We advance the concept that tautomerism is crucial for the understanding of the chemical behavior of tetracycline. Indeed, considering four deprotonations, there are 64 different possible tautomers to be considered for tetracycline. Our results indicate that tetracycline is a very adaptive molecule, capable of easily modifying itself through tautomerism in response to various chemical environments. Indeed, its situation in solution can be more accurately pictured as an equilibrium among a diversity of tautomeric species-an equilibrium that can be easily displaced depending on the various possible chemical perturbations, such as varying the pH or the dielectric constant of the solvent. Moreover, we also show that tetracycline could undergo four deprotonations and predict for it a fourth pKa of 13 and refer to our experimental determination of this parameter, which yielded the value of 12. We conclude that tautomerism is essential to the comprehension of the chemical behavior of tetracycline as determined by the semiempirical method AM1 as well as by the self-consistent reaction field method, which estimates the effects of the solvent on the tautomers. All tautomers in their different conformations have been fully optimized for each of the possible degrees of protonation of this molecule. Thus, the relative stabilities of the different tautomeric species have been computed.

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The equilibria between aqueous vanadate and N-hydroxyacetamide (acetohydroxamic acid, HL, CH 3 CONHOH) have been studied at 25 ЊC in 0.15 mol dm Ϫ3 NaCl medium by combined potentiometric, spectrophotometric, 51 V and 17 O NMR methods. Complexes form in the range pH 3 to 11, and many of their formation constants have been determined. The neutral species HV(HL) and HV(HL) 2 predominate below pH 4.5 and with HL : V > 2 : 1, where V is an abbreviation for [H 2 VO 4 ] Ϫ . These notional formulae do not specify the possible co-ordination or loss of water molecules from the complex anions and the charges shown are the overall species charges rather than any algebraic sum. From pH 4.5 to 8.5 the main species are [V(HL) 2 ] Ϫ and, to a lesser extent, [V(HL)] Ϫ . Oxygen-17 NMR data strongly suggest that [V(HL) 2 ] Ϫ has octahedral co-ordination, with a cis-VO 2 ϩ unit and two bidentate L Ϫ ligands, one of which can lose another proton above pH 8.5 to form [VL(HL)] 2Ϫ . The species [V(HL)] Ϫ also deprotonates above pH 9. The minor oligomeric species [V 2 (HL) 3 ] nϪ and [V 2 (HL) 4 ] nϪ also form, at higher concentrations of V and HL.

The influence of pH during hydrolysis of titanium(IV) isopropoxide on the morphological and electronic properties of TiO2nanoparticles prepared by the sol-gel method is investigated and correlated to the photoelectrochemical parameters of dye-sensitized solar cells (DSCs) based on TiO2films. Nanoparticles prepared under acid pH exhibit smaller particle size and higher surface area, which result in higher dye loadings and better short-circuit current densities than DSCs based on alkaline TiO2-processed films. On the other hand, the product of charge collection and separation quantum yields in films with TiO2obtained by alkaline hydrolysis is c.a. 27% higher than for the acid TiO2films. The combination of acid and alkaline TiO2nanoparticles as mesoporous layer in DSCs results in a synergic effect with overall efficiencies up to 6.3%, which is better than the results found for devices employing one of the nanoparticles separately. These distinct nanoparticles can be also combined by using the layer-by-layer technique (LbL) to prepare compact TiO2films applied before the mesoporous layer. DSCs employing photoanodes with 30 TiO2bilayers have shown efficiencies up to 12% higher than the nontreated photoanode ones. These results can be conveniently used to develop optimized synthetic procedures of TiO2nanoparticles for several dye-sensitized solar cell applications.