title: Macroscopic pKa Calculations for Fluorescein and Its Derivatives.
abstract: This study describes the calculation of the microscopic dissociation and tautomerization constants of fluorescein and its derivatives, 2',7'-dichlorofluorescein (DCF) and 2',7'-difluorofluorescein (DFF), in an aqueous environment. In vacuo free energies were obtained using complete basis set (CBS) and DFT-based methods, while free energies of solvation were calculated with the CPCM implicit solvation protocol using the UAHF, UAKS, and Pauling radii sets. Our results indicate that the different vacuum protocols give free energy changes upon dissociation within 1 kcal/mol of each other for a given molecule. Therefore, we suggest that the computationally less intensive PBE1PBE/6-311+G(2d,2p)//PBE1PBE/6-31+G(d) model chemistry may reasonably be used in pKa calculations of larger molecules. The calculations also provided a rigorous test of the implicit solvation models. Relative calculations of dissociation constants gave results in good agreement with experiment; absolute values deviated from experimental data by 1-3 pKa units. Consistently better results were obtained with the Pauling radii set. The influence of geometry relaxation on going from vacuum to solvent is negligible for pKa2 and larger for pKa1 but still smaller than the variation due to the radii set. Calculation of tautomerization constants gave more variable results, with none of the solvation methods able to reproduce experimental values consistently, although certain individual constants were correctly calculated.
authors.affiliation: Biomolecular Modelling Laboratory, Cancer Research UK London Research Institute Lincoln's Inn Fields Laboratories, 44 Lincoln's Inn Fields, London WC2A 3PX, UK, Department of Bioinformatics and Telemedicine, Collegium Medicum, Jagiellonian University, Kopernika 17, Kraków 31-501, Poland, and Grey Cancer Insitute, Free Radicals Research Group, Mount Vernon Hospital, P.O. Box 100, Northwood, Middlesex HA6 2JR, UK.
authors.forename: Marcin
authors.initials: M
authors.lastname: Król
authors.affiliation: Biomolecular Modelling Laboratory, Cancer Research UK London Research Institute Lincoln's Inn Fields Laboratories, 44 Lincoln's Inn Fields, London WC2A 3PX, UK, Department of Bioinformatics and Telemedicine, Collegium Medicum, Jagiellonian University, Kopernika 17, Kraków 31-501, Poland, and Grey Cancer Insitute, Free Radicals Research Group, Mount Vernon Hospital, P.O. Box 100, Northwood, Middlesex HA6 2JR, UK.
authors.forename: Marta
authors.initials: M
authors.lastname: Wrona
authors.affiliation: Biomolecular Modelling Laboratory, Cancer Research UK London Research Institute Lincoln's Inn Fields Laboratories, 44 Lincoln's Inn Fields, London WC2A 3PX, UK, Department of Bioinformatics and Telemedicine, Collegium Medicum, Jagiellonian University, Kopernika 17, Kraków 31-501, Poland, and Grey Cancer Insitute, Free Radicals Research Group, Mount Vernon Hospital, P.O. Box 100, Northwood, Middlesex HA6 2JR, UK.
authors.forename: Christopher S
authors.initials: CS
authors.lastname: Page
authors.affiliation: Biomolecular Modelling Laboratory, Cancer Research UK London Research Institute Lincoln's Inn Fields Laboratories, 44 Lincoln's Inn Fields, London WC2A 3PX, UK, Department of Bioinformatics and Telemedicine, Collegium Medicum, Jagiellonian University, Kopernika 17, Kraków 31-501, Poland, and Grey Cancer Insitute, Free Radicals Research Group, Mount Vernon Hospital, P.O. Box 100, Northwood, Middlesex HA6 2JR, UK.
authors.forename: Paul A
authors.initials: PA
authors.lastname: Bates