclodipeptide Synthases from a Marine Actinomycete Catalyze Biosynthesis of the Same Diketopiperazine Natural Product

Diketopiperazine natural products are structurally diverse and offer many biological activities. Cyclodipeptide synthases (CDPSs) were recently unveiled as a novel enzyme family that employs aminoacyl-tRNAs as substrates for 2,5-diketopiperazine assembly. Here, the Nocardiopsis sp. CMB-M0232 genome is predicted to encode two CDPSs, NozA and NcdA. Metabolite profiles from E. coli expressing these genes and assays with purified recombinant enzymes revealed that NozA and NcdA catalyze cyclo(l-Trp-l-Trp) (1) biosynthesis from tryptophanyl-tRNA and do not accept other aromatic aminoacyl-tRNA substrates. Fidelity is uncommon among characterized CDPSs, making NozA and NcdA important CDPS family additions. Further, 1 was previously supported as a biosynthetic precursor of the nocardioazines; the current study suggests that Nocardiopsis sp. may derive this precursor from both NozA and NcdA. This study offers a rare example of a single bacterium encoding multiple phylogenetically distinct enzymes that yield the same secondary metabolite and provides tools for chemoenzymatic syntheses of indole alkaloid diketopiperazines.

Correction: Synergism between genome sequencing, tandem mass spectrometry and bio-inspired synthesis reveals insights into nocardioazine B biogenesis

Correction for 'Synergism between genome sequencing, tandem mass spectrometry and bio-inspired synthesis reveals insights into nocardioazine B biogenesis' by Norah Alqahtani et al., Org. Biomol. Chem., 2015, 13, 7177-7192.

Abstract 4459: Accelerated drug discovery platform yields synthesis of novel stearoyl-CoA desaturase 1 inhibitors that demonstrate anti-tumor efficacy in several models of aggressive cancer

Background

Intra-tumor heterogeneity is the driving force behind disease progression and metastasis, drug resistance, and relapse in cancer patients. Defining personalized treatment regimens and increasing the arsenal of targeted therapies available will provide the best chance of prolonging survival and finding curative treatments. Recent work by our group and others has shed light on aberrant de novo lipogenesis as a feature of many cancers including kidney, breast, and lung cancer. Of the molecules involved in fatty acid metabolism, stearoyl-CoA desaturase 1 (SCD1) is frequently over-expressed. Targeted inhibition of SCD1 demonstrates a loss of tumor cell viability in a variety of cancer models. While there currently are specific small molecule inhibitors for SCD1 none are under investigation as anti-cancer therapeutics. In order to evaluate the efficacy of SCD1 inhibitors against ccRCC in both a preclinical and eventually a clinical setting we sought to generate unique compounds and test their biological efficacy.

Methods

Utilizing an innovative in silico approach, we designed new inhibitors for SCD1 via a scaffold hopping approach while searching for different “core” scaffolds. Critical interaction moieties from the chemical R-groups were held fixed, while generated cores were rapidly scanned for best-fit criteria (shape, docking, pharmacophore fit (QSAR)) into our Z-matrix. Following de novo compound generation and synthesis, we implemented iterative analog generation for the top hits. Using this method and diversity of chemical space, we pursued three divergent species of compounds into the nanomolar regime.

Biological testing of the novel compounds included high-throughput proliferative-based screening, oleic acid rescue, and evaluation of the endoplasmic stress response. Enzymatic inhibition of SCD1 was evaluated by LC-MS. Preclinical pharmacokinetics was established using a range of doses administered intravenously and orally. Anti-tumor response was evaluated in an ectopic model of renal carcinoma, both as a single agent and in combination with SOC.

Results

A cohort of potential unique small molecule inhibitors against SCD1 were designed and synthesized. Of these, a select group of compounds demonstrated potent and selective enzyme inhibition and reduced tumor growth in in vitro and in vivo models of ccRCC. These novel inhibitors reliably reproduced activation of ER stress markers, authenticating this biological response as a harbinger for successful abrogation of SCD1 activity.

Conclusions

We have established an effective, expedited, and economical format for in silico modeling of novel agents based on a scaffold-hopping methodology. Our findings provide compelling evidence supporting the therapeutic benefit of SCD1 inhibitors for clinical use as broad-spectrum anti-tumor agents alone or in combination with SOC.

Citation Format: Christina A. Von Roemeling, Thomas R. Caulfield, Derek C. Radisky, Ilah Bok, Laura A. Marlow, James Miller, Mojda Sidiqi, Anthony B. Pinkerton, Winston W. Tan, Amy L. Lane, Han W. Tun, John A. Copland. Accelerated drug discovery platform yields synthesis of novel stearoyl-CoA desaturase 1 inhibitors that demonstrate anti-tumor efficacy in several models of aggressive cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4459. doi:10.1158/1538-7445.AM2015-4459

Characterization of the nocardiopsin biosynthetic gene cluster reveals similarities to and differences from the rapamycin and FK-506 pathways

Macrolide-pipecolate natural products, such as rapamycin (1) and FK-506 (2), are renowned modulators of FK506-binding proteins (FKBPs). The nocardiopsins, from Nocardiopsis sp. CMB-M0232, are the newest members of this structural class. Here, the biosynthetic pathway for nocardiopsins A-D (4-7) is revealed by cloning, sequencing, and bioinformatic analyses of the nsn gene cluster. In vitro evaluation of recombinant NsnL revealed that this lysine cyclodeaminase catalyzes the conversion of L-lysine into the L-pipecolic acid incorporated into 4 and 5. Bioinformatic analyses supported the conjecture that a linear nocardiopsin precursor is equipped with the hydroxy group required for macrolide closure in a previously unobserved manner by employing a P450 epoxidase (NsnF) and limonene epoxide hydrolase homologue (NsnG). The nsn cluster also encodes candidates for tetrahydrofuran group biosynthesis. The nocardiopsin pathway provides opportunities for engineering of FKBP-binding metabolites and for probing new enzymology in nature's polyketide tailoring arsenal.

Synergism between genome sequencing, tandem mass spectrometry and bio-inspired synthesis reveals insights into nocardioazine B biogenesis

A noz gene cluster encoded by Nocardiopsis sp. CMB M0232 is presented, revealing the actinomycetes’ unique indole methylating and prenylating patterns, leading to anticancer nocardioazine alkaloids.

Characterization of an orphan diterpenoid biosynthetic operon from Salinispora arenicola

While more commonly associated with plants than microbes, diterpenoid natural products have been reported to have profound effects in marine microbe-microbe interactions. Intriguingly, the genome of the marine bacterium Salinispora arenicola CNS-205 contains a putative diterpenoid biosynthetic operon, terp1. Here recombinant expression studies are reported, indicating that this three-gene operon leads to the production of isopimara-8,15-dien-19-ol (4). Although 4 is not observed in pure cultures of S. arenicola, it is plausible that the terp1 operon is only expressed under certain physiologically relevant conditions such as in the presence of other marine organisms.

Bioactivity-guided genome mining reveals the lomaiviticin biosynthetic gene cluster in Salinispora tropica

The use of genome sequences has become routine in guiding the discovery and identification of microbial natural products and their biosynthetic pathways. In silico prediction of molecular features, such as metabolic building blocks, physico-chemical properties or biological functions, from orphan gene clusters has opened up the characterization of many new chemo- and genotypes in genome mining approaches. Here, we guided our genome mining of two predicted enediyne pathways in Salinispora tropica CNB-440 by a DNA interference bioassay to isolate DNA-targeting enediyne polyketides. An organic extract of S. tropica showed DNA-interference activity that surprisingly was not abolished in genetic mutants of the targeted enediyne pathways, ST_pks1 and spo. Instead we showed that the product of the orphan type II polyketide synthase pathway, ST_pks2, is solely responsible for the DNA-interfering activity of the parent strain. Subsequent comparative metabolic profiling revealed the lomaiviticins, glycosylated diazofluorene polyketides, as the ST_pks2 products. This study marks the first report of the 59 open reading frame lomaiviticin gene cluster (lom) and supports the biochemical logic of their dimeric construction through a pathway related to the kinamycin monomer.

Structures and comparative characterization of biosynthetic gene clusters for cyanosporasides, enediyne-derived natural products from marine actinomycetes

Cyanosporasides are marine bacterial natural products containing a chlorinated cyclopenta[a]indene core of suspected enediyne polyketide biosynthetic origin. Herein, we report the isolation and characterization of novel cyanosporasides C-F (3-6) from the marine actinomycetes Salinispora pacifica CNS-143 and Streptomyces sp. CNT-179, highlighted by the unprecedented C-2' N-acetylcysteamine functionalized hexose group of 6. Cloning, sequencing, and mutagenesis of homologous ~50 kb cyanosporaside biosynthetic gene clusters from both bacteria afforded the first genetic evidence supporting cyanosporaside's enediyne, and thereby p-benzyne biradical, biosynthetic origin and revealed the molecular basis for nitrile and glycosyl functionalization. This study provides new opportunities for bioengineering of enediyne derivatives and expands the structural diversity afforded by enediyne gene clusters.

A sea of biosynthesis: marine natural products meet the molecular age

The years 2000 through mid-2010 marked a transformational period in understanding of the biosynthesis of marine natural products. During this decade the field emerged from one largely dominated by chemical approaches to understanding biosynthetic pathways to one incorporating the full force of modern molecular biology and bioinformatics. Fusion of chemical and biological approaches yielded great advances in understanding the genetic and enzymatic basis for marine natural product biosynthesis. Progress was particularly pronounced for marine microbes, especially actinomycetes and cyanobacteria. During this single decade, both the first complete marine microbial natural product biosynthetic gene cluster sequence was released as well as the first entire genome sequence for a secondary metabolite-rich marine microbe. The decade also saw tremendous progress in recognizing the key role of marine microbial symbionts of invertebrates in natural product biosynthesis. Application of genetic and enzymatic knowledge led to genetic engineering of novel “unnatural” natural products during this time, as well as opportunities for discovery of novel natural products through genome mining. The current review highlights selected seminal studies from 2000 through to June 2010 that illustrate breakthroughs in understanding of marine natural product biosynthesis at the genetic, enzymatic, and small-molecule natural product levels. A total of 154 references are cited.

Voyager 2 encounter with the jovian system

An overview of the Voyager 2 encounter with Jupiter is presented, including a brief discussion of the trajectory, the major sequence modifications performed because of the Jupiter measurements obtained with Voyager 1, and high-lights of the results that are described in the subsequent reports.

Biochemical warfare on the reef: the role of glutathione transferases in consumer tolerance of dietary prostaglandins

BACKGROUND

Despite the profound variation among marine consumers in tolerance for allelochemically-rich foods, few studies have examined the biochemical adaptations underlying diet choice. Here we examine the role of glutathione S-transferases (GSTs) in the detoxification of dietary allelochemicals in the digestive gland of the predatory gastropod Cyphoma gibbosum, a generalist consumer of gorgonian corals. Controlled laboratory feeding experiments were used to investigate the influence of gorgonian diet on Cyphoma GST activity and isoform expression. Gorgonian extracts and semi-purified fractions were also screened to identify inhibitors and possible substrates of Cyphoma GSTs. In addition, we investigated the inhibitory properties of prostaglandins (PGs) structurally similar to antipredatory PGs found in high concentrations in the Caribbean gorgonian Plexaura homomalla.

PRINCIPAL FINDINGS

Cyphoma GST subunit composition was invariant and activity was constitutively high regardless of gorgonian diet. Bioassay-guided fractionation of gorgonian extracts revealed that moderately hydrophobic fractions from all eight gorgonian species examined contained putative GST substrates/inhibitors. LC-MS and NMR spectral analysis of the most inhibitory fraction from P. homomalla subsequently identified prostaglandin A(2) (PGA(2)) as the dominant component. A similar screening of commercially available prostaglandins in series A, E, and F revealed that those prostaglandins most abundant in gorgonian tissues (e.g., PGA(2)) were also the most potent inhibitors. In vivo estimates of PGA(2) concentration in digestive gland tissues calculated from snail grazing rates revealed that Cyphoma GSTs would be saturated with respect to PGA(2) and operating at or near physiological capacity.

SIGNIFICANCE

The high, constitutive activity of Cyphoma GSTs is likely necessitated by the ubiquitous presence of GST substrates and/or inhibitors in this consumer's gorgonian diet. This generalist's GSTs may operate as 'all-purpose' detoxification enzymes, capable of conjugating or sequestering a broad range of lipophilic gorgonian compounds, thereby allowing this predator to exploit a range of chemically-defended prey, resulting in a competitive dietary advantage for this species.

Ecological leads for natural product discovery: Novel sesquiterpene hydroquinones from the red macroalga Peyssonnelia sp

Pharmacologically-motivated marine natural product investigations have yielded a large variety of structurally unique compounds with interesting biomedical properties, but the natural roles of these molecules often remain unknown. While secondary metabolites may function as antimicrobial chemical defenses, few studies have examined this hypothesis. In the present investigation, chromatographic fractions from 69 collections of Fijian red macroalgae representing at least 43 species were evaluated for growth inhibition of three microbial pathogens and saprophytes of marine macrophytes. At least one microbe was suppressed by fraction(s) of all evaluated algae, suggesting that antimicrobial defenses are common among tropical seaweeds. From these leads, peyssonoic acids A-B (1-2), novel sesquiterpene hydroquinones, were isolated from the crustose red alga Peyssonnelia sp. At ecologically realistic concentrations, both compounds inhibited growth of Pseudoalteromonas bacteriolytica, a bacterial pathogen of marine algae, and Lindra thalassiae, a fungal pathogen of marine algae, and exhibited modest antineoplastic activity against ovarian cancer cells. The peyssonoic acids included one novel carbon skeleton and illustrated the utility of ecological studies in natural product discovery.

Antimalarial bromophycolides J-Q from the Fijian red alga Callophycus serratus

Bromophycolides J-Q (1-8) were isolated from extracts of the Fijian red alga Callophycus serratus and identified with 1D and 2D NMR spectroscopy and mass spectral analyses. These diterpene-benzoate macrolides represent two novel carbon skeletons and add to the 10 previously reported bromophycolides (9-18) from this alga. Among these 18 bromophycolides, several exhibited activities in the low micromolar range against the human malaria parasite Plasmodium falciparum.

Reactive desorption electrospray ionization mass spectrometry (DESI-MS) of natural products of a marine alga

Presented here is the optimization and development of a desorption electrospray ionization mass spectrometry (DESI-MS) method for detecting natural products on tissue surfaces. Bromophycolides are algal diterpene-benzoate macrolide natural products that have been shown to inhibit growth of the marine fungal pathogen Lindra thalassiae. As such, they have been implicated in antimicrobial chemical defense. However, the defense mechanisms are not yet completely understood. Precise detection of these compounds on algal tissue surfaces under ambient conditions without any disruptive sample processing could shed more light onto the processes involved in chemical defense of marine organisms. Conventional DESI-MS directly on algal tissue showed relatively low sensitivity for bromophycolide detection. Sensitivity was greatly improved by the addition of various anions including Cl(-), Br(-), and CF(3)COO(-) into the DESI spray solvent. Chloride adduction gave the highest sensitivity for all assayed anions. Density functional optimization of the bromophycolide anionic complexes produced during DESI supported this observation by showing that the chloride complex has the most favorable binding energy. Optimized DESI protocols allowed the direct and unambiguous detection of bromophycolides, including A, B, and E, from the surface of untreated algal tissue.

Antibacterial neurymenolides from the Fijian red alga Neurymenia fraxinifolia

Two novel alpha-pyrone macrolides, neurymenolides A (1) and B (2), were isolated from the Fijian red alga Neurymenia fraxinifolia and characterized using a combination of NMR and mass spectral analyses. These molecules represent only the second example of alpha-pyrone macrolides, with 1 existing as interchanging atropisomers due to restricted rotation about the alpha-pyrone ring system. Neurymenolide A (1) displayed moderately potent activities against methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus faecium (VREF).

Structures and absolute configurations of sulfate-conjugated triterpenoids including an antifungal chemical defense of the green macroalga Tydemania expeditionis

Cytotoxicity-guided fractionation of the green macroalga Tydemania expeditionis led to isolation of four sulfate-conjugated triterpenoids including one new lanostane-type triterpenoid disulfate, lanosta-8-en-3,29-diol-23-oxo-3,29-disodium sulfate (1), and three known cycloartane-type triterpenoid disulfates, cycloartan-3,29-diol-23-one 3,29-disodium sulfate (2), cycloart-24-en-3,29-diol-23-one 3,29-disodium sulfate (3), and cycloartan-3,23,29-triol 3,29-disodium sulfate (4). Extensive 1D and 2D NMR analyses in combination with X-ray crystallography established the structure and absolute configuration of 1 and allowed determination of the absolute configurations of 2-4 with a revision of previously assigned configuration at C-5. Each natural product was moderately cytotoxic in tumor cell and invertebrate toxicity assays. Of the natural products, only 4 exhibited significant antifungal activity at whole-tissue natural concentrations against the marine pathogen Lindra thalassiae. Comparison of the biological activities of natural products with their desulfated derivatives indicated that sulfation does not appear to confer cytotoxicity or antifungal activity.

Callophycoic acids and callophycols from the Fijian red alga Callophycus serratus

Callophycoic acids A-H (1-8) and callophycols A and B (9 and 10) were isolated from extracts of the Fijian red alga Callophycus serratus, and identified by NMR, X-ray, and mass spectral analyses. These natural products represent four novel carbon skeletons, providing the first examples of diterpene-benzoic acids and diterpene-phenols in macroalgae. Compounds 1-10 exhibited antibacterial, antimalarial, and anticancer activity, although they are less bioactive than diterpene-benzoate macrolides previously isolated from this red alga.

Structure-activity relationship of chemical defenses from the freshwater plant Micranthemum umbrosum

Vascular plants produce a variety of molecules of phenylpropanoid biosynthetic origin, including lignoids. Recent investigations indicated that in freshwater plants, some of these natural products function as chemical defenses against generalist consumers such as crayfish. Certain structural features are shared among several of these anti-herbivore compounds, including phenolic, methoxy, methylenedioxy, and lactone functional groups. To test the relative importance of various functional groups in contributing to the feeding deterrence of phenylpropanoid-based natural products, we compared the feeding behavior of crayfish offered artificial diets containing analogs of elemicin (1) and beta-apopicropodophyllin (2), chemical defenses of the freshwater macrophyte Micranthemum umbrosum. Both allyl and methoxy moieties of 1 contributed to feeding deterrence. Disruption of the lactone moiety of 2 reduced its deterrence. Finally, feeding assays testing effects of 1 and 2 at multiple concentrations established that these two natural products interact additively in deterring crayfish feeding.

The Mars oxidant experiment (MOx) for Mars '96

The MOx instrument was developed to characterize the reactive nature of the martian soil. The objectives of MOx were: (1) to measure the rate of degradation of organics in the martian environment; (2) to determine if the reactions seen by the Viking biology experiments were caused by a soil oxidant and measure the reactivity of the soil and atmosphere: (3) to monitor the degradation, when exposed to the martian environment, of materials of potential use in future missions; and, finally, (4) to develop technologies and approaches that can be part of future soil analysis instrumentation. The basic approach taken in the MOx instrument was to place a variety of materials composed as thin films in contact with the soil and monitor the physical and chemical changes that result. The optical reflectance of the thin films was the primary sensing-mode. Thin films of organic materials, metals, and semiconductors were prepared. Laboratory simulations demonstrated the response of thin films to active oxidants.

Detection of ozone on Ganymede

An absorption band at 260 nanometers on the trailing hemisphere of Ganymede, identified as the Hartley band of Ozone (O3), was measured with the Hubble Space Telescope. The column abundance of ozone, 4.5 x 10(16) per square centimeter, can be produced by ion impacts or by photochemical equilibrium with previously detected molecular oxygen (O2). An estimated number density ratio of [O3]/[O2] = 10(-4) to 10(-3) requires an atmospheric density orders of magnitude higher than upper limits from spacecraft occultation experiments. Apparently, this O2-O3 "atmosphere" is trapped in Ganymede's surface ice, an inference consistent with the shift and broadening of the band compared with the gas-phase O3 band.

Galileo Ultraviolet Spectrometer Experiment: Initial Venus and Interplanetary Cruise Results

The Galileo Extreme Ultraviolet Spectrometer obtained a spectrum of Venus atmospheric emissions in the 55.0- to 125.0-nanometer (nm) wavelength region. Emissions of helium (58.4 nm), ionized atomic oxygen (83.4 nm), and atomic hydrogen (121.6 nm), as well as a blended spectral feature of atomic hydrogen (Lyman-beta) and atomic oxygen (102.5 nm), were observed at 3.5-nm resolution. During the Galileo spacecraft cruise from Venus to Earth, Lyman-alpha emission from solar system atomic hydrogen (121.6 nm) was measured. The dominant source of the Lyman-alpha emission is atomic hydrogen from the interstellar medium. A model of Galileo observations at solar maximum indicates a decrease in the solar Lyman-alpha flux near the solar poles. A strong day-to-day variation also occurs with the 27-day periodicity of the rotation of the sun.

Temperature and Thermal Emissivity of the Surface of Neptune's Satellite Triton

Analysis of the preliminary results from the Voyager mission to the Neptune system has provided the scientific community with several methods by which the temperature of Neptune's satellite Triton may be determined. If the 37.5 K surface temperature reported by several Voyager investigations is correct, then the photometry reported by the imaging experiment on Voyager requires that Triton's surface have a remarkably low emissivity. Such a low emissivity is not required in order to explain the photometry from the photopolarimeter experiment on Voyager. A low emissivity would be inconsistent with Triton having a rough surface at the approximately 100-microm scale as might be expected given the active renewal processes which appear to dominate Triton's surface.

Photometry from Voyager 2: Initial Results from the Neptunian Atmosphere, Satellites, and Rings

The Voyager photopolarimeter successfully accomplished its objectives for the Neptune encounter, performing measurements on the planet, several of its satellites, and its ring system. A photometric map of Neptune at 0.26 micrometer (microm) shows the planet to be bland, with no obvious contrast features. No polar haze was observed. At 0.75 microm, contrast features are observed, with the Great Dark Spot appearing as a low-albedo region and the bright companion as being substantially brighter than its surroundings, implying it to be at a higher altitude than the Great Dark Spot. Triton's linear phase coefficients of 0.011 magnitudes per degree at 0.26 microm and 0.013 magnitudes per degree at 0.75 microm are consistent with a solid-surface object possessing high reflectivity. Preliminary geometric albedos for Triton, Nereid, and 1989N2 were obtained at 0.26 and 0.75 microm. Triton's rotational phase curve shows evidence of two major compositional units on its surface. A single stellar occultation of the Neptune ring system elucidated an internal structure in 1989N1R, in the approximately 50-kilometer region of modest optical depth. 1989N2R may have been detected. The deficiency of material in the Neptune ring system, when compared to Uranus', may imply the lack of a "recent" moon-shattering event.

Photometry from Voyager 2: Initial Results from the Uranian Atmosphere, Satellites, and Rings

The Voyager 2 photopolarimeter successfully completed the Uranus encounter, acquiring new data on the planet's atmosphere, its principal satellites, and its ring system. Spatially resolved photometry of the atmosphere at 0.27 micrometer shows no enhancement in absorption toward the pole, unlike the case for Jupiter and Saturn. Stellar occultation measurements indicate the temperature at the 1-millibar level over the north pole is near 90 kelvins. The geometric albedos of the five large satellites of Uranus were measured at 0.27 and 0.75 micrometer and indicate the presence of low albedo, spetrally flat absorbing material. Titania seems to have a fluffy surface, as indicated by its phase curve. The nine ground-based rings were detected, and their internal structure, optical depths, and positions were determined. The sharp edges of the in ring made it possible to measure its edge thickness (less than 150 meters) and particle sizes (less than 30 meters); little or no dust was detcted. New narrow rings and partial rings (arcs) were measured, and the narrow component of the eta ring was found to be discontinuous.

Comparison of several control standard endotoxins to the National Reference Standard Endotoxin--an HIMA collaborative study

A collaborative study, initiated under the auspices of the Health Industry Manufacturers Association (HIMA), was designed to establish the relationship of Escherichia coli O55:B5 endotoxin (the control standard endotoxin of HIMA and the Food and Drug Administration's Office of Medical Devices) to the U.S. National Reference Standard Endotoxin and to two internationally used control standard endotoxins. By using two Limulus amoebocyte lysate test systems, it was established that the E. coli O55:B5 endotoxin lot originally used by HIMA and the Office of Medical Devices to establish Limulus amoebocyte lysate release test criteria for pyrogen testing of medical devices contains approximately 4.5 endotoxin units (EU) per ng. Thus, the 1.0-ng/kg endotoxin dose limit currently established for medical devices is approximately the same as the 5.0-EU/kg endotoxin limit (on an activity basis) established by several other Food and Drug Administration agencies for human and animal parenteral drugs and biological products.

Eccentric Ringlet in the Maxwell Gap at 1.45 Saturn Radii: Multi-Instrument Voyager Observations

The Voyager spacecraft observed a narrow, eccentric ringlet in the Maxwell gap (1.45 Saturn radii) in Saturn's rings. Intercomparison of the Voyager imaging, photopolarimeter, ultraviolet spectrometer, and radio science observations yields results not available from individual observations. The width of the ringlet varies from about 30 to about 100 kilometers, its edges are sharp on a radial scale < 1 kilometer, and its opacity exhibits a double peak near the center. The shape and width of the ringlet are consistent with a set of uniformly precessing, confocal ellipses with foci at Saturn's center of mass. The ringlet precesses as a unit at a rate consistent with the known dynamical oblateness of Saturn; the lack of differential precession across the ringlet yields a ringlet mass of about 5 x 10(18) grams. The ratio of surface mass density to particle cross-sectional area is about five times smaller than values obtained elsewhere in the Saturn ring system, indicating a relatively larger fraction of small particles. Also, comparison of the measured transmission of the ringlet at radio, visible, and ultraviolet wavelengths indicates that about half of the total extinction is due to particles smaller than 1 centimeter in radius, in contrast even with nearby regions of the C ring. However, the color and brightness of the ringlet material are not measurably different from those of nearby C ring particles. We find this ringlet is similar to several of the rings of Uranus.

Photopolarimetry from Voyager 2; Preliminary Results on Saturn, Titan, and the Rings

The Voyager 2 photopolarimeter was reprogrammed prior to the August 1981 Saturn encounter to perform orthogonal-polarization, two-color measurements on Saturn, Titan, and the rings. Saturn's atmosphere has ultraviolet limb brightening in the mid-latitudes and pronounced polar darkening north of 65 degrees N. Titan's opaque atmosphere shows strong positive polarization at all phase angles (2.7 degrees to 154 degrees ), and no single-size spherical particle model appears to fit the data. A single radial stellar occultation of the darkened, shadowed rings indicated a ring thickness of less than 200 meters at several locations and clear evidence for density waves caused by satellite resonances. Multiple, very narrow strands of material were found in the Encke division and within the brightest single strand of the F ring.

Photometric Observations of Jupiter at 2400 Angstroms

The photopolarimeter instrument on Voyager 2 was used to obtain a map of Jupiter at an effective wavelength of 2400 angstroms. Analysis of a typical north-south swath used to make this map shows strong absorption at high latitudes by a molecular or particulate constituent in the Jovian atmosphere. At 65 degrees north latitude, the absorbing constituent extends to altitudes above the 50-millibar pressure level.

Voyager 1 Encounter with the Jovian System

An overview of the Voyager 1 encounter with Jupiter is presented, including a brief discussion of the characteristics of the spacecraft and trajectory and highlights of the results which are described in the subsequent reports.

The Composition of Phobos: Evidence for Carbonaceous Chondrite Surface from Spectral Analysis

A reflectance spectrum of Phobos (from 200 to 1100 nanometers) has been compiled from the Mariner 9 ultraviolet spectrometer, Viking lander imaging, and ground-based photometric data. The reflectance of the martian satellite is approximately constant at 5 percent from 1100 to 400 nanometers but drops sharply below 400 nanometers, reaching a value of 1 percent at 200 nanometers. The spectral albedo of Phobos bears a striking resemblance to that of asteroids (1) Ceres and (2) Pallas. Comparison of the reflectance spectra of asteroids with those of meteorites has shown that the spectral signature of Ceres is indicative of a carbonaceous chondritic composition. A physical explanation of how the compositional information is imposed on the reflectance spectrum is given. On the basis of a good match between the reflectance spectra of Phobos and Ceres and the extensive research that has been done to infer the composition of Ceres, it seems reasonable to believe that the surface composition of Phobos is similar to that of carbonaceous chondrites. This suggestion is consistent with the recently determined low density of Mars's inner satellite. Our result and recent Viking noble gas measurements suggest different modes of origin for Mars and Phobos.

Multicolor Observations of Phobos with the Viking Lander Cameras: Evidence for a Carbonaceous Chondritic Composition

The reflectivity of Phobos has been determined in the spectral region from 0.4 to 1.1 micrometers from images taken with a Viking lander camera. The reflectivity curve is flat in this spectral interval and the geometric albedo equals 0.05 +/- 0.01. These results, together with Phobos's reflectivity spectrum in the ultraviolet, are compared with laboratory spectra of carbonaceous chondrites and basalts. The spectra of carbonaceous chondrites are consistent with the observations, whereas the basalt spectra are not. These findings raise the possibility that Phobos may be a captured object rather than a natural satellite of Mars.

Mariner 9 Ultraviolet Spectrometer Experiment: Seasonal Variation of Ozone on Mars

Ozone is observed to be present in the polar regions of Mars and to have a seasonal variation. In the summer, the amount present in the polar atmosphere is less than 3 micrometer-atmospheres. In the fall, ozone increases in amount and is found in association with the formation of the polar hood. In winter, the maximum amount of ozone is present, 57 micrometer-atmospheres over the polar hood and 16 over the polar cap. In spring, the amount over the polar cap decreases monotonically until by the beginning of summer the ozone disappears. Ozone is not observed in the equatorial region during any season.

Mariner 9 Ultraviolet Spectrometer Experiment: Initial Results

The ultraviolet airglow spectrum of Mars has been measured from an orbiting spacecraft during a 30-day period in November-December 1971. The emission rates of the carbon monoxide Cameron and fourth positive bands, the atomic oxygen 1304-angstrom line and the atomic hydrogen 1216-angstrom line have been measured as a function of altitude. Significant variations in the scale height of the CO Cameron band airglow have been observed during a period of variable solar activity; however, the atomic oxygen and hydrogen airglow lines are present during all the observations. Measurements of the reflectance of the lower atmosphere of Mars show the spectral characteristics of particle scattering and a magnitude that is about 50 percent of that measured during the Mariner 6 and 7 experiments in 1969. The variation of reflectance across the planet may be represented by a model in which the dominant scatterer is dust that absorbs in the ultraviolet and has an optical depth greater than 1. The atmosphere above the polar region is clearer than over the rest of the planet.

Mariner 9 Ultraviolet Spectrometer Experiment: Stellar Observations

Photoelectric spectra have been obtained for a number of early-type stars in the 1100- to 2000-angstrom region with the Mariner 9 ultraviolet spectrometer. The resonance lines of H I, Si IV, and C IV are easily identified, as are features due to C II, C III, Si III, Fe II, N IV. The absolute energy distribution derived from the data lie about 20 percent below those of OAO-2 in the 1200- to 2000-angstrom region.

Mariner 6 and Mariner 7 Ultraviolet Spectrometer: In-Flight Measurements of Simulated Jupiter Atmosphere

An experiment has been performed in interplanetary space which closely simulates the observations that would be madef by an ultraviolet spectrometer observing the atmospheres of the jovian planets. A mixture of ammonia, nitrogen, and hydrogen was released from the Mariner spacecraft, and spectra were recorded while these gases were illuminated by sunlight. The principal emissions observed were the HI 1216-angstrom Lyman-alpha line, the H(2) B-X Lyman bands, and the NH c-a and A-X bands.