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Questions and Answers
What is the term used to describe molecules that bear charged groups of opposite polarity?
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.
Amino acids are limited to only 20 for all organisms.
False (B)
What is the result of the specific modification of an amino acid residue after the polypeptide chain has been synthesized?
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.
The pK value of the ______________ group is represented by pK1.
What is the type of modification that occurs in amino acid side chains?
What is the type of modification that occurs in amino acid side chains?
Amino acids exist as neutral molecules at physiological pH.
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?
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:
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?
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.
At a very low pH (pH ~ 0), all of the ionizable groups of alanine are deprotonated.
What is the isoelectric point (pI) of alanine?
What is the isoelectric point (pI) of alanine?
The pK values of the α – amino group are represented by _______.
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?
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:
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.
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 is the formula to calculate the isoelectric point (pI) of a diprotic acid?
What are the three major types of amino acids?
What are the three major types of amino acids?
Cysteine has a thiol group that can form a disulfide bond with another cysteine.
Cysteine has a thiol group that can form a disulfide bond with another cysteine.
What is the characteristic of nonpolar, hydrophobic side chains?
What is the characteristic of nonpolar, hydrophobic side chains?
Polar, uncharged side chains have at least one ______________________ with electron pairs available for H-bonding.
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?
Which of the following amino acids has a charged side chain?
All amino acids have charged side chains.
All amino acids have charged side chains.
Match the following amino acid types with their characteristics:
Match the following amino acid types with their characteristics:
What is the unique feature of cysteine among the 20 amino acids?
What is the unique feature of cysteine among the 20 amino acids?
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.
