1.2 Acid–Base Chemistry of Amino Acids Flashcards

1
Q

What happens to amphoteric species under low and high pH?

A

at low pH, ionizable groups tend to be protonated; at high pH, they tend to be deprotonated.

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2
Q

How do pH and pKa affect protonation of an AA?

A

If the pH is less than the pKa, a
majority of the species will be protonated. If the pH is higher than the pKa, a majority of the species will be deprotonated

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3
Q

How many pKa values are there for AAs?

A

2, one for the carboxyl (COOH) and one for amino group (NH2)

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4
Q

What are the 1st and 2nd pKa’s?

A
  • pKa1 for the carboxyl group is around 2
  • pKa2 for amino group is usually between 9 and 10
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5
Q

What happens to AA’s under very acidic (pH 1) conditions?

A
  • AA’s tend to be positively charged
  • NH2 -> NH3+
  • COOH becomes neutral
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6
Q

What happens to AA’s under intermediate (pH 7.4) conditions?

A
  • AA’s are zwitterions
  • NH2 -> NH3+
  • COOH -> COO-
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7
Q

What happens to AA’s under Basic (pH 10.5) conditions?

A
  • AA’s are negative
  • NH3+ -> NH2
  • Remains COO-
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8
Q

What does titration curve of AA look like?

A
  • like a combination of two monoprotic acid titration curves (or three curves, if the side chain is charged)
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9
Q

When does the solution act as a buffer?

A
  • When the pH of a solution is approximately equal to the pKa of the solute.
  • The titration curve is relatively flat
  • As we add more base, the carboxylate group goes from half-deprotonated to fully deprotonated. The amino acid stops acting like a buffer, and pH starts to increase rapidly during this phase
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10
Q

Isoelectric point (pI)

A

the pH at which the molecule is electrically neutral

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11
Q

What does the titration curve of a neutral molecule look like?

A
  • When the molecule is neutral, it is especially sensitive to pH changes, and the titration curve is nearly vertical
  • As we continue adding base, glycine passes through a second buffering phase as the amino group deprotonates; again, the pH remains relatively constant.
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12
Q

What’s glutamic acids charge when fully protonated?

A

+1, the amino group gets protonated (NH3+)

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13
Q

What’s lysines charge when fully protonated?

A

+2, amino groups get protonated

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14
Q

How do basic and acidic side chains affect isoelectric points?

A

amino acids with acidic side chains (COOH) have relatively
low isoelectric points, while those with basic side chains (NH2) have relatively high
ones.

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15
Q

How to find Isoelectric point of AA’s?

A
  • Add the pKas of side chains and divide by 2
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