José A. Gavín

On the Configuration of Five-Membered Rings: A Spin-Spin Coupling Constant Approach

Five-membered rings are clearly among the most common structural motifs found in chemistry and biology. Nevertheless, the configuration of conformationally mobile five-membered rings is often difficult to assign from nuclear magnetic resonance (NMR) data. A simple, reliable, and efficient approach for the stereochemical analysis of five-membered rings based on the measurement of NMR coupling constants is presented. Density functional theory calculations using representative conformations of the full conformational space available to rings with different substitution patterns were used to identify differences between the accessible coupling constant values for cis and trans relative orientations of the substituents. The calculations were assessed experimentally using NMR data obtained from a number of models. This approach can be easily used to analyze different five-membered rings, such as oxolanes, cyclopentanes, furanosides and pyrrolidines, and their relative configuration can be determined without the need for making further conformational considerations.

Several new squalene-derived triterpenes from Laurencia

Abstract Four new triterpenoids with a squalene carbon skeleton martiriol 7, pseudodehydrothyrsiferol 8, dioxepandehydrothyrsiferol 9 and 16-epihydroxydehydrothyrsiferol 10 have been isolated from the red alga Laurencia viridis. The structures were determined through the interpretation of their spectral data and the relative stereochemistries were proposed on the basis of ROESY and NOEDIFF data. Their cytotoxic activities were evaluated.

A practical approach to the implementation of selectivity in homonuclear multidimensional NMR with frequency selective-filtering techniques. Application to the chemical structure elucidation of complex oligosaccharides†

This review describes the use of frequency‐selective filters for the design of selective NMR experiments. Frequency‐selective filters constitute ‘user‐friendly’ and efficient alternatives to selective excitation in most applications. Different selective schemes are discussed, which are based either on selective inversion or selective refocusing of the frequencies of interest. Selective filters can be used to transform 2D (or 3D) experiments into 1D (or 2D) analogues, or to restrict the matrix size in higher dimensional spectra. Accurate NMR parameters can be easily extracted from such spectra in short measuring times, with high digital resolution and minimum data storage. For these reasons, these experiments are a very useful alternative to conventional multidimensional experiments in structural and/or conformational studies of small‐ and medium‐sized molecules. Moreover, using DANTE procedures for selectivity, they can be easily implemented on ‘routine’ spectrometers, without the need for sophisticated hardware. These methods have been tested on a complex heterosidic compound, where they prove to afford outstanding interest in achieving proton resonance assignment and chemical structure delineation. Copyright

Macrocyclization of Peptide Side Chains by the Ugi Reaction: Achieving Peptide Folding and Exocyclic N-Functionalization in One Shot.

The cyclization of peptide side chains has been traditionally used to either induce or stabilize secondary structures (β-strands, helices, reverse turns) in short peptide sequences. So far, classic peptide coupling, nucleophilic substitution, olefin metathesis, and click reactions have been the methods of choice to fold synthetic peptides by means of macrocyclization. This article describes the utilization of the Ugi reaction for the side chain-to-side chain and side chain-to-termini macrocyclization of peptides, thus enabling not only access to stable folded structures but also the incorporation of exocyclic functionalities as N-substituents. Analysis of the NMR-derived structures revealed the formation of helical turns, β-bulges, and α-turns in cyclic peptides cross-linked at i, i + 3 and i, i + 4 positions, proving the folding effect of the multicomponent Ugi macrocyclization. Molecular dynamics simulation provided further insights on the stability and molecular motion of the side chain cross-linked peptides.

Towards a Structural Basis for the Relationship Between Blood Group and the Severity of El Tor Cholera

It has long been known that people with blood group O are more severely affected by El Tor cholera than those with blood groups A or B. Microcalorimetry and NMR spectroscopy are used to evaluate the ability of the B-subunits of cholera toxin and E. coli heat-labile toxin to bind to selected blood group oligosaccharides.

A Multicomponent Conjugation Strategy to Unique N‐Steroidal Peptides: First Evidence of the Steroidal Nucleus as a β‐Turn Inducer in Acyclic Peptides

Constraining small peptides into specific secondary structures has been a major challenge in peptide ligand design. So far, the major solution for decreasing the conformational flexibility in small peptides has been cyclization. An alternative is the use of topological templates, which are able to induce and/or stabilize peptide secondary structures by means of covalent attachment to the peptide. Herein a multicomponent strategy and structural analysis of a new type of peptidosteroid architecture having the steroid as N-substituent of an internal amide bond is reported. The approach comprises the one-pot conjugation of two peptide chains (or amino acid derivatives) to aminosteroids by means of the Ugi reaction to give a unique family of N-steroidal peptides. The conjugation efficiency of a variety of peptide sequences and steroidal amines, as well as their consecutive head-to-tail cyclization to produce chimeric cyclopeptide-steroid conjugates, that is, macrocyclic lipopeptides, was assessed. Determination of the three-dimensional structure of an acyclic N-steroidal peptide in solution proved that the bulky, rigid steroidal template is capable of both increasing significantly the conformational rigidity, even in a peptide sequence as short as five amino acid residues, and inducing a β-turn secondary structure even in the all-s-trans isomer. This report provides the first evidence of the steroid skeleton as β-turn inducer in linear peptide sequences.

On the unusual 2J C 2H f coupling dependence on syn/anti CHO conformation in 5‐X‐furan‐2‐carboxaldehydes

A remarkable difference for 2J  C 2H f coupling constant in syn and anti conformers of 5‐X‐furan‐2‐carboxaldehydes (X = CH3, Ph, NO2, Br) and a rationalization of this difference are reported. On the basis of the current knowledge of the Fermi‐contact term transmission, a rather unusual dual‐coupling pathway in the syn conformer is presented. The additional coupling pathway resembles somewhat that of the JHH in homoallylic couplings, which are transmitted by hyperconjugative interactions involving the πCC electronic system. The homoallylic coupling pathway can be labeled as σ*CH ← πCC → σ*CH. In the present case, this additional coupling pathway, using an analogous notation, can be labeled as σ*  C 2C C ← LP1(O1)··· LP2(OC) →σ*  C CH f (σ*  C 2C C ) where O1 and OC stand for the ring and carbonyl O atoms, respectively. This additional coupling pathway is not activated in the anti conformers since both oxygen lone pairs do not overlap. Copyright

Stereochemistry of complex marine natural products by quantum mechanical calculations of NMR chemical shifts: solvent and conformational effects on okadaic acid.

Marine organisms are an increasingly important source of novel metabolites, some of which have already inspired or become new drugs. In addition, many of these molecules show a high degree of novelty from a structural and/or pharmacological point of view. Structure determination is generally achieved by the use of a variety of spectroscopic methods, among which NMR (nuclear magnetic resonance) plays a major role and determination of the stereochemical relationships within every new molecule is generally the most challenging part in structural determination. In this communication, we have chosen okadaic acid as a model compound to perform a computational chemistry study to predict 1H and 13C NMR chemical shifts. The effect of two different solvents and conformation on the ability of DFT (density functional theory) calculations to predict the correct stereoisomer has been studied.

In vitro Cytotoxicity of Norditerpenoid Alkaloids

Abstract Forty-three norditerpenoid alkaloids isolated from Aconitum, Delphinium and Consolida species have been evaluated for their cytotoxic effects on the tumor cell lines CT26 (murine colon adenocarcinoma), SW480 (human colon adenocarcinoma), HeLa (human cervical adenocarcinoma), SkMel25 (human melanoma) and SkMel28 (human malignant melanoma) with several multidrug resistance mechanisms and the non-tumor cell line CHO (Chinese hamster ovary cells). Neoline (5), 8-O-methylcolumbianine (6), 1,14-diacetylcardiopetaline (9), 18-O-demethylpubescenine (13), 14-deacetylpubescenine (14), pubescenine (15), 14-deacetylajadine (25), lycoctonine (26), browniine (28), delphatine (29), dehydrotakaosamine (34), and ajadelphinine (37) exhibited selective cytotoxicity to cancerous versus non-cancerous cells. Some of these compounds had an irreversible effect on SW480 (5, 15, 25, 26, and 34), HeLa (15, 34, and 37) and SkMel25 (15 and 34) cell lines. In order to gain insights into the mechanism of irreversible cytotoxic action of these compounds we compared the cell viability by means of the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) and the acid phosphatase (AP) methods. Our results suggest that the effects of these compounds could be related to the inhibition of ATP production

Drug-dendrimer supramolecular complexation studied from molecular dynamics simulations and NMR spectroscopy

Fully atomistic molecular dynamics (MD) simulations and NMR spectroscopy were employed to get insights about the molecular details of drug-dendrimer supramolecular association phenomena, using piroxicam (PRX) and the third generation poly(amido amine) (PAMAM-G3) dendrimer as model systems. Theoretical results concerning the complex stoichiometry suggest that PRX forms drug-dendrimer complexes of the type 24:1 at pH 7.0. This result was validated with the experimental quantities obtained from aqueous solubility profiles, which led to an empiric stoichiometry of 23:1 for the PRX:PAMAM-G3 system. The predicted binding mode between PRX and PAMAM-G3 accounts for the preferred encapsulation of the drug inside dendrimer cavities, which is mainly driven by van der Waals and hydrogen bonding interactions, and to a lesser extent, for the external association of the guest through electrostatic contacts with the positively charged amino groups of PAMAM periphery. The binding mode obtained from MD simulations was confirmed with 2D-NOESY experiments, which evidence the preferred internal complexation of PRX with PAMAM-G3. The predominance of internal encapsulation over external contacts in the PRX:PAMAM-G3 system differs from the general behaviour expected for acidic anionic guests, for which external electrostatic interactions with the positively charged PAMAM surface have been postulated as the most relevant factor for drug association.