I'm synthesizing a peptide using Fmoc-Cys(Trt)-Trt-PEG Resin. To deprotect the Fmoc group from the resin, I used 40% piperidine in DMF for 5 minutes and 20% piperidine in DMF for 15 minutes. To confirm the presence of free amine, I used TNBS. If there is a free amine, TNBS will give a red color to the resin beads. However, I couldn't observe such colour change. I've tried swelling the resin in DMF and NMP overnight or for 4 hours, but the problem persists. I have not faced this issue with other resins. Can anybody explain as to why the colour change is not happening with this resin?