Unformatted text preview: why/how it works.
Isoelectric point=isoelectric pH, the pH at which the average charge of apolyprotic species is 0. For the
amino acid leucine, represented by the neutral, zwitterionic HL and having equilibria At the isoelectric point, and . In contrast, the isoionic point (or isoionic pH) is the pH obtained when the pure, neutral polyprotic acid HL is dissolved in water. The
only ions in solution are H2A+, A-, H+ and OH- and
See isoelectric focusing or the textbook and classroom discussion of the surface charge of glass. 2. [10 marks] Define zwitterion. Why is it important?
A zwitterions is a species that has two moieties within the molecule bearing opposite formal charges.
For an amino acid such as alanine, the zwitterions is the net neutral species. The sites within an amino
acid have different pKa’s allowing their protonation state to change as a function of pH, to make the
amino acid overall positive, neutral or negative. The zwitterionic nature of amino acids allows them to
hold charge in different parts of protein molecules and thereby to determine the charge distribution o f a
protein, which in part governs pr...
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This document was uploaded on 03/12/2014 for the course CHM 212 at Rhode Island.
- Fall '11