Abstract
Mixed dissociation constants of four drug acids, i.e. silychristin, silybinin, silydianin and mycophenolate at various ionic strengths I of range 0.01 and 0.30 and at temperatures of 25 and 37 °C were determined using the SQUAD(84) regression analysis program applied to pH-spectrophotometric titration data. The proposed strategy of an efficient experimentation in a protonation constants determination, followed by a computational strategy for the chemical model with a protonation constants determination, is presented on the protonation equilibria of silychristin. The thermodynamic dissociation constant pK a T was estimated by non-linear regression of { pK a , I} data at 25 and 37 °C: for silychristin pK a,1 T =6.52(16) and 6.62(1), pK a,2 T =7.22(13) and 7.41(5), pK a,3 T =8.96(9) and 8.94(9), pK a,4 T =10.17(7) and 10.03(8), pK a,5 T =11.89(4) and 11.63(7); for silybin pK a,1 T =7.00(4) and 6.86(5), pK a,2 T =8.77(11) and 8.77(3), pK a,3 T =9.57(8) and 9.62(1), pK a,4 T =11.66(3) and 11.38(1); for silydianin pK a,1 T =6.64(7) and 7.10(6), pK a,2 T =7.78(5) and 8.93(1), pK a,3 T =9.66(9) and 10.06(11), pK a,4 T =10.71(7) and 10.77(7), pK a,5 T =12.26(5) and 12.14(5); for mycophenolate pK a T =8.32(1) and 8.14(1). Goodness-of-fit tests for various regression diagnostics enabled the reliability of parameter estimates to be found.
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