chemistry journal of moldova

V. Boldescu et al./Chem.J.Mold. 2008, 3 (1), 85-88ExperimentalPreparation of the SANG--CD systems for the spectrophotometric studiesThe studied systems of SANG at constant concentration 0.03 mM and CD at increasing concentrations 0.03,0.06, 0.10, 0.30, 0.50, 0.80, 1.00, 1.30, 1.50, 1.80, 2.00 mM have been prepared with and without addition of 0.1% (w/v)PVP in phosphate buffer-alcoholic solution (95:5 v/v) at different pH. The obtained solutions were mixed for 2 hours at35 °C wrapped in the aluminium foil to protect them from the direct light and then left for 24-hour equilibration.UV-vis spectrophotometry. UV-vis absorption spectra were recorded on a Jasco UV/VIS spectrophotometer,model V-550 equipped with a quartz cells with 1.0 cm optical path length.Results and discussionsThe spectrophotometric studies of the SANG--CD systems with and without addition of 0.1% (w/v) PVP atpH 5.0 did not show any evidence of stable complex formation. These results can be explained by the prevalence ofiminium form (Figure 1) of SANG in the studied systems at this level of pH. The iminium form is charged and is morehydrophilic than ‘pseudobase’ form, therefore, it can not be included into the hydrophobic cavity of the -CD.At the pH 8.0, close to physiological levels, SANG--CD systems with and without addition of 0.1% (w/v)PVP showed several significant evidences of the inclusion complex formation: the hyperchromic and the hypochromiceffects and the presence of the isosbestic point in the region of 200 – 210 nm (Figure 2. (a) and (b)).(a)Figure 2. (a) UV-VIS absorption spectra of 0.03 mM SANG at different concentrations of CD: 0.03, 0.06, 0.10, 0.30,0.50, 0.80, 1.00, 1.30, 1.50, 1.80, 2.00 mM, in phosphate buffer-alcoholic solution (95:5 v/v), pH 8.0; (b) UV-VISabsorption spectra of 0.03 mM SANG at different concentrations of CD: 0.03, 0.06, 0.10, 0.30, 0.50, 0.80, 1.00,1.30, 1.50, 1.80, 2.00 mM in the presence of 0.1% PVP, in phosphate buffer-alcoholic solution (95:5 v/v), pH 8.0(b)The simulated curves of the spectrophotometric titration (at 283 nm) of 0.03 mM SANG with CD in phosphatebuffer-alcoholic solution (95:5 v/v) with and without addition of 0.1% PVP, at pH 8.0, are shown in the Figure 3. (a)and (b). The hyperbolic character of the obtained curves is one of the indicators of the 1:1 stoichiometric ratio in theinclusion complex formation between the host (CD) and the guest molecule (SANG), according to the reaction shownin Scheme 1.SANG + CD SANG-CDThe curves resulted from the spectrophotometric titration of SANG with CD (at 283 nm) were processed toobtain the values of the association constant K aof SANG-CD systems, in the presence and without of 0.1% PVP. Theconstants were calculated using different linear methods: Benesi-Hildebrand, Scott, Scatchard [15]. The plots of thesethree methods for the studied host-guest complexes at pH 8.0, are shown in the Figures 4, 5 and 6. Among them, theScott method showed the highest levels of the correlation coefficients of the obtained results, ranging in the values 0.98– 0.99. The correlation coefficients for the Benesi-Hildebrand and the Scatchard methods were in the limits of 0.96 –0.98 and 0.93 – 0.94 respectively.86