Isoelectric Point and Amino Acids Quiz

What is the isoelectric point (PI) of a molecule?

• The pH at which the molecule is most acidic

• The pH at which the molecule has a net charge of zero

• The pH at which the molecule has a net negative charge

• The pH at which the molecule has a net positive charge

What happens to alanine at a very low pH?

• It becomes deprotonated with a net charge of minus one

• It remains neutral with no charge

• It becomes protonated with a net charge of plus one

• It becomes deprotonated with a net charge of zero

How do you determine the PI of a molecule with two pKa values?

• By multiplying the two pKa values

• By dividing the two pKa values

• By averaging the two pKa values

• By subtracting the two pKa values

What is the first step in calculating the PI of a peptide with multiple ionizable groups?

• Deprotonate every ionizable group

• Protonate every ionizable group

• Arrange pKa values in order from high to low

• Arrange pKa values in order from low to high

What is the net charge of a peptide when all ionizable groups are protonated?

• Zero

• Plus one

• Plus two

• Minus one

In the PI calculation example, what happens when the pH is raised above the first pKa?

• The amine group is deprotonated

• The carboxy group is deprotonated

• The carboxy group is protonated

• The sulfur group is protonated

What is the final PI value calculated for the peptide example with pKa values of 8 and 10?

• 7.5

• 8.5

• 9.0

• 9.5

In the complex peptide example, what is the estimated pKa value for the parent carboxy group?

• 12

• 4

• 2

• 9

What is the net charge of the complex peptide when all groups are protonated?

• Plus one

• Plus two

• Zero

• Minus one

What is the final PI value calculated for the complex peptide example?

• 7.5

• 7.0

• 6.5

• 6.0