Can carbonic acid protonate biological bases? Support from base protonation in methanol solvent
Résumé
In separate contributions, we have focussed on demonstrating that carbonic acid (CA) - historically considered too unstable to be a viable protonating agent - is able to protonate several types of pH indicators while behaving as a regular, moderately strong, carboxylic acid. Together with the experimental support we found for considering CA as a regular carboxylic acid are theoretical calculations demonstrating CA’s ability to protonate methylamine within 25 fs when forming with it a contact reactive complex. Here we briefly discuss a further aspect of this focus, involving the measurement of the lifetime and pKa of CA in pure methanol. The lifetime in methanol was found to be about 12-fold longer than in water, showing that the decomposition reaction of CA is solvent-dependent. The pKa change upon transferring CA from water to methanol was found to be 4.7 ± 0.1 pKa units, changing from 3.49 ± 0.03 to 8.16 ± 0.05: this change is similar to the pKa change observed for common stable carboxylic acids when these are transferred from water to methanol. These results add further support of our earlier proposal that CA can be an important protonating agent of biological bases in the blood plasma.