Hi,
I am trying to label my protein of interest with monobromobimane or alexa 350 maleimide after introducing cysteine at the required position. But in the presence and absence of tryptophan (in proximity to the cysteine according to the crystal structure) which is supposed to quench the fluorescence, I am not observing any difference. The distance between the two positions is 4.5Angstroms. Can someone please let me know what is going wrong? cysteine-less wild type shows less than 10% fluorescence.

thanking you in advance