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An attempt has been made to identify the characteristic positions, and their eventual displacements with changing co-ordination, of the charge-transfer bands involving donor groups located on molecules of biologicaI interest [and N-benzoylglycyl-L-histidyl-L-leucine) such as carboxylate, alcoholate, amine, deprotonated peptide and imidazole. Information about the species formed in solution was obtained by means of pH-metric readings while ultraviolet-circular dichroism spectra were recorded, at fixed pH values, 298 K and I = 0. I mol dm-3, for the proton-ligand and proton~opper(lr)-ligand systems, in order to evaluate a spectrum for each complex formed in solution. Intraligand and charge-transfer bands were assigned for each spectrum with the aim of relating spectral features to the structure of the species formed in solution.Ultraviolet-circular dichroism (UVjCD) spectra have been employed for the determination of the or-helix content of proteins,' of the influence on ligands of metal ions' and for structural definition in micellar aggregates of a series of chiral surfactants. When metal-ligand interactions are investigated the nature of the donor groups involved in the co-ordination sphere of a given metal ion might be detected. Both visible and ultravioletxircular dichroism spectra can provide very useful structural information for metal complexes in solution. The results obtained by means of the two techniques are complementary; nevertheless from UVjCD spectra it is also possible to study compiex formation (i) with colourless metal ions,4 (ii) with metal ions having low molar absorption in the visible, such as octahedral complexes of nicke1(11), (iii) between chiral anionic ligands (such as anions of carboxylic acids) and cationic ligands (such as protonated amines), and (io) in real samples of biological or environmental interest (by using suitable techniques for chemical separation and for data elaboration).The UVjCD spectra obtained from several systems containing copper(r1) and simple model ligands should allow us to propose a correlation between spectral features and complex structures, as the spectrum ofeach species in solution (including the free and protonated forms of the pro-ligand) can be calculated from experimental UVjCD data recorded on soiutions of different known compositions and from known values of formation constants. It is necessary to distinguish between intraligand (i.1.) bands, which can be modified, in intensity, sign and position, as a consequence of co-ordination and charge-transfer (c.t.) bands, which should be attributed to the donor groups.In the literature there are many articIes in which UVjCD spectra are used for structural definitions in solution but the attributions are directly based on experimental spectra, without taking into account either the superposition of the different metal complexes at a given pH (often only the structures of the predominant species in solution are discussed) or the contribution of the differently protonated forms of the proligand and, moreover, wi...

Alkali-metal and calcium(ti) complexes of monocarboxylate A -(acetate and salicylate), dicarboxylate A2-(malonate, maleate, succinate, malate, tartrate, phthalate, and oxydiacetate), or amino acid HA (glycine or L-histidine) ligands have been studied potentiometrically, using a glasssaturated calomel electrode, at different temperatures and ionic strengths. The monocarboxylate ligands form [MA] and the dicarboxylates [MA] and [ M (HA)] species (charges omitted) with both alkali metals and calcium. Glycine and L-histidine form [MA] + and [M(HA)]*+ {and [M(H2A)I3+ for L-histidine} complexes with Ca*+, whilst alkali metals form only [M(HA)]+ with glycine. Some interesting regularities in the formation constants are pointed out. From the dependence on temperature of formation constants, values of AH* and AS* have been determined. The function log fl = f ( 1 ) has been carefully studied in the range 0.02 < / < 1 mol dm-3. The reliability of a new model for the dependence on ionic strength of formation constants (when dealing with weak complexes it is in practice impossible to use the constant ionic medium method) is widely discussed. Two methods of calculation are described and the more general method has been checked by simulated curves.Recently, an interesting lecture by Bjerrum ' was based on the question: 'How far is it possible to determine small stability constants in aqueous solution?' This question arises from the uncertainties in determining activity coefficients when working with high and varying concentrations. In the last five years some of us have been concerned with the study of weak alkali 2-9 and alkaline-earth ' O-' metal complexes of low molecular weight * Acetate and suainate: experimental data and calculations from refs. 4, 12, 14, and 15; Na' and Caz+ complex formation constants were checked by means of four titration curves per system: previously reported values were fully confirmed. Malate, tartrate, oxydiacetate, Lhistidinate, and salicylate: new experimental data and calculations (salicylic acid was considered as monoprotic, pH < 4). Malonate, maleate, and phthalate: experimental data from refs. 4, 12, and 13; calculations repeated.

In this paper the overall antioxidant power, expressed as Briggs-Rauscher antioxidant index, of decoction or cold infusions of dried Hibiscus sabdariffa flowers was determined at 25 and 37°C, to compare the scavenger ability of the beverages at either room or physiological temperature. Total polyphenol contents and the absorbance of anthocyanin pigments were also determined, and the trend with the overall antioxidant capability is considered. Combined photometric and pH-metric titrations were acquired to obtain information on the colour-total acidity relationship of the product. The results show that the decoction preparation protocol provides karkadè with the highest nutritional value and that the polyphenol content can account for the antioxidant capability of H. sabdariffa-based beverages. Moreover, a quantitative relationship between acid-base and redox chemistry was found. The H. sabdariffa-based drinks can be considered as protective beverages and a regular consumption of karkadè might be proposed to ensure protection against free radicals.