Two structures of alanine can be drawn that have a net charge of zero, but the...

Two structures of alanine can be drawn that have a net charge of zero, but the predominant form of alanine at its pI is zwitterionic. Why is alanine predominantly zwitterionic rather than completely uncharged at its pI? What percent (to one significant figure) of alanine molecules would be in the completely uncharged form at the pI? Hint: pK1 of alanine is 2.35 and pK2 is 9.87.

Expert Solution

The PI of alanine occurs at a pH well above the pKa of the -carboxyl group and well below the pKa of the -amino group. Hence, both the groupf are present predominantly in their charged (ionized) form.

B)

The pI of alanine is 6.01, midway between the two pKa values, 2.35 and 9.87. From the Henderson - Hasselbach equation,

i.e.

that is, one molecule in 4570 is still in the form -COOH. Similarly at pH = pI, one molecule is 4570 is in the form -NH₂. Thus, the percent of molecules with both groups uncharged ( - COOH and - NH₂) is 1 in 4570 x 4570, or 1 in 2.08*10⁷.

queen_honey_blossom answered 2 years ago

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