Reaction of BTNA, DMSO, and phosphate?

I prepared a solution of n-benzoyl L-tyrosine p-nitroanilide (BTNA) in dimethyl sulfoxide for use in an enzymatic assay. However, mixing it with a solution of phosphate buffer produced a color change for unknown reasons. Here is what I did:


* A 1.4mM solution of BTNA in DMSO was prepared.
* 5mM phosphate buffer was prepared. This was done by dissolving dipotassium phosphate in distilled water and adjusting the pH to 7.59 by addition of hydrochloric acid.
* Into a cuvette was added 0.4mL of the BTNA solution, followed by addition of 0.1mL of the phosphate buffer solution.


When the BTNA solution and phosphate buffer were mixed, the mixture took on a yellow color. I tried mixing the BTNA solution with distilled water without any phosphate added to see if, perhaps, the act of mixing water with the BTNA solution is responsible for the color change. In this case, the mixture did not change color.


My question is thus: Why did mixing 0.4mL of BTNA solution in DMSO with 0.1mL of 7.59 phosphate buffer result in the mixture taking on a yellow color, whereas mixing BTNA solution with water did not result in a color change?


Sidenote: None of the solutions were tested for contaminants.


