HELP!!!
umichundergrad
someone could help me.
This is the question.
At a pH equal to the isoelectric point of alanine, the net charge on
alanine is zero. There are two structures that have a net charge of
zero. (a) What are these two structures? (b) Why is alanine
predominantly zwiterionic rather than completely uncharged at its pI?
(c) What fraction of alanine is in the completely uncharged form at
its pI? Justify any assumptions you make. The pKa1 = 2.35 and pKa2 =
9.87
a) I am thinking that the two structures are the amino group and the
carboxyl group.
b) I know that the pI of alanine is 6.11. Zwitterionic means dipolar.
Any help would be appreciated. Thank you!
Bob
wrote:
First, it helps -- helps you -- if you give your msg a meaningful
title. Not everyone reads every msg, and titles like yours will be a
turn off to many.
>I am having problems with a Biochem question and I was wondering if
>someone could help me.
>
>This is the question.
>
>At a pH equal to the isoelectric point of alanine, the net charge on
>alanine is zero. There are two structures that have a net charge of
>zero. (a) What are these two structures? (b) Why is alanine
>predominantly zwiterionic rather than completely uncharged at its pI?
>(c) What fraction of alanine is in the completely uncharged form at
>its pI? Justify any assumptions you make. The pKa1 = 2.35 and pKa2 =
>9.87
>
>a) I am thinking that the two structures are the amino group and the
>carboxyl group.
"two structures" here refers to two ways of writing the entire amino
acid. Example... CH3CH3 is one "structure" for ethane.
The two things you named are the two ionizable functional groups.
Hint: See part b.
>
>b) I know that the pI of alanine is 6.11. Zwitterionic means dipolar.
More specifically here, means that it carries both + and - charges,
which balance.
The pI is the pH at which it has no net charge.
Which group has pKa = 2? What does that mean? What happens to this
group at lower pH? higher pH? Therefore, in what form is this group at
pH 6 (the pI). Etc. This logic will tell you the predominant form at
pH 6.
Part c is simply the quantitative calculation -- somewhat messy, but
straightforward from Chem 1.
Hope that aims you. If you want to follow up, would you please provide
some context. The basic background material for this is basic org chem
(the functional groups and their pKs), and most any biochem book would
have a good explanation of the structure of amino acids vs pH. But
sometimes a general biol book tries to go too far too fast.
bob
Uncle Al
Take a year of organic chemistry before starting biochemistry.
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--
Uncle Al
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