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Questions and Answers
Which type of amino acid R-group is characterized by being non-polar and hydrophobic?
Which type of amino acid R-group is characterized by being non-polar and hydrophobic?
Which classification does an amino acid with a carboxyl group in its side chain belong to?
Which classification does an amino acid with a carboxyl group in its side chain belong to?
What characterizes polar-neutral amino acids?
What characterizes polar-neutral amino acids?
Which of the following amino acid classifications is characterized by aromatic side chains?
Which of the following amino acid classifications is characterized by aromatic side chains?
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Which of the following statements about the shorthand notations for amino acids is true?
Which of the following statements about the shorthand notations for amino acids is true?
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What characterizes a zwitterion?
What characterizes a zwitterion?
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At which pH is an amino acid likely to be a zwitterion?
At which pH is an amino acid likely to be a zwitterion?
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What occurs to amino acids in solution at high pH?
What occurs to amino acids in solution at high pH?
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What is the isoelectric point (pI) of an amino acid?
What is the isoelectric point (pI) of an amino acid?
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Which statement best describes amino acids in relation to pH changes?
Which statement best describes amino acids in relation to pH changes?
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Study Notes
Amino Acids Classification
- Amino acids are classified into five groups based on R-groups, according to their polarity and interaction with water.
- Non-polar, aliphatic R groups are neutral, hydrophobic, and found in the protein's interior.
- Non-polar, aromatic R groups are also neutral, non-polar, and engage in hydrophobic interactions.
- Polar amino acids are hydrophilic and are divided into:
- Polar-neutral: more soluble in water.
- Polar acidic: contain carboxyl groups.
- Polar basic: contain amino groups.
Nomenclature and Forms
- Amino acids have common names and shorthand notations; three-letter and one-letter abbreviations are widely used.
- The first letter of the amino acid name is capitalized; for certain amino acids, the first three letters may include only the first letter capitalized.
- α-amino acids are typically written in zwitterion form, representing their internal salt structure.
Acid-Base Properties
- Amino acids exist as zwitterions—molecules with both positive and negative charges and a net zero charge.
- Carboxyl groups release protons (H+) while amino groups accept protons to create positive charges.
- The net charge of an amino acid in solution varies with pH, shifting between zwitterions, positive ions, and negative ions.
Isoelectric Point (pI)
- The isoelectric point is the specific pH where an amino acid has a net charge of zero.
- Amino acids are not attracted to electric fields at their pI.
- Different amino acids have unique isoelectric points depending on their structure and side chains.
Guidelines for Amino Acid Forms
- At low pH: all carboxyl and amino groups are protonated (net + charge).
- At high pH: all groups are deprotonated (net - charge).
- At neutral pH: carboxyl groups are deprotonated; amino groups are protonated (net 0 charge).
pKa Values and Peptide Behavior
- Different amino acids have varying pKa values that affect their ionization and charge at specified pH levels.
- Proteins and peptides act as zwitterions and have isoelectric points; their solubility changes with pH, affecting precipitation.
Formation of Peptides
- Emil Fischer proposed that proteins are chains of amino acids connected by peptide bonds (amide bonds).
- The peptide bond forms between the α-carboxyl group of one amino acid and the α-amino group of another.
Writing Peptides
- Peptides are depicted from left to right, starting with the N-terminal (free -NH3+) to the C-terminal (free -COO-).
- N-terminal and C-terminal designations indicate positions in the amino acid sequence.
Peptide Nomenclature
- The IUPAC naming convention replaces amino acid name endings with the suffix "-yl," except for the C-terminal amino acid, which retains its full name.
- Example notation for a peptide includes the sequence: Ala-Phe-Ser; abbreviation: AFS.
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Description
Test your knowledge on the classification of standard amino acids based on their R-group properties. This quiz focuses on groups defined by polarity and their behavior in biological systems. Understand how these classifications influence protein structure and function.