PL aijq: Amino acids; peptide bonding; optical isomerism Flashcards
Aminocarboxylic acids are more commonly known as what?
α-amino acids
Draw the general structure of an α-amino acid.
alpha carbon attached to both -NH2 + -COOH groups
![](https://s3.amazonaws.com/brainscape-prod/system/cm/331/318/678/a_image_thumb.png?1593346536)
Amino acids are bifunctional compounds. What is meant by “bifunctional”?
Has 2 functional groups (i.e. NH2 + COOH)
Name a property of amino acids which is derived from their:
- amine group
- carboxyl group
- Amine: basic: react with acids to form salts
- Carboxyl: acidic: react with alkalis to form salts
What are zwitterions?
Neutral molecules containing both negatively and positively charged groups.
How do amino acids behave when dissolved in water?
- Form a (mostly) zwitterion solution, which is neutral (unless extra COOH / NH2 in R group)
- Highly soluble since effectively ionic
COOH is proton-donating + NH2 is proton-accepting
![](https://s3.amazonaws.com/brainscape-prod/system/cm/331/318/683/a_image_thumb.png?1593352142)
Give the ionic form in which amino acids exist in:
- neutral solution
- acid solution
- alkaline solution
- Neutral: H3N+-CHR-COO-
- Acid: H3N+-CHR-COOH
- Alkaline: H2N-CHR-COO-
Adding small quantities of acid or alkali to an amino acid solution has little effect on the pH. Explain why, using equations to support your answer.
Solutions of amino acids are buffer solutions, since they contain zwitterions, which can either donate or accept protons.
H3N+-CHR-COO- + OH- → H2O + H2N-CHR-COO-
H3N+-CHR-COO- + H3O+ → H2O + H3N+-CHR-COOH
Why do amino acids exhbit optical isomerism?
There are 4 different groups arranged tetrahedrally around a carbon atom.
What is the other term for “optical isomer”?
Enantiomer
Explain why 2-aminopropanoic acid has an enantiomer but aminoethanoic acid doesn’t.
All 4 groups on the former are different, whereas the latter’s carbon is bonded to 2 hydrogens.
State what is meant by the term “chiral molecule”.
A molecule with non-superimposable mirror images.
What is the chiral centre?
The carbon in a chiral molecule which is bonded to 4 different groups.
Proteins are built from only one enantiomer of each amino acid. What are these known as?
L-enantiomers
How do enantiomers compare in their properties?
- Most physical properties (melting/boiling point, solubility, density) and chemical properties are the same
- Behaviour differs in presence of other chiral molecules
- Sometimes smell/taste different (due to enzymes also being chiral)