acetylation of amino acids

[rorosingsong]

Hi, I've just started studying molecular biology this semester and I'm a little stumped on a homework problem - would really appreciate some insight.

 

I have a hypothetical hexapeptide with N-terminal Glutamine and C-terminal Arginine.

 

The question is "If the N-terminal was acetylated how would this affect pI?"

 

I know acetylation is the addition of an acetyl group (C₂H₃O), and that glutamine is a polar amino acid -- however it's uncharged at pH 7 and I can't see how the addition of an acetyl group would affect the charge of the molecule or the pI in anyway.

We haven't covered acetylation in any of our lectures, so I'm really not sure about this at all.

 

Please help!

 

Thanks in advance,

rorosingsong.

[mississippichem]

Think about what effect an acetyl group might have on an amine nitrogen with respect to electron density. Should the acetyl group be electron-withdrawing or electron-donating to the amine-N of glutamine? How do electron withdrawing and donating groups affect the ionizability of a proton? What's more basic...an amine or an amide? Just a few questions to ask yourself that might point in the right direction.

[rorosingsong]

Hi Mississipichem, thanks for your prompt reply!

Um, I'm just a beginner so stop me if I'm wrong, but I'm guessing that the acetyl group will be elctron-withdrawing (due to the electronegativity of the carbonyl group), thus making the amine N of glutamine more likely to give up its proton...? This would make the glutamine more acidic and thus lower the pI....?

 

Wait, amines are more basic than amides right?