I can't get any precipitation of tripeptide when Wang Resin is used as solid support. I also tried to get precipitate with tertiary butyl ether instead of di-ethyl ether but can't get any precipitation. I need acid group at the C - terminal of peptide, so I have to use Wang Resin. Any one with any suggestion???
I do small peptide synthesis as well and use the the same resin.
Di- and Tri- peptides are tricky to precipitate because they are so small and will dissolve in anything remotely similar in polarity. One thing you can try is precipitate w/ a mixture of ether+hexane which worked well for my dipeptides - this causes other contaminants to precipitate out as well, but that can be cleaned up using HPLC.
Also, TFA removal is very important. Make sure to remove as much TFA as possible before adding in the solvents. Another option is to make sure that your solvents are thoroughly dried (no water, molecular sieves should work fine) and are free from contaminants.
Yes. I agree with Patrick on this completely-make sure you azeotrope off the TFA with repeated evaporations from DCM (toluene is even better) and precipitate in ehter/hexanes. To be honest, if you only have a free amine and a free carboxylate with aliphatic amino acids (or uncharged ones at least) you might consider a short silica plug for purification. Or simply triturate in hexanes. The smaller the molecule the harder it is to get a solid as you might be near the Tg and any trace solvent (like TFA or DMF) can help it stay a thick gel/oil.
Thanks for your suggestions John & Patrick.... I will see if the method is working or not...
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