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>It is not surprising that your bradford and BCA assays don't agree if you >have no aromatic amino acids in your protein. Bradford dye binds to >hydrophobic residues, mainly aromatics, so I would guess your bradford is >consistantly giving lower measurements than the BCA assay. I also wouldn't >be surprised if the results of your Bradford vary significantly between >replicates. The BCA assay reagent interacts with the backbone amides, not >with any sidechains, so I would tend to believe that measurement more than >anything else you have done. > >I work with a protein that has very few hydrophobics (only one aromatic - >a Phe) and I have found that Bradfords are unreliable, but the BCA assay >tends to be consistent. Bradford reagent is colloidal Coomassie G-250 and it's binding to proteins is very complex, depending on local structure, hydrophobic interaction and basic charges (mainly Arg residues). So yes, it is quite variable protein to protein but it is not a simple function of aromatics. - Dima