. How Much Alanine Is Present as the Com Uncharged Species? At a pll equal to th
ID: 562877 • Letter: #
Question
. How Much Alanine Is Present as the Com Uncharged Species? At a pll equal to the isoelectric poirt alanine, the net charge on alanine is zero. Two structureg be drawn that have a net charge of zero, but the predoni form of alanine at its pl is zwitterionic. Completely CH O H2N-C-C H OFH Uncharged Zwitterionic (a) Why is alanine predominantly zwitterionic rather than completely uncharged at its pl? (b) What fraction of alanine is in the completely uncharged form at its pl? Justify your assumptions.Explanation / Answer
(a) We know that Alanine is - amino acid having both - carboxyl and - amino groups on either side.
As pI is very much significantly greater than pKa of the -carboxyl group and for -amino group
pI is very much less than pKa. Hence both the groups on the either side of alanine structure remain
charged (COO- and NH4+) with opposite charges of course, which makes it zwitterionic rather than
completely uncharged at its pI.
(b) pKa1 = 2.34 ; pKa2 = 9.69
pI = 6.11
Use Henderson Hasselbalcch equation,
pH = pKa1 + log [ charged / uncharged form]
6.11 = 2.34 + log [ COO- / COOH]
3.77 = log [ COO- / COOH]
103.77 = [ COO- / COOH] = 5888.4365 = [charged / uncharged form]
Check the ratio of uncharged to charged form for amino group with pKa 9.69 to justify
the assumption.
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