I'm doing an SPPS synthesis on Wang resin with Fmoc/tBu strategy. I use Oxyma and DIC as coupling agents in DMF, and when coupling Fmoc-Thr(tBu)-OH I consequently observe a developing purple colour (red in the beginning, see the attached photos). I have done this synthesis before and I never observed a colour change like this, it used to be bright yellow. Now the only difference is the temperature in the lab is 35 °C, because we don't have an air conditioner (it used to be 25 °C normally). What can be this colorful side product? It also seems t fluoresce. Thank you in advance!
I use Fmoc-Thr(tBu)-OH in my solid phase protocol, but I perform the synthesis using 2-Chlorotrityl resin and HATU, HOAt and collidine. I've never observed the developping of any colour in this condition, but I notice the resin changing colour and become purple/red during the acidic resin cleavage. Are you sure not to have a small amount of acid in the vessel, which may cause a premature cleavage? Or, maybe, you have strong basic condition, which may cause a premature Fmoc cleavage (maybe due to the different temperature)? The by-product of Fmoc cleavage can develop different colour depending on the other reagents present in the vessel! :)
Dear Kelly, tank you for your answer :)
I don't think that I have any acid contamination in the vessel. The weirdest thing is this red colour only develops when coupling Fmoc-Thr(tBu)-OH. I couple every amino acid the same way, consequently. I also have Fmoc-Thr(PO(OBzl)OH)-OH in the sequence that doesn't develop any strange colour, but Fmoc-Thr(tBu)-OH develops it every time. So I don't know how to explain it, maybe the Fmoc-Thr(tBu)-OH is contaminated with something.....Luckily the sequence was good after cleavage from the resin :)
Eszter Szombati yes, I was thinking that maybe this amino acid is contaminated, too. Strange things happen during SPPS! :) Hope your peptide will have the right sequence :)
I also noticed the red color using the Fmoc strategy (TFA:TIS:DDT:H20) for an amide resin cleavage. It's a small peptide with 3 arginines and 2 phenylalanine.
The interesting thing was that after returning to stop the reaction, the color changed to dark green. At least that's the perception of my eye. Has this happened to any of you?
