Hi, I have been trying to dissolve a very hydrophobic peptide (containing cysteine) with DMF, however, the solution looks very cloudy when I add DMF, and further dilutions with water doesn't help much. Does anyone have any tips on this issue? In addition, I'm use NanoDrop to measure concentration, but when I use DMF as blank, it shows "bad blank". By the way, I'm measuring the concentration at A280. Thanks!
I can think of several potential problems for you. But first let me support Kun Yun's recommendation to avoid DMSO for Cys peptides. DMSO can oxidize Cys to oxygen-containing states.
DMF has a UV absorption tail that extends up into the mid300, so it is not acceptable for quantitation via A280. I recommend RPHPLC for quantitation, although you'll want a standard.
As you may know, one handy property of DMF is its vapor pressure. It's fairly easily removed via evaporation. If you add water you can conveniently lyophilize, but the DMF will come out in your vacuum pump exhaust.
If you reckon most of your peptide went into solution, then the cloudiness you observe may not be your target product. Crude synthetic peptides containing Cys commonly undergo many side reactions during the final acid cleavage and workup steps, resulting in relatively low yield and significant junk. So think about centrifuging or even filtering the solution.
Also, depending on the sequence, Cys may rapidly dimerize in some peptides, when exposed to atmospheric oxygen. And if your peptide has more than one Cys, oxidation can lead to formation of peptide multimers, especially in high concentration (>10mg/mL). If your peptide has low solubility, dimers and multimers probably have even lower solubility. This problem can be addressed by including a small amount (0.01% w/v) of DTT or other reducing agent in all solutions. If that is not practical sparge them thoroughly.
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