13C-NMR Chemical Shifts in 1,3-Benzazoles as a Tautomeric Ratio Criterion

Benzimidazole is an important heterocyclic fragment, present in many biologically active compounds with a great variety of therapeutic purposes. Most of the benzimidazole activities are explained through the existence of 1,3-tautomeric equilibrium. As the binding affinity of each tautomer to a protein target depends on an established bioactive conformation, the effect of tautomers on the ligand protein binding mechanism is determinant. In this work, we searched and analyzed a series of reported ¹³C-NMR spectra of benzazoles and benzazolidine-2-thiones with the purpose of estimating their tautomeric equilibrium. Herein, several approaches to determine this problem are presented, which makes it a good initial introduction to the non-expert reader. This chemical shift difference and C4/C7 signals of benzimidazolidine-2-thione and 1-methyl-2-thiomethylbenzimidazole as references were used in this work to quantitatively calculate, in solution, the pyrrole–pyridine tautomeric ratio in equilibrium. The analysis will help researchers to correctly assign the chemical shifts of benzimidazoles and to calculate their intracyclic or exocyclic tautomeric ratio as well as mesomeric proportion in benzimidazoles.     

Interference of ruthenium red analogues at photosystem II of spinach thylakoids

The effect of ruthenium red analogues on several thylakoid photosynthetic activities has been investigated. RR, RV, RRPh1, RRPh2 and Ph inhibit ATP synthesis and electron flow from water to MV (basal, phosphorylating and uncoupled) as their concentration increases, thus, they act as a Hill reaction inhibitor. They inhibit uncoupled electron transport through PSII from water to DCPIP and partially from DPC to DCPIP. However, these compounds do not affect uncoupled PSI electron transport from DCPIP to MV. Therefore, the target of interaction is at the level of OEC and the span P(680) to Q(A) for RR, RRPh1 and RRPh2. Chlorophyll a fluorescence studies corroborate the already found interference sites and may affect the disconnection between chlorophyll molecules within the LHCII and/or between antennae and RCs, or decreases the exciton to reach the RC and inhibition of PSII occurs. RRPh2 is six times more active than RR. Finally, Ph inhibits electron flow interacting at the level of Q(B).     

Regioselective synthesis of 2,4,6-triaminopyridines

A regioselective synthesis of 2,4,6-trisubstituted pyridine is described starting from 2,6-dibromo-4-nitropyridine. All three different regioisomers of the 2,4,6-triamino substituted pyridine have been synthesized in four to five steps. The method described is a general route to unsymmetrical 2,4,6-trisubstituted amino pyridines.     

Characterization of disk media

It has been shown that the scalar properties of recording media can be characterized by a moving model that includes reversible and irreversible magnetizing processes. A test of this model for various media is reported in this paper. It is found that the model is capable of calculating accurate major and minor loops.     

Pictorial representation of three-dimensional electron distributions through a perspective view of contour diagrams in a set of parallel planes

A method of graphically exhibiting detailed information about a three-dimensional electron distribution function f(x,y,z) is described. Contour lines f = constant are drawn on a set of equidistant parallel planes that intersect the distribution, and a perspective view of all contours on all planes is displayed. Representative examples are given.     

Galois module structure of Tate modules

Dedicated to Professor Peter Roquette Received 1 February 1996; in final form 26 April 1996