This perplexed me the first time I saw it. I prepared a solution of Vancomycin in DI water with concentration of 5 mg/ml. The solution was used as a dipping solution where the substrate was previously dipped in the polymer, PLA. Thus, it is possible for leftover pieces of PLA to be in Vancomycin + water.

After I finished using it, I forgot to clean up one of the vial from the fridge and 1 month later I came back to a 'precipitated' Vancomycin(?). The solid mostly adhere to the vial's surface. The pH measured was around 4, which was a drop from its initial of around 5. My assumption at first was that the pieces of PLA that was left in the solution during dipping degraded into LA and dropped the pH to decrease Vancomyin's solubility; however, I'm not certain as there is no change in charge but only increase in proton amount. Does an increase in H+ hinders Vancomycin's ability to dissolve in water? Additionally, I believe it might also be the degradation/change in Vancomycin's structure to something else. At this point, Vancomycin's color is yellow.

Since I thought pH was the cause initially, I added in some NaOh. Surprisingly, the precipitates dissolved. This was confusing as increasing pH should decrease Vancomycin's solubility if above 8 because of the neutral net charge. I thought it was fine until the next day, I checked and the solution became red. After a few days, it turned back to yellow. So I believe it is a change/degradation in structure, but I'm not sure how. I forgot to check its wavelength, but I believe it changes from Vancomycin's original wavelength of 381 nm.

Sorry if this sounds quite nonsense, but I was curious as to what caused this to happen.