The effect of substitution on protonation sites: evidence for protonation at N(3) in N(7)-substituted adenine

Several experimental and theoretical studies have shown that N(1) is the first site for protonation in adenine and N(9)-substituted adenine derivatives. N(7) is considered the site for the second protonation to yield dipositive cations. Results are reported here which indicate that this protonation pattern is altered in N(7)-substituted adenine derivatives. In particular, an X-ray diffraction analysis of the structure of 7-methyladenine dihydrochloride, [C₆N₅H₇]Cl₂, definitively shows that the sites for protonation are N(3), as opposed to N(1), and N(9). Theoretical calculations of the molecular electrostatic potential in various systems suggest that such changes in preferred protonation sites should be expected, in general, when a modification of the molecular structure creates adjacent sites with similar reactivity, such as the N(3) and N(9) sites in 7-methyladenine.     

VISUALIZATION OF ULTRAVIOLET LIGHT-INDUCED THYMINE DIMERS IN DNA BY IMMUNOELECTRON MICROSCOPY

Abstract— To see the damage of DNA due to ultravoilet-B more distinctly, immunoelectron microscopic studies using a monoclonal antibody against cyclobutane-type thymine dimers were performed. As a result, we could detect the existence of thymine dimers on human genomic DNA and _pUC18 plasmid DNA visually. This technique can be useful to locate the photoproducts formed on DNA.     

Sequencing wasp venom peptides by endopeptidase digestion and nested collision-induced dissociation/post-source decay methods

A method incorporating nested collision-induced dissociation/post-source decay (CID/PSD) combined with endopeptidase digestion is described as an approach to determine the sequence of N-terminally modified peptides. The information from immonium and related ions observed in the CID/PSD spectrum was used for the selection of a suitable endopeptidase for the digestion of peptides. Rapid and reliable assignment of peptide sequence was performed by the comparison of CID/PSD spectra of both intact and endopeptidese-digested peptide fragments, since the assignments of the observed fragment ions to either N- or C-terminal ions can thus be carried out unambiguously. This nested CID/PSD method was applied to the sequence determination of two peptides from the solitary wasps Anoplius samariensis and Batozonellus maculifrons (pompilid wasps), which could not be sequenced by the Edman method due to N-terminal modification.     

Theoretical assessment of the elastic constants and hydrogen storage capacity of some metal-organic framework materials

Metal-organic frameworks (MOFs) are promising materials for applications such as separation, catalysis, and gas storage. A key indicator of their structural stability is the shear modulus. By density functional theory calculations in a 106-atom supercell, under the local density approximation, we find c(11)=29.2 GPa and c(12)=13.1 GPa for Zn-based MOF 5. However, we find c(44) of MOF-5 to be exceedingly small, only 1.4 GPa at T=0 K. The binding energy E(ads) of a single hydrogen molecule in MOF-5 is evaluated using the same setup. We find it to be -0.069 to -0.086 eVH(2) near the benzene linker and -0.106 to -0.160 eVH(2) near the Zn(4)O tetrahedra. Substitutions of chlorine and hydroxyl in the benzene linker have negligible effect on the physisorption energies. Pentacoordinated copper (and aluminum) in a framework structure similar to MOF-2 gives E(ads) approximately -0.291 eVH(2) (and -0.230 eVH(2)), and substitution of nitrogen in benzene (pyrazine) further enhances E(ads) near the organic linker to -0.16 eVH(2), according to density functional theory with local density approximation.     

Synthesis and application of a new thiazolylazo reagent for cloud point extraction and determination of cobalt in pharmaceutical preparations

The synthesis and characterization of the reagent 2-(5-bromothiazolylazo)-4-chlorophenol and its application in the development of a preconcentration procedure for cobalt determination using flame atomic absorption spectrometry after cloud point extraction is presented. This procedure is based on cobalt complexing and entrapment of the metal chelates into micelles of a surfactant-rich phase of Triton X-114. The preconcentration procedure was optimized by using a response surface methodology through the application of the Box-Behnken matrix. Under optimum conditions, the procedure determined the presence of cobalt with an LOD of 2.8 microg/L and LOQ of 9.3 microg/L. The enrichment factor obtained was 25. The precision was evaluated as the RSD, which was 5.5% for 10 microg/L cobalt and 6.9% for 30 microg/L. The accuracy of the procedure was assessed by comparing the results with those found using inductively coupled plasma-optical emission spectrometry. After validation, the procedure was applied to the determination of cobalt in pharmaceutical preparation samples containing cobalamin (vitamin B12).     

Studies on the synthese of heterocyclic compounds. Part DXCV. The stereochemistry of methyl O-(4-hydroxy-3-methoxycinnamoyl)reserpate

The stereochemistry of the title compound (1), which has an antihypertensive activity, was confirmed by chemical and spectroscopical evidences.     

Synthesis and application of a functionalized resin in on-line system for copper preconcentration and determination in foods by flame atomic absorption spectrometry

An on-line system for preconcentration and determination of copper at μg l⁻¹ level by flame atomic absorption spectrometry (FAAS) is proposed. Amberlite XAD-2 functionalized with 3,4-dihydroxybenzoic acid packed in a minicolumn was used as sorbent. Copper(II) ions were sorbed in the minicolumn, from which it could be eluted by hydrochloric acid solution directly to the nebulizer-burner system of the FAAS. Eluent solution was carried by water at a flow rate of 5.00 ml min⁻¹. Signals were measured as peak height by using an instrument software. Achieved sampling rate was 27 samples per hour. Analytical parameters were evaluated and the results demonstrated that copper can be determined, with acetate buffer to adjust the sample pH at 6.0, preconcentration time of 120 s and a sample flow rate of 6.50 ml min⁻¹. The desorption was carried out with 30 μl of a 1.0 mol l⁻¹ hydrochloric acid solution. An enrichment factor of 33 in 13.00 ml of sample (120 s preconcentration time) was achieved by using the time-based technique. The detection limit (DL) (3 s) was 0.27 μg l⁻¹ and the precision (assessed as the relative standard deviation) reached values of 5.7-1.1% in copper solutions of 5.00 to 50.00 μg l⁻¹ concentration, respectively. The accuracy of procedure was confirmed by copper determination in certified reference materials. Recoveries of spike additions (1.0 or 2.0 μg g⁻¹) to food samples were quantitative (90.0-110.0%). These results proved also that the procedure is not affected by matrix interference and can be applied satisfactorily for copper determination in rice flour and starch samples.     

Thiazolylazo dyes and their application in analytical methods

An overview is presented on thiazolylazo dyes and their analytical applications in the determination, preconcentration or separation of trace metal ions. The article summarizes conventional analytical methods based on spectrophotometry, solid phase extraction, liquid chromatography and liquid–liquid and cloud point extraction. Both in-batch and on-line procedures are considered. General properties about these ligands are also commented on. Finally, the use of thiazolylazo dyes in procedures involving electroanalytical and other methods is presented.     

Spectrophotometric determination of aluminum with m-carboxyphenylfluorone, a novel chemical probe, and its application.

A highly spectrophotometric method for the determination of aluminum was developed. This method used the color reaction between m-carboxyphenylfluorone (MCPF) as a novel chemical probe and aluminum in the presence of a surfactant, poly(N-vinylpyrrolidone) (PVP, K-90) (0.03 - 1.40 microg of aluminum in a final volume of 10 ml at 561 nm). The proposed method showed excellent sensitivity (molar absorptibity of 1.70 x 10(5) l mol(-1) cm(-1)) and reproducibility (within-day precision: RSD = 0.35% n = 6, between-day precision: RSD = 0.44% n = 6). Linearity was achieved over the range 3 - 140 microg L(-1) with a correlation coefficient of 0.9999, and the effects of foreign substances were low.     

Estimation of molecular weights of peptides by determination of height equivalent to a theoretical plate in size-exclusion chromatography

The analysis of the flow-rate required to obtain the optimum height equivalent to a theoretical plate, HETP, has been used to estimate the molecular weight, MW, of peptides in size-exclusion chromatography. A straight line is obtained when log MW is plotted against the flow-rate which gives the optimum HETP. This relationship holds even for peptides which adsorb to column packings. The typical quantity of peptide required is 1 nmol and analysis time was less than 2 h. The method is applied to peptides of 200–10,000, and the precision is ±20%. The method may be applicable to the analysis of MW for all classes of compounds.