It may depend on the properties you are interested because at least protein tertiary structure may change wit pH. For structural and termodynamic properties of globular proteins the answer is clearly yes. For example, the ranges of pH where lysozime and other proteins are stable in solution, aggregates, crystallizes in tetragonal crystals or in needles is well illustrated in the works of Vekilov, for example. The influence of pH, pMg,… on the equilibrium thermodynamics is very clearly established in the series of papers by R.Alberti.

Thank you for the rapid answer. I deduce that changing the pH can modify protein structure and biological activity. Anyway, if one is interesed in protein only as bio-compatible dispersant, their biological activity is not required. Do you think pH changes can also cause problem on cell vitality?

I am a physico-chemist and I do not know anything at all about cell vitality. However, if the protein is only used as dispersant, the problem would rather be a problem of macromolecular crowding. The key person in this field is Allen Minton but I cannot say whether he has worked with proteins as crowders. In my own experience, proteins should modify the molecular activity due mainly to its size and not to the charge but in so a complicate environment as a cell, who knows?. Please let me know your own results as you consider convenient

I only observed aggregation of nanotubes dispersed with lysozyme in culture media, but not in nanotubes dispersed with BSA. I wonder if the protein charge may be the cause. Thus, it should be worth studying BSA with a different charge state by changing pH. I do not know if the latter condition can be unsafe for cells.

The isoelectric point of lysozyme is 11.35; that's for chicken egg lysozyme, which is what I assume you are using, since it is so cheap. The isoelectric point of BSA is 4.7. The charge character of these two proteins are rather different. That could possibly explain why you see aggregation with lysozyme and not with Bovine Serum Albumin.

With respect to changing the pH, I need more information. Are you doing this experiment in a test tube without any cells? If that is the case, then pH changes should not affect any cells. However, if you are using genetically engineered bacteria to overproduce the proteins, then changing the pH will definitely affect cell viability. E. coli (if that''s what you are using), likes slightly alkaline pH ranges and low or high pH ranges will inhibit growth of your bacterial cells.

Thank you Michael for your answer. The question arises because if I change the pH in the protein-stabilized dispersions, then I should change in the pH also in the culture medium with the cells. The goal is to investigate citotoxicity of carbon nanotubes dispersions. However, I am not a biologist and I do not know if it is possible working with cells culture. Is it also a question of what type of cell I use?

Hi Franco. First, its not a good idea to tamper with the pH of cells in tissue culture. It will affect their metabolism and viability.

I don't have access to the full article you linked to, but my guess is that they used excess protein for the CNT coating.

Proteins often denature when they are adsorbed to CNTs. Also, most proteins do not bind CNTs with high affinity. This could lead to an increase in the concentration of denatured proteins in the solution and aggregation.

Steingrimur is right. Tissue culture cells are grown in rather carefully buffered media. If you drop the pH of their growth medium, the cells will give up the ghost. I cannot think of a single popular cell line (Hela, NIH 3T3, MDCK, CHO, S2, etc.) that will grow in a low pH medium.

What about using Kato III cells? They are a gastric cell line and they might tolerate low pH rather well.

Thank you all for your valuable suggestions. Michael, I will think about Kato III cells.