Spectrophotometric study of competitive complexation equilibria involving overlapped spectral responding species: Determination of the stability constant of bismuth-pyrophosphate complex

Spectrophotometric study of competitive complex formation equilibria involving overlapped spectral responding species applying a simple and versatile algorithm was carried out. The algorithm involves multivariable regression for calculation of equilibrium concentrations from multiwavelength data and mass action law for the stability constant calculation. The used regression functions are part of common statistical software. Stability constants and complex stoichiometry of competing equilibria were simultaneously determined. The species concentration profiles at several spectral overlapping and ??-coefficient of competing reaction were obtained. Non-absorbing bismuth ?? pyrophosphate (PPh) system was studied as a competitive reaction of bismuth ?? 4-(2-Pyridylazo) resorcinol (PAR) complex. The formation of Bi-PPh complex with 1:1 stoichiometry was proved in the studied concentration region (C_Bi = 1×10^?5 mol L^?1; C_PPh = 5×10^?6 ? 1×10^?4 mol L^?1). The stability constant of the complex at pH 1 and ?? = 1.0 have been determined: log?? = 4.2±0.2.      

Comparative Hydrophobic Core Dynamics Between Wild-Type Amyloid-β Fibrils,Glutamate-3 Truncation,and Serine-8 Phosphorylation

Post-translational modifications (PTMs) of amyloid-β (Aβ) species are implicated in the modulation of overall toxicities and aggregation propensities. We investigated the internal dynamics in the hydrophobic core of the truncated ΔE3 mutant fibrils of Aβ_1–40 and compared them with prior and new data for wild-type fibrils as well as with phosphorylated S8 fibrils. Deuteron static solid-state NMR techniques, spanning line-shape analysis, longitudinal relaxation, and chemical exchange saturation transfer methods, were employed to assess the rotameric jumps of several methyl-bearing and aromatic groups in the core of the fibrils. Taken together, the results indicate the rather significant influence of the PTMs on the hydrophobic core dynamics, which propagates far beyond the local site of the chemical modification. The phosphorylated S8 fibrils display an overall rigidifying of the core based on the higher activation barriers of motions than the wild-type fibrils, whereas the ΔE3 fibrils induce a broader variety of changes, some of which are thermodynamic in nature rather than the kinetic ones.     

Crystal Structure of an i-Motif from the HRAS Oncogene Promoter

An i-motif is a non-canonical DNA structure implicated in gene regulation and linked to cancers. The C-rich strand of the HRAS oncogene, 5′-CGCCC GTGCCC TGCGCCC GCAACCC GA-3′ (herein referred to as iHRAS), forms an i-motif in vitro but its exact structure was unknown. HRAS is a member of the RAS proto-oncogene family. About 19 % of US cancer patients carry mutations in RAS genes. We solved the structure of iHRAS at 1.77 Å resolution. The structure reveals that iHRAS folds into a double hairpin. The two double hairpins associate in an antiparallel fashion, forming an i-motif dimer capped by two loops on each end and linked by a connecting region. Six C−C⁺ base pairs form each i-motif core, and the core regions are extended by a G−G base pair and a cytosine stacking. Extensive canonical and non-canonical base pairing and stacking stabilizes the connecting region and loops. The iHRAS structure is the first atomic resolution structure of an i-motif from a human oncogene. This structure sheds light on i-motifs folding and function in the cell.     

Soft template synthesis of (2,8-dithio-3,7-diaza-5-oxanonandithioamide-1,9) nickel(II) and (2,7-dithio-3,6-diazaoctadien-3,5-dithioamide-1,8)nickel(II) using a nickel(II)hexacyanoferrate(II) gelatin-immobilized matrix

Complexing processes leading under specific conditions to nickel(II)hexacyanoferrate(II) gelatin-immobilized matrix systems in contact with aqueous-alkaline solutions (pH12) containing dithiooxamide and formaldehyde or glyoxal, have been studied. It has been shown that template synthesis of macrocyclic co-ordination compounds, (2,8-dithio-3,7-diaza-5-oxanonandithioamide-1,9)nickel(II) and (2,7-dithio-3,6-diazaoctadien-3,5-dithioamide-1,8)nickel(II), occurs. Dithiooxamide, formaldehyde and glyoxal act as ligand synthons in this process.     

Exploring the photoexcited triplet states of aluminum and tin corroles by time-resolved Q-band EPR

The photoexcited triplet states of three 5,10, 15-tris(pentafluorophenyl)corroles (tpfc), hosting Sn(IV) and Al(III) in their core, namely, Sn(Cl)(tpfc), Al(pyr)₂(tpfc) and Al(pyr)₂(tpfc-Br₈), were studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy in the nematic liquid crystal E7. Only two of these metallocorroles, namely, Sn(Cl)(tpfc) and Al(pyr)₂(tpfc-Br₈), exhibit TREPR spectra following pulsed laser excitation. This result is rationalized in terms of a very low quantum yield of triplet formation in Al(pyr)₂(tpfc). Analysis of the spin polarized Q-band (34 GHz) EPR spectra of Sn(Cl)(tpfc) and Al(pyr)₂(tpfc-Br₈) provides detailed information on the magnetic and kinetic parameters of the triplet states as well as on the molecular ordering of the complexes in the liquid crystal. With the assignment of the zero-field splitting parameterD<0 for the Sn(Cl)(tpfc) and Al(pyr)₂(tpfc-Br₈), one can evaluate the dominant intersystem crossing path for these metallocorroles. Analysis reveals that in Sn(Cl)(tpfc) the in-plane triplet sublevels are preferentially populated, i.e.,A _X, A_Y?A _Z. This can be rationalized in terms of weak electronic interactions between the Sn(IV) ion and the corrole π-system, consistent with the domed structure of Sn(Cl)(tpfc). In Al(pyr)₂(tpfc-Br₈), however, the out-of-plane triplet sublevel is predominantly populated, i.e.,A _Z>A _X, A_Y, which is attributed to a large increase in the spin-orbit coupling strength arising from the peripheral bromine atoms on the corrole skeleton.     

New Sulfur and Phosphorus Containing Metal Complexes on the Basis of Trithiophosphites. Synthesis and Properties

Abstract

New sulfur- and phosphorus containing metal complexes have been obtained by means of reactions of trialkyl- and triaryltrithiophosphites with transition metal halides. Their reactions with proton containing and carbonyl compounds have been studied.     

Reaction of bicyclo[2.2.1]hept‐5‐ene‐endo‐2‐ylmethylamine and nitrophenyl glycidyl ethers

Reactions of 2‐nitro‐, 4‐nitro‐ and 2,4‐dinitrophenylglycidyl ethers with bicyclo[2.2.1]hept‐5‐ene‐endo‐2‐ylmethylamine in isopropanol have been studied. The mixtures of products were chromatographed on silica gel and eluted with ether or ether/2‐propanol (1:1), the structures of individual products have been confirmed by IR spectra, NMR ¹H, ¹³C spectra, using experiments that involve homonuclear and heteronuclear scalar coupling interactions (COSY, TOCSY, HMQC, HMBC), and mass spectrometry. Amino alcohols as the major products of regioselective aminolysis of epoxides (according to the Krasusky rule) have been obtained. The minor products were the compounds with two hydroxyalkyl fragments at the nitrogen atom. In case of dinitrophenylglycidyl ether, it was the minor product of aryl nucleophilic substitution (S_NAr). The abnormal course of aminolysis has been confirmed by the results of quantum‐chemical calculations of activation barries and Free Gibbs energies of the competitive reactions of epoxides (at the B3LYP/6‐311 + G(d,p) level of theory). Copyright © 2010 John Wiley & Sons, Ltd.