Dear All
I am wondering if someone has an good explanation for the "nanobuffering" behavior of pH responsive polymers. Here is some literature on the topic:
Article Increasing Enzyme Cascade Throughput by pH-Engineering the M...
Article Nanobuffering of pH-Responsive Polymers: A Known but Sometim...
In brief, the phenomenon is explained as due to a negatively charged polyelectrolyte attracting protons as counterions, thereby lowering the pH locally in its vicinity. My problem with this explanation is that the concentration of other cations is much higher than that of protons. For instance, the research paper above appears to achieve nanobuffering at a bulk pH of 7 and physiological ionic strength. At pH 7 we have [H+] = 10^-7 M, which is more than 6 orders of magnitude less than the concentration of other cations (Na+, K+ etc). So less than one in a million of the negative charges on the polyelectrolyte will attract a proton, meaning that there couldn't possibly be any significant change in pH (even if we are talking small volumes).
I'm not questioning the results of any published work, just trying to understand what is going on.
Update: In the enzyme paper, they actually mention that the polyelectrolytes in the protonated state are responsible for the local pH change:
"Because of the high grafting density, 50% of the carboxyl acid groups are still undissociated at pH 7. The concentrated protonated carboxyl groups create a microenvironment that is more acidic than the bulk solution, so that the activity of conjugated Cyt C displays a higher activity at alkaline pH."
However, I cannot follow this argumentation either. I understand the shift in pKa, but pH is the concentration of protons in solution, not of undissociated COOH groups. How can they lower the pH if the protons do not dossociate?
This is my idea and i do not want to reject or confirm the published papers. About the buffer, buffer means one weak acid and its conjugate weak base or reverse. I did not read the papers carefully, but, as they said, Because of the high grafting density, 50% of the carboxyl acid groups are not still dissociated at pH 7.00, How they know! The solution and its pH reaches to equilibrium very fast. About the carboxyl acid group,and its dissociation , it depends on pka, not density. Another point, If the polymer has negative charge such as a some nano particles. In the solution, all ions such H+, Na+ can interact with negative surface charges, also in the presence of organic dyes (with positive charge) they also can adsorb on the surface and color of solution can change, it is reversible process , just it needs a pH variation(You can find it in the ligatures). In addition, I am not agree with them,” the H+ concentration in proximity to the polymer segments is higher than that in the bulk”, it might happens in a short time after adding polymers to the solution but after a while the system or solution reaches to a equilibrium.
Thank Amir. I'm not sure I understand fully your point but in my opinion the altered pKa for dense films of polyacids compared to the same polymer free in solution is fairly easy to understand. However, that is something else than the nanobuffering, which is a local change in free proton concentration. As far as I understand it is supposed to be the equilibrium state.
I found this paper about interfacial pH for lipid membranes. They use a bulk salt of 400 mM, even more than physiological! Yet they claim protons are attracted by the negatively charged head groups in the membrane (or repelled by positive ones). The strength of the work is that they acknowledge that the pKa of the probe is different depending on its environment and that this is another effect than the local pH change (free proton concentration). They calculate the actual interfacial pH by going through two different previous papers, one on the role of permittivity for probe pKa and one on how to estimate permittivity from spectral peak wavelength.
I will not comment on how precise this method is, but even if it is correct that the interfacial pH is shifted from the bulk by more than one unit, I see no good explanation. Again, why cannot Na+ act as counterions?
Article Evaluation of pH at Charged Lipid/Water Interfaces by Hetero...
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