Test cleavage followed by MALDI analysis confirmed the peptide was formed. It did not show any precipitate in super cold ether when I tried to cleave it from rink amide resin using cocktail 90:5:5 TFA/TIPS/H2O. It is a 9 residue peptide containing all hydrophobic residue like Aib. It does not contain any side chain protecting amino acid. Is there any other methods to solve this particular problem?
it appened sometime that the peptide doesn't precipitate in ether after clevage. In this case we prefer to evaporate the cocktail of cleveage under vacuo and see if there is any product inside. If there is product you can try at this point to triturate by ether. Or just purify it by hplc
I can second John Offer's suggestion for the Hmb backbone protection. It will work. However, the second problem indicated in his answer is also correct. Once Hmb or any other backbone protection is removed, you will have a very insoluble peptide again. I have had luck in retaining hydrophobic peptides in solution using 10-20% trifluoroethanol in a standard phosphate or ammonium bicarbonate buffer. Use 100% trifluoroethanol with the hydrophobic peptide if it is precipitated, then add buffer to dissolve. It doesn't always work, but I usually get some proportion of peptide into solution this way. Be careful that the trifluoroethanol percentage doesn't cause your buffer to precipitate. Also, this much trifluoroethanol can also cause purification problems (peptide not sticking to the column) if you are using reversed-phase HPLC, but I would not recommend such an HPLC purification for very hydrophobic peptides. You can easily end up all peptide stuck to the column. Purify at the Hmb-protected stage, then remove Hmb in the final step. These types of peptides are tough to work with, so I hope this helps and best of luck.
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