Computationally Assisted Analysis of NMR Chemical Shifts as a Tool in Conformational Analysis.

A key to understanding the properties of functional molecules is to determine their conformation in solution. A conformational analysis procedure that relies on quantum mechanical calculations and the widely used DP4+ probability was evaluated to decipher the structural information encoded in NMR chemical shifts. The results underscore the potential utility of using NMR chemical shifts in advancing conformational analysis studies of complex molecules in solution.

Antiproliferative effect of natural and semisynthetic polyethers from Laurencia viridis.

Squalene-derived polyethers are a unique class of compounds that display a great diversity of structures and a broad array of bioactivities, among which its notable antiproliferative activity stands out against various types of cancer cell lines. In this study, eighteen triterpene squalene-derived polyethers, including twelve natural products and six synthetic derivatives, obtained from the red alga Laurencia viridis Gil-Rodríguez & Haroun were screened for their antiproliferative activity against six cancer cell lines: A549, HBL-100, HeLa, SW1573, T-47D, and WiDr; and a structure-activity relationship (SAR) study was established.

Identification and characterization of novel marine oxasqualenoid yucatecone against Naegleria fowleri.

Naegleria fowleri is an opportunistic protozoan, belonging to the free-living amoeba group, that can be found in warm water bodies. It is causative agent the primary amoebic meningoencephalitis, a fulminant disease with a rapid progression that affects the central nervous system. However, no 100% effective treatments are available and those that are currently used involve the appearance of severe side effects, therefore, there is an urgent need to find novel antiamoebic compounds with low toxicity. In this study, the in vitro activity of six oxasqualenoids obtained from the red algae Laurencia viridis was evaluated against two different strains of N. fowleri (ATCC® 30808 and ATCC® 30215) as well as their cytotoxicity against murine macrophages. Yucatecone was the molecule with the highest selectivity index (>2.98 and 5.23 respectively) and it was selected to continue with the cell death type determination assays. Results showed that yucatone induced programmed cell death like responses in treated amoebae causing DNA condensation and cellular membrane damage among others. In this family of oxasqualenoids, it seems that the most significative structural feature to induce activity against N. fowleri is the presence of a ketone at C-18. This punctual oxidation transforms an inactive compound into a lead compound as the yucatecone and 18-ketodehydrotyrsiferol with IC50 values of 16.25 and 12.70 µM, respectively. The assessment of in silico ADME/Tox analysis revealed that the active compounds showed good Human Oral Absorption and demonstrate that are found to be within the limit of approved drug parameter range. Hence, the study highlights promising potential of yucatone to be tested for therapeutic use against primary amoebic meningoencephalitis.

Acaricidal activity of Mexican plants against Rhipicephalus microplus resistant to amitraz and cypermethrin.

This study evaluates the acaricidal activity of Mexican native plants against a Rhipicephalus microplus population resistant to both amitraz and cypermethrin. To explore the activities of plants, the larvicidal effects of 72 Kupchan fractions (Hex, DCM, AcOEt and MeOH:H2O) of 18 plant species collected in Veracruz, Mexico were studied. The evaluation of the Kupchan fractions against double resistant R. microplus indicated that the fractions of Annona globiflora (Hex, LC50 = 0.007% w/v; DCM, LC50 = 0.007% w/v), Annona scleroderma (Hex, LC50 = 0.08% w/v; DCM, LC50 = 0.02% w/v), and Litchi chinensis (Hex, LC50 = 0.79% w/v; DCM, LC50 = 0.54% w/v) showed the greatest larvicidal activities. To identify the presence of additive, synergistic, or antagonistic effects, the acaricidal activity of binary mixtures of the most active fractions of A. globiflora, A. scleroderma, L. chinensis and Citrus sinensis were also evaluated. The latter was chosen as it is easy to obtain due to its substantial presence in agricultural activity locally and globally. The results showed synergistic interaction of the fraction Hex of C. sinensis with the fractions of Hex and DCM of L. chinensis. The most active larvicidal fractions were tested against engorged ticks of R. microplus by adult immersion test at concentration of 2.5% w/v. The Hex and DCM fractions from A. globiflora and A. scleroderma were the most active, causing 100% mortality. The Hex and DCM fractions of L. chinensis exhibited approximately 50% mortality, while the other evaluated fractions did not show efficacy at this concentration. Therefore, it is evident that the fractions of these plants have the potential to be used in either combined or single form as effective alternatives in the control of R. microplus zoonoses.

Ectyoplasin, a novel cytotoxic cyclic peptide from Ectyoplasia ferox sponge.

A new cyclic heptapeptide, ectyoplasin (1), was isolated from a methanol extract of the sponge Ectyoplasia ferox. The planar structure of 1, cyclo(-Leu1-Asn2-Ala3-Val4-Thr5-Pro6-Gly7-), was determined by one and two-dimensional NMR spectroscopy and high-resolution tandem mass spectrometry. Its absolute stereochemistry was solved by Marfey's method. The in vitro assays show that ectyoplasin (1) possess significant cytotoxic activity (2.9 - 23.5 µM) against the cell lines, DU-145 (human prostate cancer), Jurkat (human T-cell acute leukaemia), MM144 (human multiple myeloma), HeLa (human cervical carcinoma) and CADO-ES1 (human Ewing's sarcoma). The DU-145 cell line showed apoptotic cell death in response to ectyoplasin (1) treatment.

Squamins C-F, four cyclopeptides from the seeds of Annona globiflora.

Four cyclic octapeptides, squamins C-F, were isolated from the seeds of Annona globiflora Schltdl. These compounds share part of their amino acid sequence, -Pro-Met(O)-Tyr-Gly-Thr-, with previously reported squamins A and B. Their structures were determined using NMR spectroscopic techniques together with quantum mechanical calculations (QM-NMR), ESI-HRMS data and a modified version of Marfey's chromatographic method. All compounds showed cytotoxic activity against DU-145 (human prostate cancer) and HeLa (human cervical carcinoma) cell lines. Clearly, A. globiflora is an important source of bioactive molecules, which could promote the sustainable exploitation of this undervalued specie.

Antiamoeboid activity of squamins C-F, cyclooctapeptides from Annona globifora.

Free-living amoebae of Acanthamoeba spp. are causative agents of human infections such as granulomatous amoebic encephalitis (GAE) and Acanthamoeba keratitis (AK). The exploration of innovative chemical entities from natural sources that induce intrinsic apoptotic pathway or a Programmed Cell Death (PCD) in Acanthamoeba protozoa is essential to develop new therapeutic strategies. In this work, the antiamoeboid activity of squamins C-F (1-4), four cyclooctapeptides isolated from Annona globiflora was tested in vitro against Acanthamoeba castellanii Neff, A. polyphaga, A. quina, and A. griffini, and a structure-activity relationship was also established. The most sensitive strain against all tested cyclooctapeptides was A. castellanii Neff being the R conformers of the S-oxo-methionine residue, squamins D (2) and F (4), the most active against the trophozoite stage. It is remarkable that all four peptides showed no cytotoxic effects against murine macrophages cell line J774A.1. The analysis of the mode of action of squamins C-F against A. castellanii indicate that these cyclopeptides induced the mechanisms of programmed cell death (PCD). All peptides trigger mitochondrial damages, significant inhibition of ATP production compared to the negative control, chromatin condensation and slight damages in membrane that affects its permeability despite it conserves integrity at the IC90 for 24 h. An increase in reactive oxygen species (ROS) was observed in all cases.

Structure and Computational Basis for Backbone Rearrangement in Marine Oxasqualenoids.

Six novel oxasqualenoids (polyether triterpenes) were isolated from the red alga Laurencia viridis. Laurokanols A-E (1-5) comprise an unreported tricyclic core with a [6,6]-spiroketal system. Yucatecone (6) shows a biogenetically intriguing epimerization at C14. Quantum mechanical calculations were used to corroborate their structures and to explain key steps involved in the biogenetic mechanisms proposed for the formation of oxasqualenoids.

Studies on the bioactive flavonoids isolated from Azadirachta indica.

Two novel natural metabolites, 3-O-butyl-(-)-epicatechin (1) and 3-O-butyl-(-)-epigallocatechin (2), as well as several known substances, (-)-epicatechin (3), (+)-gallocatechin (4), (-)-epigallocatechin (5), azadirachtin A (6), trilinolein (7) and octadecanoic acid-tetrahydrofuran-3,4-diyl ester (8), were isolated from the bark of Azadirachta indica. The structures of all compounds were established by comprehensive and comparative spectroscopic analysis of NMR and ESI-HRMS data. The new metabolites 1 and 2 represent one of the few examples of natural compounds with a butyl ether group moiety. The acaricidal activity of the compounds was tested using a standard Shaw larval immersion assay. All the compounds, except 7, possess a LD50 value less than or equal to 7.2 mM.

Antiprotozoal activities of marine polyether triterpenoids.

Chagas disease and leishmaniasis are tropical neglected diseases caused by kinetoplastids protozoan parasites of Trypanosoma and Leishmania genera, and a public health burden with high morbidity and mortality rates in developing countries. Among difficulties with their epidemiological control, a major problem is their limited and toxic treatments to attend the affected populations; therefore, new therapies are needed in order to find new active molecules. In this work, sixteen Laurencia oxasqualenoid metabolites, natural compounds 1-11 and semisynthetic derivatives 12-16, were tested against Leishmania amazonensis, Leishmania donovani and Trypanosoma cruzi. The results obtained point out that eight substances possess potent activities, with IC50 values in the range of 5.40-46.45 µM. The antikinetoplastid action mode of the main metabolite dehydrothyrsiferol (1) was developed, also supported by AFM images. The semi-synthetic active compound 28-iodosaiyacenol B (15) showed an IC50 5.40 µM against Leishmania amazonensis, turned to be non-toxic against the murine macrophage cell line J774A.1 (CC50 > 100). These values are comparable with the reference compound miltefosine IC50 6.48 ±â€¯0.24 and CC50 72.19 ±â€¯3.06 µM, suggesting that this substance could be scaffold for development of new antikinetoplastid drugs.

Evaluation of Oxasqualenoids from the Red Alga Laurencia viridis against Acanthamoeba.

Acanthamoeba genus is a widely distributed and opportunistic parasite with increasing importance worldwide as an emerging pathogen in the past decades. This protozoan has an active trophozoite stage, a cyst stage, and is dormant and very resistant. It can cause Acanthamoeba keratitis, an ocular sight-threatening disease, and granulomatous amoebic encephalitis, a chronic, very fatal brain pathology. In this study, the amoebicidal activity of sixteen Laurencia oxasqualenoid metabolites and semisynthetic derivatives were tested against Acanthamoeba castellanii Neff. The results obtained point out that iubol (3) and dehydrothyrsiferol (1) possess potent activities, with IC50 values of 5.30 and 12.83 µM, respectively. The hydroxylated congeners thyrsiferol (2) and 22-hydroxydehydrothyrsiferol (4), active in the same value range at IC50 13.97 and 17.00 µM, are not toxic against murine macrophages; thus, they are solid candidates for the development of new amoebicidal therapies.

Sclerin, a New Cytotoxic Cyclononapeptide from Annona scleroderma.

A new cytotoxic cyclononapeptide, sclerin, cyclo(⁻Dab¹â»Ser²â»Tyr³â»Gly4⁻Thr5⁻Val6⁻Ala7⁻ Ile8⁻Pro8⁻) (1), was isolated from the methanol extract of the seeds of Annona scleroderma, together with the known metabolite, cyclosenegalin A, cyclo(⁻Pro¹â»Gly²â»Leu³â»Ser4⁻Ala5⁻Val6⁻Thr7⁻) (2). The planar structures for the two compounds were established by comprehensive analysis of NMR and ESI-HRMS data, and the absolute stereochemistry was stablished by Marfey's method. Compound 1 showed moderate cytotoxic activity against the human prostate carcinoma cell line DU-145 at µM concentration.

Marine Longilenes, Oxasqualenoids with Ser-Thr Protein Phosphatase 2A Inhibition Activity.

The red seaweed Laurencia viridis is a rich source of oxygenated secondary metabolites that were derived from squalene. We report here the structures of three novel compounds, (+)-longilene peroxide (1), longilene (2), and (+)-prelongilene (3) that were isolated from this alga, in addition to other substances, 4 and 5, resulting from their acid-mediated degradation. The effect of compounds 1 and 3 against Ser-Thr protein phosphatase type 2A (PP2A) was evaluated, showing that (+)-longilene peroxide (1) inhibited PP2A (IC50 11.3 μM). In order to explain the interaction between PP2A and compounds 1 and 3, molecular docking simulations onto the PP2A enzyme-binding region were used.

Oxasqualenoids from Laurencia viridis: Combined Spectroscopic-Computational Analysis and Antifouling Potential.

The chemical study of the red alga Laurencia viridis has led to the isolation of four new polyether triterpenoids: 28-hydroxysaiyacenol B (2), saiyacenol C (3), 15,16-epoxythyrsiferol A (4), and 15,16-epoxythyrsiferol B (5). The structures of 2 and 3 were established mainly by NMR data analysis and comparison with the well-known metabolite dehydrothyrsiferol (1). However, due to the existence of a nonprotonated carbon within the epoxide functionality, stereochemical assignments in 4 and 5 required an in-depth structural study that included NOESY data, J-based configuration analysis, comparison with synthetic models, and DFT calculations. The biological activities of the new metabolites and other related oxasqualenoids were evaluated for the first time against a panel of relevant biofouling marine organisms, and structure-activity conclusions were obtained.

New oxidized zoanthamines from a Canary Islands Zoanthus sp.

Three new norzoanthamine-type alkaloids, named 2-hydroxy-11-ketonorzoan thamide B (1), norzoanthamide B (2) and 15-hydroxynorzoanthamine (3), were isolated from Zoanthus sp. specimens collected at the Canary Islands. Their structures were determined by interpretation of NMR and HR-ESIMS data. Relative configurations of their chiral centers were proposed on the basis of ROESY spectra and by comparison of their spectroscopic data with those of the well-known compound, norzoanthamine.

Zoaramine, a zoanthamine-like alkaloid with a new skeleton.

Chemical investigation of an Atlantic variety of Zoanthus sp. led to the isolation of two new metabolites, zoaramine and zoarenone. Their structures were deduced by the use of NMR spectroscopy and computational calculation of (1)H and (13)C chemical shifts. The core of these novel compounds resembles the structure of norzoanthamine alkaloids, and their isolation represents an important step toward a better understanding of the biogenetic origin of this group of antiosteoporotic molecules.

Connecting discrete stereoclusters by using DFT and NMR spectroscopy: the case of nivariol.

The structural determination of small organic molecules is mainly undertaken by using NMR techniques, although it is increasingly supplemented by using computational methods. NMR parameters, such as chemical shifts and coupling constants, are extremely sensitive indicators of local molecular conformation and are a source of structural evidence. However, their interpretation is fairly challenging in many circumstances, such as the case of the new polyether squalene derivative nivariol, the structure of which was elucidated by means of NMR spectroscopy and DFT calculations. The potential flexibility of this molecule and the high number of quaternary carbon atoms that it contains make its configurational assignment very difficult. Moreover, the relative configuration of four separated stereoclusters was established and subsequently connected by using NOE and J-based analysis, as well as by a comparison of its experimental (13)C NMR chemical shifts with the corresponding population-weighted values, as calculated by using DFT methods. Limitations of these used approaches became apparent but were overcome by combining the two methods.

Cytotoxic oxasqualenoids from the red alga Laurencia viridis.

Three new polyether squalene derivatives 15-dehydroxythyrsenol A (2), prethyrsenol A (3) and 13-hydroxyprethyrsenol A (4) have been isolated from the red alga Laurencia viridis. Their structures were determined through the interpretation of NMR spectroscopic data and chemical correlations. In addition, four semi-synthetic compounds modulating the solubility of the lead compound dehydrothyrsiferol (1) were prepared without loss of activity. The cytotoxicity of the new compounds exhibited low µM activities. In order to explain their biological properties, docking simulations of the natural and synthetic compounds onto the αvß3 integrin binding region were carried out.

Studies on polyethers produced by red algae.

Two novel squalene-derived triterpenes, spirodehydrovenustatriol (3) and 14-keto-dehydrothyrsiferol (4) were isolated from the red alga Laurencia viridis, together with two new and unusual C(17) terpenoids, adejen A (5) and B (6). These truncated structures possess structural similarities with other known squalene metabolites and their biogenetic origin has been proposed on the basis of an oxidative process of the squalene skeleton. All the structures were elucidated by extensive use of 2D NMR spectroscopic methods.