Amino Acids Flashcards
Describe the 3 properties of amino acids
- Amino acids are alpha-amino acids because the amino group is attached to the carbon alpha to the hydroxyl group
- Amino acids acids are primary amines (except for proline, which is a cyclic secondary amine)
- Amino acids have an R carbon (except for glycine, where R = H)
Describe acid catalysed hydrolysis of amino acids (nucelophilic acyl substitution)
Water is the nucleophile (requires heat)
Describe base catalysed hydrolysis of amino acids
Describe the unique stereochemistry of amino acids
Natural amino acids that are chiral are L-amino acids (glycine is achiral)
- L corresponds to S configuration (except for cysteine, where L = R configuration)
State the 9 nonpolar/hydrophobic side chains
- Glycine
- Alanine
- Valine
- Isoleucine
- Leucine
- Phenylalanine
- Tryptophan
- Methionine
- Proline
State the 6 polar side chains
- Serine
- Threonine
- Cysteine
- Asparagine
- Glutamine
- Tyrosine
State the 2 acidic side chains
Acid: ionized at physiological pH and are negative
- Aspartic acid
- Glutamic acid
State the 3 basic side chains
Base: protonated at physiological pH and are positive
- Histidine
- Lysine
- Arginine
State the Henderson-Hasselbalch equation
- State the pKa of NH2 and COOH
- When protonated, the amine group -NH3+ goes from 35 to 9
- When protonated, the carboxyl group -COOH goes from 5 to 2
Describe the titration curve
How can pKa be used to determine the charge state of functional groups?
If pH < pKa, protonated
If pH > pKa, deprotonated
Describe the acidic and basic properties of amino acids as well as their solubility
- Amino acids are bifunctional/amphoteric: can act as an acid or a base
- They are not very soluble because they form ion pairs
- But, when amino acids have net charges, ion pairs cannot form
- Thus, amino acids are extremely soluble in acidic or basic conditions
What is the isoelectric point (pI)?
How is it calculated?
pI: the pH of the solution at which the collection of molecules does not have a net charge
- For amino acids with neutral side chains, the pI is the average of the pKa values for the ammonium and carboxyl
- For amino acids with acidic/basic side chains, the pI is the average of the 2 pKa values closest to each other
What does the pl tell us?
- pH > pI = net negative charge
- pH < pI = net positive charge
How are amino acids identified by electrophoresis?
The sample is added to the middle and a current is applied
Net positive amino acids will move to the cathode, and net negative amino acids will move to the anode
