Biochem 1.3 Acid-Base Chemistry of Amino Acids

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Questions and Answers

What is the charge of an amino acid backbone at physiological pH?

Negative charge

Positive charge

Variable charge depending on the side chain

Neutral charge (correct)

Which group tends to become positively charged when the pH is low?

Side chain groups

Amino groups (correct)

Carboxyl groups

Zwitterion groups

Which of the following amino acids is classified as ionizable?

Arginine (correct)

Glycine

Serine

Alanine

What effect does increasing the pH have on the carboxyl and amino groups of an amino acid?

Both groups lose protons Signup and view all the answers

Which statement best describes the ionization state of amino acids at high pH?

They are predominantly anions Signup and view all the answers

Which amino acids have the potential to carry a negative charge at physiological pH?

Glutamate and Aspartate Signup and view all the answers

What happens to amino acids at low pH?

They gain protons and may become cations Signup and view all the answers

Which of the following options is NOT an ionizable amino acid?

Valine Signup and view all the answers

What is the average charge of histidine at its isoelectric point of 7.8?

0 Signup and view all the answers

What is the predominant ionization state of histidine between the pH values of 6.0 and 9.6?

0 Signup and view all the answers

How is the isoelectric point (pI) of an amino acid determined?

By averaging the pKa values of the acidic and basic groups. Signup and view all the answers

What happens to the average charge of histidine as the pH decreases?

The average charge increases. Signup and view all the answers

Which of the following statements about histidine at its isoelectric point is true?

The charged forms of histidine equalize, resulting in a net charge of zero. Signup and view all the answers

What effect does an adjacent negative charge have on a carboxyl group in terms of proton acceptance?

It increases the likelihood of proton acceptance. Signup and view all the answers

Which ionizable amino acids are typically positively charged when protonated?

Arg, Lys, His Signup and view all the answers

When deprotonated, which ionizable amino acids become negatively charged?

Tyr, Cys, Glu, Asp Signup and view all the answers

Which statement about serine and threonine is true in the context of ionization?

They are typically omitted from the list of ionizable amino acids. Signup and view all the answers

What largely determines the protonation state of an ionizable amino acid side chain?

The local environmental context of the amino acid. Signup and view all the answers

What type of ionizable side chains can be neutral or positive depending on their protonation state?

Side chains containing nitrogen. Signup and view all the answers

Which of the following statements is false regarding the percent ionization of a chemical group?

A single ionizable site can be partially protonated. Signup and view all the answers

How does an alcohol group behave when placed near a positive charge?

It loses a proton and becomes more acidic. Signup and view all the answers

Which amino acids are neutral when protonated?

Tyr, Cys, Glu, Asp Signup and view all the answers

What is the primary factor for determining the charge of side chains that contain oxygen or sulfur?

Protonation state. Signup and view all the answers

What occurs to the percentage of deprotonated groups as pH increases?

It increases. Signup and view all the answers

At what pH is the population of a protonated ionizable group 50%?

pH = pKa Signup and view all the answers

Which statement is true when pH is below the pKa of an ionizable group?

More than 50% of the population is protonated. Signup and view all the answers

What is the approximate pKa value of the α-carboxyl group in amino acids?

2.2 Signup and view all the answers

According to the equation pH = pKa + log[A⁻]/[HA], what does [A⁻] represent?

Concentration of deprotonated form Signup and view all the answers

Which side chain has the highest pKa value among the ionizable side chains listed?

Arginine Signup and view all the answers

What happens to the protonation state of an amino acid when the pH is raised above its pKa?

More than 50% of the population becomes deprotonated. Signup and view all the answers

What is the pKa value of the histidine side chain?

6.0 Signup and view all the answers

Which factor directly affects the protonation state of an ionizable group?

pH of the solution Signup and view all the answers

What can be concluded when the pH is exactly the same as the pKa?

The concentration of protonated and deprotonated forms is equal. Signup and view all the answers

What does the equation $pH = pK_a + log\frac{[A^-]}{[HA]}$ express?

The relationship between pH, pK_a, and the concentrations of protonated and deprotonated forms. Signup and view all the answers

How can the percent ionization of an amino acid be calculated?

% ionization = $\frac{[HA]}{[A^-] + [HA]}$ × 100% Signup and view all the answers

What does the isoelectric point (pI) of an amino acid represent?

The pH at which the population of amino acids has an average net charge of 0. Signup and view all the answers

At what pH is histidine predominantly positive?

Below pH 6.0 Signup and view all the answers

How does pH impact the net charge of amino acid populations?

Decreasing pH can create net positive charges in amino acids. Signup and view all the answers

What occurs to histidine molecules at physiological pH?

Most histidine molecules are neutral with a slight positive charge on average. Signup and view all the answers

To calculate the pI of any amino acid, you average which two values?

The pKa below which the amino acid is predominantly positive and the pKa above which it is predominantly negative. Signup and view all the answers

In what scenario can a population of amino acids collectively have a net negative charge?

By sufficiently increasing the pH. Signup and view all the answers

What is a characteristic of amino acids at low pH?

They tend to be predominantly positively charged. Signup and view all the answers

When examining amino acid ionization, which is an important factor?

The surrounding environmental conditions, such as pH. Signup and view all the answers

Study Notes

Acid-Base Chemistry of Amino Acids

Amino acids can exchange protons with their environment
Protonation/deprotonation affects chemical groups
Ionizable groups can become positive or negative
Amino acids have at least two ionizable groups: backbone amino and carboxyl
Zwitterion: neutral state at physiological pH (protonated amino group, deprotonated carboxyl group, net charge of 0)
pH adjustments change protonation states
- Lower pH: more protons available, carboxyl and amino groups more likely to be protonated, overall positive charge
- Higher pH: fewer free protons, carboxyl and amino groups more likely to be deprotonated, overall negative charge

Ionizable Amino Acid Side Chains

Side chains of many amino acids are electrically neutral at physiological pH
Some side chains can become charged (positive or negative)
Ionizable amino acids: arginine, lysine, histidine, tyrosine, cysteine, glutamate, aspartate
Side chain ionization affected by environment
- Negative charge nearby: carboxyl group more likely to accept a proton, becoming more basic
- Positive charge nearby: alcohol more likely to lose a proton, becoming more acidic

Amino Acid Percent Ionization

Percentage of ionizable groups in a population that are protonated depends on the pKa (the pH at which 50% of the ionizable group is protonated/deprotonated) and the pH of the solution
Lower pH than pKa: More likely to be protonated
Higher pH than pKa: More likely to be deprotonated

Isoelectric Points (pI)

The pH at which an amino acid population has an average net charge of zero (each positive charge balances out a negative charge)
Calculated by averaging pKa values where amino acid is predominantly positive and negative
Histidine example: pI = (6.0 + 9.6) / 2 = 7.8

Amino Acid Titration Curves

Titration: Solution of amino acid brought to a low pH, strong base (NaOH) added gradually, pH plotted against amount of base added
Reveals identity of unknown amino acids
Buffer regions: pH ranges around pKa values where the amount of base added does not result in a large change in pH

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Description

Explore the intricate acid-base chemistry of amino acids and their protonation states in various pH environments. This quiz covers the characteristics of zwitterions and the ionizable side chains of amino acids, which affect their overall charge. Understand the significance of ionizable groups in biochemical contexts.