Amino Acids pH Titration

Questions and Answers

What is the term used to describe molecules that bear charged groups of opposite polarity?

Amino acids

Zwitterions

Both A and B (correct)

Dipolar ions

Amino acids are limited to only 20 for all organisms.

False

What is the result of the specific modification of an amino acid residue after the polypeptide chain has been synthesized?

Uncommon amino acids

The pK value of the ______________ group is represented by pK1.

carboxylic acid Signup and view all the answers

What is the type of modification that occurs in amino acid side chains?

All of the above Signup and view all the answers

Amino acids exist as neutral molecules at physiological pH.

False Signup and view all the answers

What is the term used to describe the simple addition of small chemical groups to certain amino acid side chains?

Modification Signup and view all the answers

Match the following types of amino acid modifications with their descriptions:

Hydroxylation = Addition of a hydroxyl group Methylation = Addition of a methyl group Acetylation = Addition of an acetyl group Carboxylation = Addition of a carboxyl group Signup and view all the answers

What is the pH at which the –NH2 group of alanine starts to deprotonate?

9.7 Signup and view all the answers

At a very low pH (pH ~ 0), all of the ionizable groups of alanine are deprotonated.

False Signup and view all the answers

What is the isoelectric point (pI) of alanine?

6 Signup and view all the answers

The pK values of the α – amino group are represented by _______.

pK2 Signup and view all the answers

What is the effect of increasing the pH of the solution to greater than 9.7 on the ionizable groups of alanine?

All ionizable groups are deprotonated. Signup and view all the answers

Match the following ionizable groups with their corresponding pK values:

α – amino group = pK2 –COOH group = pK1 R group = pKR None of the above = None Signup and view all the answers

Amino acids can be titrated with equivalent amounts of acid to give a titration curve.

False Signup and view all the answers

What is the formula to calculate the isoelectric point (pI) of a diprotic acid?

(pK1 + pK2) / 2 Signup and view all the answers

What are the three major types of amino acids?

Nonpolar, uncharged polar, and charged polar Signup and view all the answers

Cysteine has a thiol group that can form a disulfide bond with another cysteine.

True Signup and view all the answers

What is the characteristic of nonpolar, hydrophobic side chains?

They have carbon-hydrogen containing aliphatic or aromatic groups and have nonpolar character Signup and view all the answers

Polar, uncharged side chains have at least one ______________________ with electron pairs available for H-bonding.

heteroatom (O or S) Signup and view all the answers

Which of the following amino acids has a charged side chain?

Arginine Signup and view all the answers

All amino acids have charged side chains.

False Signup and view all the answers

Match the following amino acid types with their characteristics:

Nonpolar, hydrophobic = Have carbon-hydrogen containing aliphatic or aromatic groups Polar, uncharged = Have at least one heteroatom (O or S) with electron pairs available for H-bonding Charged, polar = Can be classified as either basic or acidic Signup and view all the answers

What is the unique feature of cysteine among the 20 amino acids?

It has a thiol group that can form a disulfide bond with another cysteine through the oxidation of the two thiol groups Signup and view all the answers

Study Notes

Amino Acids Are Dipolar Ions

Amino acids can be titrated with equivalent amounts of base (e.g., NaOH) to give a titration curve.
At a very low pH (pH ~ 0), all ionizable groups of amino acids are protonated.
As pH increases, the proton of the –COOH group ionizes (pH = pK1), and the protonated and deprotonated forms are in equilibrium.
Further increasing the pH (pH = pK2) deprotonates the –NH2 group, and the protonated and deprotonated forms are in equilibrium.

Isoelectric Point

Defined as the pH at which a molecule carries no net electrical charge or is electrically neutral.
Often designated by pI.
For a diprotic acid, pI = (pK1 + pK2) / 2.
For alanine, pI = (2.3 + 9.7) / 2 = 6.

Calculation of pI

Draw out the complete ionization of an amino acid.
Determine the net charge on each ionized form.
Calculate pI using the formula: pI = (pK1 + pK2) / 2.

Classification of Amino Acids

Three major types of amino acids:
- Nonpolar (hydrophobic) side chains
- Polar, uncharged side chains
- Charged, polar side chains
Examples of classification:
- Aliphatic amino acids: glycine, alanine, valine
- Aromatic amino acids: phenylalanine, tyrosine, tryptophan
- OH-containing amino acids: serine, threonine
- Basic amino acids: lysine, arginine, histidine
- Sulfur-containing amino acids: cysteine, methionine

Uncommon Amino Acids

20 common amino acids are not the only ones that occur in biological systems.
"Nonstandard" amino acid residues are often important constituents of proteins and biologically active peptides.
These unusual amino acids result from specific modifications of an amino acid residue after the polypeptide chain has been synthesized.
Examples of modifications: hydroxylation, methylation, acetylation, carboxylation, and phosphorylation.

Dipolar Ions

Amino acids exist as dipolar ionic species at physiological pH.
The amino and carboxylic acid groups of amino acids readily ionize.
Molecules such as amino acids, which bear charged groups of opposite polarity, are known as dipolar ions or zwitterions.

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Description

Learn how amino acids behave as dipolar ions and understand their titration curves, isoelectric points, and protonation/deprotonation equilibria at different pH levels.

Amino Acids pH Titration

Questions and Answers

What is the term used to describe molecules that bear charged groups of opposite polarity?

Amino acids are limited to only 20 for all organisms.

What is the result of the specific modification of an amino acid residue after the polypeptide chain has been synthesized?

The pK value of the ______________ group is represented by pK1.

What is the type of modification that occurs in amino acid side chains?

Amino acids exist as neutral molecules at physiological pH.

What is the term used to describe the simple addition of small chemical groups to certain amino acid side chains?

Match the following types of amino acid modifications with their descriptions:

What is the pH at which the –NH2 group of alanine starts to deprotonate?

At a very low pH (pH ~ 0), all of the ionizable groups of alanine are deprotonated.

What is the isoelectric point (pI) of alanine?

The pK values of the α – amino group are represented by _______.

What is the effect of increasing the pH of the solution to greater than 9.7 on the ionizable groups of alanine?

Match the following ionizable groups with their corresponding pK values:

Amino acids can be titrated with equivalent amounts of acid to give a titration curve.

What is the formula to calculate the isoelectric point (pI) of a diprotic acid?

What are the three major types of amino acids?

Cysteine has a thiol group that can form a disulfide bond with another cysteine.

What is the characteristic of nonpolar, hydrophobic side chains?

Polar, uncharged side chains have at least one ______________________ with electron pairs available for H-bonding.

Which of the following amino acids has a charged side chain?

All amino acids have charged side chains.

Match the following amino acid types with their characteristics:

What is the unique feature of cysteine among the 20 amino acids?

Study Notes

Amino Acids Are Dipolar Ions

Isoelectric Point

Calculation of pI

Classification of Amino Acids

Uncommon Amino Acids

Dipolar Ions

Studying That Suits You

Description

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