Easy Biochemistry question

[Stacker]

Would you expect the carboxylate side chain of aspartic acid (pk = 3.65) to be protonated or deprotonated in an aqueous solution of pH 7?

My guess is that it is deprotonated.

Would you expect the pK of the carboxylate side chain of aspartic acid to increase, decrease, or stay the same in an organic solution of pH 7?

Stay the same?

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[Chrome19]

Would you expect the carboxylate side chain of aspartic acid (pk = 3.65) to be protonated or deprotonated in an aqueous solution of pH 7?

My guess is that it is deprotonated.

Would you expect the pK of the carboxylate side chain of aspartic acid to increase, decrease, or stay the same in an organic solution of pH 7?

Stay the same?

Your second answer is generally incorrect. pKa varies depending on the chemical environment. The pKa of amino acid side chains can vary depending on the solvent accessibility of the residue. In an organic solvent (truly hydrophobic locale), an acid will be unlikely to release its proton thus its pKa has increased. Then again, a residue nearby can alter this effect by being a willing proton acceptor in an otherwise hydrophobic region, a common feature in many enzymes.

I doubt your second scenario makes for a good MCAT question, unless of course it's in a passage.

 

[SN2ed]

Thread moved. These types of questions belong in the Study Q&A forum only.