Biomedical subjects

Ana Sayago

Publications and source records attributed to Ana Sayago.

3 recordsLinked to original sources

PubMed2006

Spectrophotometric evaluation of stability constants of 1:1 weak complexes from continuous variation data.

A bilogarithmic hyperbolic cosine method for the spectrophotometric evaluation of stability constants of 1:1 weak complexes from continuous variation data has been devised and applied to literature data. A weighting scheme, however, is necessary in order to take into account the transformation for linearization. The method may be considered a useful alternative to methods in which one variable is involved on both sides of the basic equation (i.e. Heller and Schwarzenbach, Likussar and Adsul and Ramanathan). Classical least squares lead in those instances to biased and approximate stability constants and limiting absorbance values. The advantages of the proposed method are: the method gives a clear indication of the existence of only one complex in solution, it is flexible enough to allow for weighting of measurements and the computation procedure yield the best value of logbeta11 and its limit of error. The agreement between the values obtained by applying the weighted hyperbolic cosine method and the non-linear regression (NLR) method is good, being in both cases the mean quadratic error at a minimum.

Drug Stability↗

PubMed2006

A bilogarithmic method for the spectrophotometric evaluation of stability constants of 1:1 weak complexes from mole ratio data.

The absorbance changes that occur when the mole ratio of the components of ligand complex equilibria is varied while the concentration of one component is kept constant (mole ratio method) allow evaluating stability constants in favourable conditions. Values of the corresponding stability (association) constants are normally assigned on the basis of spectrophotometric analysis. Determination of stability constants can be performed by a number of linear procedures, but most of these, suffer from theoretical and practical drawbacks, e.g., linear transformation of the rectangular hyperbola type of binding constants, is valid only when one of the two species is present in a large excess. A rigorous treatment of the experimental mole ratio data for 1:1 weak complexes is carried out in this paper with the aim of eliminating some of the assumptions involved in the other methods usually applied for evaluating stability constants. Orthogonal regression is required in order to take into account the error in both axes. The method has been applied to literature data for the iron(III)-thiocyanate and nickel(II)-selenocyanate systems, as well as to a number of host-guest cyclodextrin complexes.

Algorithms↗

PubMed2005

Continuous variation data: 1:1 or 2:2 weak complexes?

An attempt to distinguish between 1:1 and 2: 2 weak complexes from continuous variation data is given. A modification of the Heller and Schwarzenbach's method which involves the use of the absorbance ratio y=A/A(lim) (once the limiting absorbance, A(lim) is known) may be applied to 1:1 weak complexes. This allows checking if the slope obtained is close to the theoretical value of -0.25. A curvature appears and the residuals have a pattern when a wrong 1:1 model is selected for a true 2:2 stoichiometry.

Drug Stability↗