Equilibria of polyoxometalates in aqueous solution

In most aqueous polyoxometalate systems, numerous, often highly negatively charged species, are formed. To establish the speciation in such complex polyanion systems, many experimental methods and techniques must be used. Moreover, it is of vital importance that the experimental data are of the highest accuracy and that the data collected from different methods are treated simultaneously with an appropriate computer program. In systems containing one or more sensitive NMR nuclei, a combined EMF-NMR method has been shown to be extremely powerful.This article firstly gives some general comments on equilibria in aqueous polyoxometalate systems including ionic medium effects, and then describes an equilibrium EMF-NMR (³¹P and⁵¹V) study of the five component system H⁺-Mo(VI)-V(V)-P(V)-e^–. The study has been focused on so-called Keggin ratio solutions, Mo+V):P=12:1, since these are commonly used in selective oxidation processes. Special attention has been given to Mo₁₀V₂P and Mo₉V₃P solutions, where positional isomers occur. We have been able to identify and characterize all the five possible isomers of -Mo₁₀V₂PO ₄₀ ^5– at 25 and 90 °C. Besides the results from the five component system, some interesting findings from the binary, ternary and quarternary sybsystems are also reported.     

New aspects of NMR spectroscopy of polyoxometallates

A range of 1D and 2D NMR methods has been developed for the investigation of polyoxometallate solutions, to complement and extend the insights obtained from crystallography and potentiometry. Static, dynamic and structural information can be obtained from¹H,¹⁷O,⁵¹V and¹⁸³W NMR. Applications are given in the elucidation of the structures and chemical interrelationships of isopolytungstates and vanadates, molybdotungstates, tungstovanadates and molybdovanadates.     

Chemodiversity and α-Glucosidase Activity of Eucalyptus Species from Northwestern Himalaya,India

The aim of current work was to determine essential oils (EOs) composition from three Eucalyptus species, including E. citriodora, E. camaldulensis and E. globulus and assess their α-glucosidase inhibitory activity. The EOs were collected using the hydrodistillation technique and characterized by GC/MS, GC-FID and NMR. The isolated EOs from leaves parts of Eucalyptus species varied from 0.56 to 1.0 % on fresh weight basis. The content of the EOs was distinct according to the species. The most abundant metabolites were identified as citronellal (0–83.0 %), 1,8-cineole (0.2–44.8 %), spathulenol (0.4–16.1 %) α-pinene (0.4–15.9 %), p-cymene (3.7–11.9 %), citronellol (0–8.6 %), β-eudesmol (5.3–8.6 %) and β-pinene (0–7.1 %). The EOs obtained from targeted samples exhibited strong α-glucosidase inhibitory activity. These results are encouraging and underline that the EOs of Eucalyptus species may be a promising alternative source of natural antidiabetic.     

Antibacterial Compound Isolation and Characterization from the Plant Cynotis axillaris Schult

A novel flavone glycoside was isolated from the methanolic extract of Cynotis axillaris Schult. Various analysis and characterization techniques were used to determine its structure and properties. The compound exhibited a melting point range of 231–232 °C and had a molecular formula of C₂₇H₃₀O₁₄. Several spectral characterization techniques were employed to establish the isolated compound's structure. These included UV-visible spectroscopy, FT-IR, LC-ESI-MS, and NMR spectroscopy. Based on these analyses, the structure of the isolated compound was determined to be 5,7,4’-trihydroxyflavone-8-α-L-rhamnopyranoside-4’-O-β-D-galactopyranosyl. This structure indicates that it is a flavone glycoside consisting of a flavone (5,7,4’-trihydroxyflavone) moiety attached to a sugar molecule (galactopyranosyl) at position 4’, which further bears a rhamnose group at position 8 of the flavone. In addition, to the structural characterization, the compound also demonstrated significant antibacterial efficacy against various bacterial pathogens, including Gram-positive bacteria such as Bacillus subtilis MTCC441 and Gram-negative bacteria such as Escherichia coli MTCC1098, Proteus vulgarize MTCC426, and Salmonella Typhimurium MTCC3224. The antimicrobial activity was evaluated by measuring the zone of inhibition in millimetres, which provides an indication of the compound's ability to inhibit bacterial growth. The study successfully identified and characterized a novel flavone glycoside from Cynotis axillaris Schult. and its antimicrobial activity.     

Insights into the local residual entropy of proteins provided by NMR relaxation.

A simple model is used to illustrate the relationship between the dynamics measured by NMR relaxation methods and the local residual entropy of proteins. The expected local dynamic behavior of well-packed extended amino acid side chains are described by employing a one-dimensional vibrator that encapsulates both the spatial and temporal character of the motion. This model is then related to entropy and to the generalized order parameter of the popular "model-free" treatment often used in the analysis of NMR relaxation data. Simulations indicate that order parameters observed for the methyl symmetry axes in, for example, human ubiquitin correspond to significant local entropies. These observations have obvious significance for the issue of the physical basis of protein structure, dynamics, and stability.     

Secondary structure of β-hydroxydecanoyl thiol ester dehydrase, a 39-kDa protein, derived from Hα, Cα, Cβ and CO signal assignments and the Chemical Shift Index: Comparison with the crystal structure

Summary Nearly complete backbone ¹H, ¹⁵N and ¹³C signal assignments are reported for -hydroxydecanoyl thiol ester dehydrase, a 39-kDa homodimer containing 342 amino acids. Although ¹⁵N relaxation data show that the protein has a rotational correlation time of 18 ns, assignments were derived from triple-resonance experiments recorded at 500 MHz and pH 6.8, without deuteration. The Chemical Shift Index, CSI, identified two long helices and numerous -strands in dehydrase. The CSI predictions are in close agreement with the secondary structure identified in the recently derived crystal structure, particularly when one takes account of the numerous bulges in the -strands. The assignment of dehydrase and a large deuterated protein [Yamazaki et al. (1994) J. Am. Chem. Soc., 116, 11655–11666] suggest that assignment of 40–60 kDa proteins is feasible. Hence, further progress in understanding the chemical shift/structure relationship could open the way to determine the structures of such large proteins. Supplementary Material is available on request, comprising Table S1 listing the spectral parameters; Table S2 listing the assignments; Fig. S1 showing the 2D ¹H–¹⁵N HSQC spectrum; Fig. S2 showing sequential NOEs, secondary shifts, H-exchange and ³J_HN data; and Fig. S3 showing plots of the H, C, CO and C Chemical Shift Indexes.To whom correspondence should be addressed.     

Possible role of a short extra loop of the long-chain flavodoxin from Azotobacter chroococcum in electron transfer to nitrogenase: Complete 1H, 15N and 13C backbone assignments and secondary solution structure of the flavodoxin

Summary The ¹H, ¹⁵N and ¹³C backbone and ¹H and ¹³C beta resonance assignments of the long-chain flavodoxin from Azotobacter chroococcum (the 20-kDa nifF product, flavodoxin-2) in its oxidized form were made at pH 6.5 and 30°C using heteronuclear multidimensional NMR spectroscopy. Analysis of the NOE connectivities, together with amide exchange rates, ³J_Hn_H coupling constants and secondary chemical shifts, provided extensive solution secondary structure information. The secondary structure consists of a five-stranded parallel -sheet and five -helices. One of the outer regions of the -sheet shows no regular extended conformation, whereas the outer strand 4/6 is interrupted by a loop, which is typically observed in long-chain flavodoxins. Two of the five -helices are nonregular at the N-terminus of the helix. Loop regions close to the FMN are identified. Negatively charged amino acid residues are found to be mainly clustered around the FMN, whereas a cluster of positively charged residues is located in one of the -helices. Titration of the flavodoxin with the Fe protein of the A. chroococcum nitrogenase enzyme complex revealed that residues Asn¹¹, Ser⁶⁸ and Asn⁷² are involved in complex formation between the flavodoxin and Fe protein. The interaction between the flavodoxin and the Fe protein is influenced by MgADP and is of electrostatic nature.Abbreviations SQ semiquinone - FMN riboflavin 5-monophosphate; nif, nitrogen fixation - TSP 3-(trimethylsilyl)propionate sodium salt - DSS 2,2-dimethyl-2-silapentane-5-sulfonate sodium salt Supplementary Material is available on request, comprising a Materials and Methods section for the expression and purification of the A. chroococcum flavodoxin, a Table S1 containing the parameters of the titration of A. chroococcum flavodoxin with the Fe protein, and a Table S2 containing the ¹⁵N, H^N, ¹³C, ¹H, ¹³C, ¹H and ¹³CO chemical shifts.To whom correspondence should be addressed.     

Through-bond correlation of adenine H2 and H8 protons in unlabeled DNA fragments by HMBC spectroscopy

Summary A new application of the HMBC experiment is presented that provides a useful means to discriminate between H2 and H8 proton resonances, to assign the base proton resonances to the various residue types and, most importantly, to correlate the H2 and H8 protons for adenine or inosine residues in natural abundance ¹³C fragments. The utility of this experiment is demonstrated for an unlabeled DNA 20-mer. Thanks to the obtained results, preliminary conclusions could be drawn regarding the molecular conformations of the non-canonical G/I-A base pairs in the hairpin formed by this fragment.