Determination of pKa in Non-Aqueous Solution using Chemical Shift Imaging
Lead Research Organisation:
University of Liverpool
Department Name: Chemistry
Abstract
pKa is a measure of the acidity of a molecule/ion (i.e. the ease with which it loses a proton). It is a widely used parameter in catalysis, pharmacology and chemical synthesis, allowing the prediction of chemical and physical behaviour of species in solution. Reactivity, solubility and the nature of chemical interaction are all governed by the protonation state of a molecule, and thereby its pKa.
pKa is usually determined in aqueous solution, but frequently a compound is too insoluble or too reactive to do so. DMSO is commonly used as an alternative, as it is a relatively inert polar aprotic solvent supporting a wide pKa range (up to 35). Water solvates both cations and anions well while DMSO solvates cations well and anions poorly, making meaningful comparisons between pKaDMSO and pKa(aq) difficult. Experimental determination of pKa in DMSO is therefore necessary.
Various methods are available to determine pKa, each with its advantages and drawbacks. Non-aqueous pKa measurements add another dimension of difficulty. In the example of potentiometric methods, equipment designed for aqueous measurements often must be adapted to the non-aqueous medium, risking equipment damage and inaccurate results.
NMR titration methods are well known in the literature, with a range of NMR active nuclei available including 1H, 13C and 31P. Pure compounds are not necessary (as they must be for conductimetry) and no equipment modification is needed to measure in non-aqueous medium. The conventional NMR titration requires the acquisition of a large series of NMR spectra taken at incrementally changing pH. This is a labour intensive procedure requiring a large sample volume and long instrument times.
As an alternative, we have developed a one-shot titration method for determining the pKa of compounds in DMSO solution. Using a set of NMR pH indicators and chemical shift imaging techniques, we can measure the pH of a sample as a function of position along a pH gradient within an NMR tube. From a single measurement we can determine the pKa of an acidic or basic analyte. The pKa of analytes determined using this method are in good agreement with literature values ( 0.1 units) and have been shown to be self-correcting for solution ionic strength.
pKa is usually determined in aqueous solution, but frequently a compound is too insoluble or too reactive to do so. DMSO is commonly used as an alternative, as it is a relatively inert polar aprotic solvent supporting a wide pKa range (up to 35). Water solvates both cations and anions well while DMSO solvates cations well and anions poorly, making meaningful comparisons between pKaDMSO and pKa(aq) difficult. Experimental determination of pKa in DMSO is therefore necessary.
Various methods are available to determine pKa, each with its advantages and drawbacks. Non-aqueous pKa measurements add another dimension of difficulty. In the example of potentiometric methods, equipment designed for aqueous measurements often must be adapted to the non-aqueous medium, risking equipment damage and inaccurate results.
NMR titration methods are well known in the literature, with a range of NMR active nuclei available including 1H, 13C and 31P. Pure compounds are not necessary (as they must be for conductimetry) and no equipment modification is needed to measure in non-aqueous medium. The conventional NMR titration requires the acquisition of a large series of NMR spectra taken at incrementally changing pH. This is a labour intensive procedure requiring a large sample volume and long instrument times.
As an alternative, we have developed a one-shot titration method for determining the pKa of compounds in DMSO solution. Using a set of NMR pH indicators and chemical shift imaging techniques, we can measure the pH of a sample as a function of position along a pH gradient within an NMR tube. From a single measurement we can determine the pKa of an acidic or basic analyte. The pKa of analytes determined using this method are in good agreement with literature values ( 0.1 units) and have been shown to be self-correcting for solution ionic strength.
Organisations
People |
ORCID iD |
| Jonathan Iggo (Primary Supervisor) | |
| George Schenck (Student) |
Studentship Projects
| Project Reference | Relationship | Related To | Start | End | Student Name |
|---|---|---|---|---|---|
| EP/N509693/1 | 01/10/2016 | 30/09/2021 | |||
| 1797785 | Studentship | EP/N509693/1 | 01/10/2016 | 31/12/2020 | George Schenck |
| Description | Through the research funding, we have designed and implemented a new method for determining pKa (a measure of acidity) in the very common laboratory solvent dimethylsulfoxide (DMSO). This is of vital importance as the common methods for determining acidity (potentiometric or conductiometric measurements) require specialist equipment not available in most laboratories and a huge amount of researcher time to set up the analysis and record the data. The method we have developed uses a Nuclear Magnetic Resonance spectrometer (available in every pharmaceutical and most other chemistry laboratories) without need for adaptation or alteration to our method. The method uses chemical shift imaging (CSI) to observe the effect of an acid concentration gradient in a sample of analyte- performing a titration in a single "one-shot" titration experiment. It requires a maximum of 45 minutes of researcher time to set up our titration and run the experiment compared to several hours and dozens of samples for a manual titration. Our acidity measurements are concurrent with literature data for a range of acids and bases. These findings are insensitive to ionic strengths of up to 0.2M and water contamination up to 2%, showing the method to be useful in a real life, imperfect laboratory setting. We have further expanded the application of CSI titrations into various water/DMSO mixtures, which is of particular interest to pharmaceutical development as these mixtures are typically used as the medium to characterise drug candidates. This development adds greatly to the researchers arsenal as there are precious few methods to determine acidity accurately in these mixtures. |
| Exploitation Route | The method we have developed has particular potential in the pharmaceutical industry, where the higher efficiency and lower labour requirements will be highly desirable. High throughput drug discovery methods commonly use DMSO and DMSO/water mixtures as solvents for characterisation, so our method will integrate well into existing discovery pipelines. A 3 month EPSRC funded innovation placement was hosted in late 2019 at C4X Discovery, a small pharmaceutical company, to apply our CSI titrations to pharmaceutical research and integrate our method into C4X's workflow. The placement was a great success, with highly useful acidity data generated in both DMSO and DMSO/water mixtures. Two researchers at the company were trained in the experimental setup and processing to allow continued use of our method. Our CSI method for determining acidity can be adapted for other solvents, depending on a researchers need, but can also be adapted to determine other equilibrium constants (e.g association, Hydrogen bonding etc). These avenues are currently being explored. |
| Sectors | Chemicals,Pharmaceuticals and Medical Biotechnology |