Ceramicines U-Z from Chisocheton ceramicus and structure-antimalarial activity relationship study.

Ceramicines are a series of limonoids which were isolated from the barks of Malaysian Chisocheton ceramicus (Meliaceae), and were known to show various biological activity. Six new limonoids, ceramicines U-Z (1-6), with a cyclopentanone[α]phenanthrene ring system with a ß-furyl ring at C-17 were isolated from the barks of C. ceramicus. Their structures were determined on the basis of the 1D and 2D NMR analyses, and their absolute configurations were investigated by CD spectroscopy. Ceramicine W (3) exhibited potent antimalarial activity against Plasmodium falciparum 3D7 strain with IC50 value of 1.2 µM. In addition, the structure-antimalarial activity relationship (SAR) of the ceramicines was investigated to identify substituent patterns that may enhance activity. It appears that ring B and the functional groups in the vicinity of rings B and C are critical for the antimalarial activity of the ceramicines. In particular, bulky ester substituents with equatorial orientation at C-7 and C-12 greatly increase the antimalarial activity.

Effects of N-Methylated Amyloid-ß30-40 Peptides on the Fibrillation of Amyloid-ß1-40.

Alzheimer's disease is a neurodegenerative disorder associated with amyloid-ß (Aß) fibrillation. N-Methylated amyloid-ß peptides are potent inhibitors of amyloid-ß fibrillation. We investigated the inhibitory effect of N-Methylated Aß30-40 peptides on Aß1-40 fibrillation. N-Methylated Aß30-40 peptides affected the fibrillation, and this effect was dependent on the concentration of N-Methylated peptide and the number and position of N-Methylated groups. N-Methylated Aß30-40 peptides were co-aggregated with Aß1-40 . Spectroscopic technique was adopted to investigate an origin of the observed dependence. Suppression of thioflavin T (ThT) fluorescence count was correlated with the dissociation constant Kd of monomer-dimer equilibrium of each N-Methylated Aß30-40 peptide. Monomeric N-Methylated peptides decreased ThT fluorescence count during Aß1-40 fibrillation. Secondary structure content was not largely different between Aß1-40 fibrils and co-aggregates. These results suggested that N-Methylated Aß30-40 peptides disrupted the regular ß-sheet structure of Aß1-40 fibrils and affected the ThT fluorescence count. The monomer-dimer equilibrium of N-Methylated peptides was (partly) responsible for the observed dependence of their inhibitory effect on the concentration of N-Methylated peptide and the number and position of N-Methylated groups. Our study provides a hint to design new N-Methylated inhibitor peptides of fibrillation.

The structure of metallo-DNA with consecutive thymine-HgII-thymine base pairs explains positive entropy for the metallo base pair formation.

We have determined the three-dimensional (3D) structure of DNA duplex that includes tandem Hg(II)-mediated T-T base pairs (thymine-Hg(II)-thymine, T-Hg(II)-T) with NMR spectroscopy in solution. This is the first 3D structure of metallo-DNA (covalently metallated DNA) composed exclusively of 'NATURAL' bases. The T-Hg(II)-T base pairs whose chemical structure was determined with the (15)N NMR spectroscopy were well accommodated in a B-form double helix, mimicking normal Watson-Crick base pairs. The Hg atoms aligned along DNA helical axis were shielded from the bulk water. The complete dehydration of Hg atoms inside DNA explained the positive reaction entropy (ΔS) for the T-Hg(II)-T base pair formation. The positive ΔS value arises owing to the Hg(II) dehydration, which was approved with the 3D structure. The 3D structure explained extraordinary affinity of thymine towards Hg(II) and revealed arrangement of T-Hg(II)-T base pairs in metallo-DNA.

Raman spectroscopic detection of the T-Hg II-T base pair and the ionic characteristics of mercury.

Developing applications for metal-mediated base pairs (metallo-base-pair) has recently become a high-priority area in nucleic acid research, and physicochemical analyses are important for designing and fine-tuning molecular devices using metallo-base-pairs. In this study, we characterized the Hg(II)-mediated T-T (T-Hg(II)-T) base pair by Raman spectroscopy, which revealed the unique physical and chemical properties of Hg(II). A characteristic Raman marker band at 1586 cm(-1) was observed and assigned to the C4=O4 stretching mode. We confirmed the assignment by the isotopic shift ((18)O-labeling at O4) and density functional theory (DFT) calculations. The unusually low wavenumber of the C4=O4 stretching suggested that the bond order of the C4=O4 bond reduced from its canonical value. This reduction of the bond order can be explained if the enolate-like structure (N3=C4-O4(-)) is involved as a resonance contributor in the thymine ring of the T-Hg(II)-T pair. This resonance includes the N-Hg(II)-bonded state (Hg(II)-N3-C4=O4) and the N-Hg(II)-dissociated state (Hg(II+) N3=C4-O4(-)), and the latter contributor reduced the bond order of N-Hg(II). Consequently, the Hg(II) nucleus in the T-Hg(II)-T pair exhibited a cationic character. Natural bond orbital (NBO) analysis supports the interpretations of the Raman experiments.

Spectroscopic analysis of AgI-DNA complex.

It has recently been reported that C-C mismatches were able to recognize Ag(I) specifically, and form Ag(I)-mediated cytosine-cytosine base pairs (C-Ag(I)-C pair). In this study, we explored DNA sequences, which are appropriate for structural studies of this new type of metal-mediated base pairs. For this experiments, we synthesized four kinds of DNA duplexes including a single C-C mismatch. By using UV spectra, we performed titration experiments of DNA duplexes with AgNO(3). Within the sequences we tested, it was found that the DNA duplex: 5'-CGCGCGTCC-3' x 5'-GGACCCGCG-3' showed 1:1 complexation of this DNA duplex and Ag(I). In addition, a DNA duplex: 5'-TAAATCTAATA-3' x 5'-TATTACATTTA-3' also exhibited a similar character. Therefore, these two DNA duplexes are interesting candidates for structural analyses.