Controlling the self-assembly of a deoxiguaninosine on mica

We report a tapping mode scanning force microscopy investigation on the growth from solutions of a lipophilic deoxiguanosine derivative on the basal plane of a muscovite mica surface. By varying the chemical composition of the substrate surface it is possible to drive the self-assembly towards either layered thin films, which are very likely to consist of guanosine quartets, or nanoribbons, comprising hydrogen bonded networks. Both these two architectures, which are peculiar to the guanine base itself, are of notable importance for the fabrication of molecular nanowires. Moreover, the first arrangement provides evidence that the guanosine is able to behave as ionophore, thus, suggesting its utilization in the development of solid-state ion selective sensors.     

Directed evolution experiments reveal mutations at cycloartenol synthase residue His477 that dramatically alter catalysis

[reaction: see text] Cycloartenol synthase cyclizes and rearranges oxidosqualene to the protosteryl cation and then specifically deprotonates from C-19. To identify mutants that deprotonate differently, randomly generated mutant cycloartenol synthases were selected in a yeast lanosterol synthase mutant. A novel His477Asn mutant was uncovered that produces 88% lanosterol and 12% parkeol. The His477Gln mutant produces 73% parkeol, 22% lanosterol, and 5% Delta(7)-lanosterol. These are the most accurate lanosterol synthase and parkeol synthase that have been generated by mutagenesis.     

Solution and X-Ray Crystal Structures of the Di- and Tetra-allyl Ether of tert- utylcalix[4]arene

The di- and tetra-allyl ethers of tert-butylcalix[4]arene 1 and 2 have been prepared by alkylation of tert-butylcalix[4]arene with allyl bromide and K₂CO₃ using different reaction times. Solution ¹H NMR measurement of the di-allyl ether 1 and X-ray crystal structures of the complexes of 1 with chloroform (1a) or methanol (1b) indicate the cone conformation of 1 in which intramolecular hydrogen bonding can be maximized. The crystalline state conformers 1a and 1b are distorted in different grades depending on the solvent. While methanol is incorporated in the macrocycle, chloroform molecules do not occupy the cage. The solution ¹H NMR spectra of tetra-allyl ether 2 show the co-existence of the cone and partial cone conformation. The partial cone conformer of 2 was investigated by X-ray crystallography. In this compound hydrogen bonding is not existent. The conformer distribution is likely affected by steric and template effects.     

Stereochemical analysis of 3,4-methylenedioxymethamphetamine and its main metabolites by gas chromatography/mass spectrometry

3,4-Methylenedioxymethamphetamine (MDMA, ecstasy) is consumed as the racemate but some metabolic steps are enantioselective. In addition, chiral properties are preserved during MDMA biotransformation. A quantitative analytical methodology using gas chromatography/mass spectrometry (GC/MS) to determine enantioselective disposition in the body of MDMA and its main metabolites including 3,4-methylenedioxyamphetamine (MDA), 4-hydroxy-3-methoxymethamphetamine (HMMA), and 4-hydroxy-3-methoxyamphetamine (HMA) was developed. Plasma and urine samples were collected from a male volunteer. The analysis of MDMA, MDA, and 4-hydroxy-3-methoxy metabolites by GC/MS required a two-step derivatization procedure. The first step consisted of derivatization of the amine with enantiomerically pure Mosher's reagent ((R)-MTPCl). Triethylamine was used as a base to neutralize hydrochloric acid formed during the reaction allowing quantitative derivatization, which resulted in a substantial improvement in the sensitivity of the method compared with other previously described techniques. Further treatment with ammonium hydroxide was required since both amine and hydroxyl groups underwent derivatization in the reaction. Ammonium hydroxide breaks bonds formed with hydroxyl groups without affecting amine derivatives. The second derivatization step using hexamethyldisilazane was needed for metabolites containing phenol residues. This derivatization method permitted the stereochemically specific study of MDMA and its main monohydroxylated metabolites by GC/MS. A detailed study of the chemical reactions involved in the derivatization steps was indispensable to develop a straightforward, sensitive, and reproducible method for the analysis of the parent drug compound and its metabolites.     

Exceptional kinetic propensity of hydroxymethyl phosphanes toward Rh(III) stabilization in water

The reactions of (HOCH2)2P(C6H4)P(CH2OH)2 (HMPB) and P(CH2OH)3 (THP) with RhCl3.xH2O in aqueous media gave water-soluble complexes cis-[RhCl2{eta2-(HOCH2)2P(C6H4)P(CH2OH)2}2]Cl (3) and fac-[RhCl3(P(CH2OH)3)3] (4) respectively in good yields, X-ray crystal structures of 3 and 4 confirmed their molecular constitution. These reactions provide the first examples demonstrating the kinetic propensity of hydroxymethyl phosphanes to stabilize Rh in +3 oxidation state in water.     

A PHYTOCHROME PHOTOTRANSFORMATION STUDY USING TWO-LASER/TWO-COLOR FLASH PHOTOLYSIS: ANALYSIS OF THE DECAY MECHANISM OF I700*

The mechanism of I₇₀₀ decay, representing an early event in the phytochrome P_r→ P_fr phototransformation, was reanalyzed in the microsecond range by conventional laser flash photolysis as well as by two-laser/two-color flash photolysis. Three kinetic models that might describe the I₇₀₀ decay mechanism following P_r excitation were considered: a parallel, a sequential, and an equilibrium model. These models were used to mathematically simulate both the one- and two-laser flash experiments in an effort to select the model best describing the I₇₀₀ decay. The sequential model could be excluded already on the basis of the one-laser flash photolysis results alone. Discussion of the two-laser/two-color flash rcsults in the context of the equilibrium and the parallel models is presented.     

EFFECTS OF BOUND MONOCLONAL ANTIBODIES ON THE DECAY OF THE PHOTOTRANSFORMATION INTERMEDIATES I1,21700 FROM NATIVE Avena PHYTOCHROME*

Abstract–Thc kinetics of the microsecond phototransformation intermediates of 124 kDa Avena phytochrome (1700^1,2) were studied in the prcsence of bound monoclonal antibodies at various temperatures. A global analysis was applied to the decays at all wavelengths at each temperature in order to derive the rate constants and the decay-associated spectra of the three decay components. Monoclonal antibodies bound to specific epitopes altered the Arrhenius parameters of both 1700^1,2 decay components. The strongest influence on these parameters was observed with OAT 8 (epitope between residues 624 and 686), which decreased by more than 50% the activation parameters of both components. This decrease is interpreted to result from an increased flexibility induced by this antibody in the ground state or in the transition state of bonds changing during the decay of both 1700 transients. Thus, the OAT 8 cpitope appears to be functionally important during the decay of the 1700^1,2 intermediates. For the case of 1100¹ bound OAT 23 and OAT 25 (epitopes between residues 1 and 66) reduced even further the relatively small flexibility of these bonds in the red light-absorbing form of phytochrome (P1) without antibodies, as reflected by the high preex-ponential factors for its decay. This resulted also in higher activation energies for this decay in the presence of the antibodies. Thus, the amino-terminus should act as a rigid spacer of the chromophore cavity without affecting it during the microsecond transformation, because the Arrhenius parameters for these decays are similar to those for small phytochrome. The possible implications of the influence of the various antibodies on the bleaching remaining after the decay of 1700^1,2 are discussed.     

Influence of silanization on voltammetry at electrodes modified with silica films of controlled porosity formed by electrochemically initiated sol-gel processing

Silica sol-gel (SG) films with templated pores were deposited on glassy carbon (GC) electrodes by an electrochemically initiated process. Generation-4 poly(amidoamine), PAMAM, dendrimer was included in the tetraethoxysilane precursor to facilitate pore formation. The PAMAM adsorbs to the GC, which blocks SG formation at those sites on the electrode. The pore size was 10?±?5?nm. After removal of the PAMAM, cyclic voltammetry of Fe(CN)₆ ^3? and Ru(NH₃)₆ ³⁺ at pH?6.2 showed that the residual negative charge on the silica attenuated the current for the former and increased the current for the latter, presumably by electrostatic repulsion and ion-exchange preconcentration, respectively. This premise was supported by repeating the measurements at the isoelectric point. Methylation of the silanol sites was used to eliminate the charge of the SG. At the end-capped SG, the voltammetry of Fe(CN)₆ ^3? and Ru(NH₃)₆ ³⁺ yielded currents that were independent of pH over the range 2.1 to 7.2. Circumventing the need for the silanization by using (3-glycidyloxypropyl)trimethoxysilane as the sol-gel precursor failed because the oxygen plasma treatment to remove the PAMAM attacked the organically modified sol-gel backbone. The resulting modified electrode mitigated the influence of proteins on the voltammetry of test species and stabilized functionalize nanoparticle catalysts under hydrodynamic conditions.     

Establishing the synthetic origin of amphetamines by 2H NMR spectroscopy

Nine samples of N-acetyl-3,4-methylenedioxyamphetamine (N-acetyl-MDA), prepared according to the most common synthetic procedures, are submitted to (2)H NMR spectroscopy. The relative deuterium content at the various sites of the molecule is shown to depend on its synthetic history. The technique provides a chemical fingerprint of N-acetyl-MDAs and it can be used to trace back the precursor materials and the synthetic pathways employed in the preparation of the samples.     

From nitrides to carbides: topotactic synthesis of the eta-carbides Fe3Mo3C and Co3Mo3C

The molybdenum bimetallic interstitial carbides Fe(3)Mo(3)C and Co(3)Mo(3)C have been synthesized by temperature-programmed reaction (TPR) between the molybdenum bimetallic interstitial nitrides Fe(3)Mo(3)N and Co(3)Mo(3)N and a flowing mixture of CH(4) and H(2) diluted in Ar. These compounds have been characterized by X-ray diffraction, laser Raman spectroscopy, elemental analysis, energy dispersive analysis of X rays, thermal analysis (in air) and scanning electron microscopy (field emission). Their structures have been refined from X-ray powder diffraction data. These carbides crystallize in the cubic system, space group Fd3m[a= 11.11376(6) and 11.0697(3)[Angstrom] for Fe and Co compounds, respectively].