The pseudo-first-order reduction of 4-Cl-nitrobenzene by Fe(II) in aqueous systems containing montmorillonite clays is investigated over the pH range 6.00-8.00. Silica and alumina is also investigated as simple analogues to aluminosilicate mineral surfaces. At pH 7.25, montmorillonite clays were found to be as much as 1000 times less effective than ferric oxides at mediating the reaction when expressed on a surface area basis. Reaction rates increase dramatically as the pH rises and at pHs above 7.5 approach those previously reported for surface bound Fe-(II) on ferric oxides at pH 7.22. This increase in reactivity is attributed to both an increase in concentration of the FeOH + ion and to the increased sorption of Fe(II) at high pH. Sorption isotherms for Fe(II) to montmorillonite clays at pH 7.00 are reported. Two surface sites are suggested on clay minerals and incorporated into a kinetics model for the pH dependence of the reaction. The overall reaction is modeled as the sum of the reactions between 4-Cl-NB and three reductants; FeOH + and Fe(II) bound to the two surface sites. FeOH + is found to be the most effective reductant in our systems. Intrinsic rate constants for both surface sites and FeOH + are presented. Although the minerals investigated are much less effective at mediating the reaction than ferric oxides, the rates are sufficiently fast to be of importance to environmental processes. At neutral pHs, half-lives are less than a week and decrease to the scale of hours above pH 7.5. This is quite rapid in the context of groundwater systems in which residence times can be months or years.
A series of rigid planar azadiindoles (8a, 8b, and 8d), benzannelated pyridodiindoles (11a, 11b, and 11d), and indolopyridoimidazoles (11c, 20, and 24) were synthesized from 4-oxo-1,2,3,4-tetrahydro-beta-carboline 5 via the Fischer indole cyclization with the appropriate arylhydrazines. These analogues were employed as probes ("molecular yardsticks") to define the spatial dimensions of the lipophilic regions of the benzodiazepine receptor (BzR) binding cleft. Benzannelated indoles 11a-d and indolopyridoimidazoles 20 and 24 were important in establishing an area of negative interaction (S1, see Figure 6, part b) in the binding cleft common to the interactions of both inverse agonists and agonists. Data from this chemical and computer-assisted analysis of the pharmacophore (see Figure 6) indicates that inverse agonists and agonists bind to the same binding region, but the pharmacophoric descriptors required for the two activities are different, in keeping with previous studies with these planar ligands. However, the hydrogen bond donating site H1 and the lipophilic region L1 in the receptor binding site are common interactions experienced by both series of ligands. The low affinities of both indolo[3,2-c]carbazole (3a) and indolo[3,2-b]isoquinoline (3b) for the BzR are consonant with the requirements of a hydrogen bond acceptor interaction at donor site H1 and a hydrogen bond donor interaction at acceptor site A2 for potent inverse agonist activity in the beta-carboline series. The hydrochloride salts of 1-aza- 8a (IC50 10.6 nM), 2-aza- 8b (IC50 51.5 nM), and 4-azadiindole 8d (IC50 11.2 nM) were found to be much more soluble in water than the corresponding salt of the parent diindole 2. Moreover, aza analogues 8a and 8b were shown to be partial inverse agonists with proconvulsant potencies comparable to that of the parent diindole 2.
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