We have been working on microwave-mediated PNA synthesis and it seems like we can't improve the yield of synthesis any futher (66% for Gly-TTT-Gly sequence). This got me thinking whether Rink amide resin with a loading capacity of 44 mmol/g may not be the right choice. Any thoughts? Thanks..
Hi Mohammed,
Literature suggests the use of either NMP or DMF and we went for DMF. We have been using HBTU/HOBt, and piperidine, for coupling and cleavage of Fmoc, respectively. Although this procedure seem very common, we still could not improve the yield, which made me think that resin could be an issue. What do you think?
Thanks,
Onur
Hi Onur,
We have seen marginal increases in yield by changing to Protide rink resin. Are you performing any tests between coupling (ie Kaiser test for amine or a test cleave and lC-MS) to check for unreacted amine/hydroxyl groups? We typically get 80% yield (at best) for short peptide (up to 10 aa) and that is sequence dependent. I don't have experience with Nucleic acid couplings but you could try different reagents (DIC, Oxyma or PYBOP, Dipea) to see if that helps. If you are having difficulty purifying the product you could try capping after each coupling.
Paddy
Hi Patrick,
So far, we did stick with HBTU/HBOt and have not consider the other coupling reagents yet. We have performed Kaiser test between couplings, there is no problem there. We began to think that either fmoc-group on PNA monomer is not fully cleaved or it causes aggregation of PNA chains, despite the fact that we have been using microwave.
I have seen some papers using resins like chemmatrix to overcome the aggregation problem. This is what got me thinking..
Onur
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