Hi Sepideh,

Curcumin is chemically stable in mild alkaline conditions, so the color change is completely reversible, as you will see if you re-neutralise the alkaline solution. Thus, the antimicrobial activity is not changed - there is still an equilibrium of tautomeric states, but of course where this equilibrium lies is pH dependent. Having said that, the website www.chemspider.com states the following under curcumin: 'Soluble in 0.1 M NaOH to 3 mg/mL-do not store more than 12 hours'. This suggests to me that the stability in strong alkaline conditions may be limited. When we used curcumin, we always mafe a stock solution in DMSO, in which curcumin is highly soluble. From this we would dilute into aqueous buffer for immediate usage. I think this worked best and the DMSO solution can be used for a considerable time when stored  in the fridge. I think we were able to use up to 100 micromolar in growth medium, when diluted 100-fold from DMSO (i.e 10 mM), as you don't want more than 1% of DMSO in a culture.

Fabio is right that the protonation or tautomeric state of drugs almost always influences the uptake by the target cell. In the case of curcumin, it remains neutral, so any diffusion across biomembranes would not be likely to be affected. But I think it is likely to be transported by a protein in the plasma membrane and that is likely to be pH dependent.

Hey, curcumin is a diarylheptanoid with two aromatic rings with phenolic OH groups and a alfa,beta unsaturated beta diketone, the change in color you are observing is due to electron delocalization as all three groups are able to deprotonate in alcaline medium. This is called bathochromic shift see for example doi:10.1016/j.saa.2011.04.016. Now to answer your question specifically, you may need to address each property individualy. Let me explain, for example in the case of antimicrobial activity the fact is that the internalization of the molecule (so it reaches its target) by the bacterial cell is highly important for activity. With this in mind one may expect a change in behavior if the compound is present as one -or the other- tautomer, though is more difficult to predict if this structural changes will have a possitive or negative impact on activity and must be assayed (this is a very common effect in antibiotics, see for example tetracycline uptake in neutral vs charge state). Hope this helps       

Hi Sepideh,

Curcumin is chemically stable in mild alkaline conditions, so the color change is completely reversible, as you will see if you re-neutralise the alkaline solution. Thus, the antimicrobial activity is not changed - there is still an equilibrium of tautomeric states, but of course where this equilibrium lies is pH dependent. Having said that, the website www.chemspider.com states the following under curcumin: 'Soluble in 0.1 M NaOH to 3 mg/mL-do not store more than 12 hours'. This suggests to me that the stability in strong alkaline conditions may be limited. When we used curcumin, we always mafe a stock solution in DMSO, in which curcumin is highly soluble. From this we would dilute into aqueous buffer for immediate usage. I think this worked best and the DMSO solution can be used for a considerable time when stored  in the fridge. I think we were able to use up to 100 micromolar in growth medium, when diluted 100-fold from DMSO (i.e 10 mM), as you don't want more than 1% of DMSO in a culture.

Fabio is right that the protonation or tautomeric state of drugs almost always influences the uptake by the target cell. In the case of curcumin, it remains neutral, so any diffusion across biomembranes would not be likely to be affected. But I think it is likely to be transported by a protein in the plasma membrane and that is likely to be pH dependent.

Hey again. I agree with Harry, one additional thing to consider is that this type of functionalized structure is able to complex metals so there you have another factor which may affect (positive or negatively) biological behaviour. Check out a couple of these interesting examples in which there is a structure-activity relationship for curcumin. I haven't been able to locate a specific paper in which antibacterial capability is addressed, though. 

I attach one paper that explores the SAR of curcumin and its analogs against mycobacteria, and 1 that gives a large SAR  analysis of curcumin analogs against the protozoan parasite Trypanosoma brucei, the agent that causes sleeping sickness. We have also a lot of data of similar compounds against Leishmania species, but that has not been published yet.

Hello everyone, can you answer my related question here please?

https://www.researchgate.net/post/Why_does_curcumin_changes_its_color_when_producing_a_thin_lipid_film_liposome_production