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Fast pKa measurements for molecules with UV absorption

Karl Box, John Comer, Ruth Allen, Martin Stuart, Gabriela Cimpan, Sirius Analytical Instruments Ltd., Forest Row Business Park, RH18 5DW, UK

Purpose: An improved method for fast measurements of pKa values has been recently developed. The method is based on the well-established Dip-Probe Absorption Spectroscopy method (D-PAS), a hybrid combining the traditional pH-metric method with spectroscopic detection. By conducting the titrations in a linear buffer solution with low UV absorbance, the improved method allows pKa measurements in only 4 minutes compared with 20-30 minutes for the traditional D-PAS method.

Methods: A set of samples was selected to exemplify different properties commonly found in drug molecules, such as strong or weak UV absorbance, poor aqueous solubility and colored samples.  All assays were performed in aqueous solutions at 25°C in the presence of a linear buffer whose pH was pre-adjusted to an initial pH of 2 or 12, Depending on the initial pH, the samples were titrated with HCl or KOH (0.5M)… solutions and the UV spectra were continuously recorded.  Some poorly water soluble samples were titrated in the presence of water-solvent mixtures.

Results: The poster includes validation data comparing the Fast D-PAS, D-PAS and pH-metric methods. Innovative data refinement allows the calculation of pKa values from the spectral changes of the sample during the titration. Some of the molecules were known to decompose at extreme pH values, making pKa measurement difficult by traditional techniques. However, reliable pKa values were obtained using this new, fast method. The validation study compares the pKa values for 31 drug molecules obtained by D-PAS and traditional methods, with a R2 = 0.999 and RMSD = 0.098. 

Conclusion: The described method can successfully be applied in physico-chemical screening as a sample can be measured in only 4 minutes.

[1] R.C. Mitchell, C.J. Salter, K.Y. Tam, J. Pharm. Biomed. Anal., 1999, 20, 289-295

[2] J. Comer, Introduction to Fast D-PAS, lecture, 5th Sirius user meeting, Barcelona, April 2005

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Last modified: 28 April 2008