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This study describes a novel electrochemical approach to effective online monitoring of electroinactive [Ca.sup.2+] and [Mg.sup.2+] in the rat brain based on the current enhancement of divalent cations toward electrocatslytic oxidation of NADH. Cyclic voltammetry for NADH oxidation at the electrodes modified with the polymerized film of toluidine blue O (TBO) reveals that the current of such an electrocatulytic oxidation process is remarkably enhanced by divalent cations such as [Ca.sup.2+] and [Mg.sup.2+]. The current enhancement is thus used to constitute an electrochemical method for the measurements of [Ca.sup.2+] and [Mg.sup.2+] in a continuous-flow system with the polyTBO-modified electrode as the detector. Upon being integrated with in vivo microdialysis, the electrochemical method is successfully applied in investigating on cerebral [Ca.sup.2+] and [Mg.sup.2+] of living animals in two aspects: (1) online simultaneous measurements of the basal levels of [Ca.sup.2+] and [Mg.sup.2+] in the brain of the freely moving rats by using ethylenegleolbis(2-aminoethylether) tetraacetic acid (EGTA) as the selective masking agent for [Ca.sup.2+] to differentiate the net current responses selectively for [Ca.sup.2+] and [Mg.sup.2+]; and (2) online continuous monitoring of the cerebral [Mg.sup.2+] following the global ischemia by using [Ca.sup.2+]-masking agent (i.e., EGTA) to completely eliminate the interference from [Ca.sup.2+]. Compared with the existing methods for the measurements of cerebral [Ca.sup.2+] and [Mg.sup.2+], the method demonstrated here is advantageous in terms of its simplicity both in instrumentation and in the experimental procedures and near real-time nature, and is thus highly anticipated to find wide applications in understanding of chemical events involved in some physiological and pathological processes. 10.1021/ac102605n

[Mg.sup.2+] ions are very effective at stabilizing tertiary structures in RNAs. In most cases, folding of an RNA is so strongly coupled to its interactions with [Mg.sup.2+] that it is difficult to separate free energies of [Mg.sup.2+]- RNA interactions from the intrinsic free energy of RNA folding. To devise quantitative models accounting for this phenomenon of [Mg.sup.2+]-induced RNA folding, it is necessary to independently determine [Mg.sup.2+]-RNA interaction free energies for folded and unfolded RNA forms. In this work, the energetics of [Mg.sup.2+]-RNA interactions are derived from an assay that measures the effective concentration of [Mg.sup.2+] in the presence of RNA. These measurements are used with other measures of RNA stability to develop an overall picture of the energetics of [Mg.sup.2+]-induced RNA folding. Two different RNAs are discussed, a pseudoknot and an rRNA fragment. Both RNAs interact strongly with [Mg.sup.2+] when partially unfolded, but the two folded RNAs differ dramatically in their inherent stability in the absence of [Mg.sup.2+] and in the free energy of their interactions with [Mg.sup.2+]. From these results, it appears that any comprehensive framework for understanding [Mg.sup.2+]-induced stabilization of RNA will have to (i) take into account the interactions of ions with the partially unfolded RNAs and (ii) identify factors responsible for the widely different strengths with which folded tertiary structures interact with [Mg.sup.2+]. cations | ion interaction coefficients | Wyman linkage relations

The NMR solution structure of the Mg(super 2+)-loaded C lobe, cTnC(81-161) in a complex with the N domain of cardiac troponin I, cTnI-(33-80) is determined and compared with a refined Ca(super 2+)-loaded structure to understand the structural consequences of Ca(super 2+)/Mg(super 2+) exchange in the C lobe. A comparison of Ca(super 2+)- and Mg(super 2+)-loaded CcTnC structures suggests mechanisms by which Ca(super 2+)/Mg(super 2+) exchange and PKC phosphorylation of NcTnI could modulate the Ca(super 2+) signal.

The NMR solution structure of the Mg(super 2+)-loaded C lobe, cTnC(81-161) in a complex with the N domain of cardiac troponin I, cTnI-(33-80) is determined and compared with a refined Ca(super 2+)-loaded structure to understand the structural consequences of Ca(super 2+)/Mg(super 2+) exchange in the C lobe. A comparison of Ca(super 2+)- and Mg(super 2+)-loaded CcTnC structures suggests mechanisms by which Ca(super 2+)/Mg(super 2+) exchange and PKC phosphorylation of NcTnI could modulate the Ca(super 2+) signal.

The physical properties that determine the carboxylate-binding mode in metalloproteins are elucidated by surveying the Protein Data Bank (PDB) and performing density functional and continuum dielectric calculations. The carboxylate-binding mode is determined by competition between the metal cation, on one hand and nonacidic neighboring ligands from the metal inner or outer coordination sphere, on the other for the second O atom of the COO(super -) moiety.

Byline: M.T. Orzaez Villanueva (1), A. Diaz Marquina (1), B. Arribas de Diego (1), y G. Blazquez Abellan (1) Keywords: Key wordsaCereals; Macroelements; Atomic absortion spectroscopy Abstract: aIn this work is presented a study of the sodium, potassium, calcium and magnesium contents in 15 samples of the breakfast cereals that are most consumed by the Spanish population. We have found that they are of varied compositions, both in relation to the number of cereals which are the basis of their formulations, and to the rest of the ingredients added to enrich these products. This results in a wide variation in the macroelements contents, although all are found in significant quantities. In all samples, sodium was the predominant cation, reaching levels above 800mg/100g, and magnesium was the most scarce cation, with values ranging between 7mg/100g and 67mg/100g. Potassium and calcium also varied greatly, the former particulary in those breakfast cereals which contained wheat, and the latter in those enriched with milk or calcium salts. Author Affiliation: (1) Dpto. Nutricion y Bromatologia II: Bromatologia, Facultad de Farmacia, Universidad Complutense de Madrid, Plaza Ramon y Cajal s/n, 28040 Madrid, Spain, ES (2) Dpto. Nutricion y Bromatologia II: Bromatologia, Facultad de Farmacia, Universidad Complutense de Madrid, Spain, e-mail: g.blazquez.000@recol.es, ES Article note: Received: 25 November 1999 / Revised version: 7 February 2000