3.13 Amino Acids and Proteins Flashcards
Why are amino acids considered amphoteric?
∵ have basic amino group and acidic carboxyl group
Amino acids are ____ molecules
Chiral
Draw an amino acid
Amino acids an exists as ______
Zwitterions
What are zwitterions?
Dipolar ions
Have both +ve and -ve charge in different parts of molecule
Zwitterions only exist near an amino acid’s ____ ___
isoelectric point
What is meant by an amino acid’s isoelectric point?
This is the pH where average overall charge on amino acid is 0
Describe what happens to an amino acid at the isoelectric point
At the isoelectric point, both carboxyl and amino group are likely to be ionised forming an ion called a zwitterion
Describe what happens to an amino acid when it’s in conditions more acidic than the isoelectric point
COO- group is likely to gain an H
Describe what happens to an amino acid when it’s in conditions more alkaline than the isoelectric point
-NH3+ group is likely to lose an H
What can you use to identify unknown amino acids?
Thin-layer chromatography
Explain how you can use thin-layer chromatography to identify different amino acids in mixture
- Draw pencil line near bottom of thin-layer chromatography plate & put concentrated spot of mixture of amino acids on it
- Dip bottom of plate (not the spot) into a solvent
- As solvent spreads up plate, different amino acids move with it, but a different rates so they separate out
- When solvent’s nearly reached the top, take out plate and mark solvent front with pencil
- Leave plate to dry
- Amino acids aren’t coloured so make spots visible
- Work out Rf value of each amino acid
- Use table of known amino acid Rf values to identify amino acids in mixture
Explain two ways how you can make amino acids visible in thin-layer chromatography
- Spray ninhydrin solution on plate = turns spots purple
- Or use special plate which has fluorescent dye added to it
- Dye glows when UV light shines on it
- Where there’s spots of chemical on plate, cover fluorescent dye so sports appear dark
- Can draw dark patches to show were spots are under UV lamp
State how you work out Rf value
Draw a condensation reaction of two amino acids joining together
Describe how to hydrolyse a protein into its individual amino acids
Add hot aqueous 6 M HCl & heat mixture under reflux for 24 hours
Describe the primary structure of protein
Sequence of amino acids in polypeptide chain
Describe the secondary structure of protein
- Peptide links form hydrogen bond sight each other
- So chain isn’t straight
- Forms α helix or β-pleated sheet
Describe the tertiary structure of protein
- Further folding of whole polypeptide chain
- More bonds form between different parts (R groups) of polypeptide chain
- Gives 3D shape
Name the 2 main types of bond that hold proteins in shape
- Hydrogen bonding
- Disulfide (or sulfur-sulfur) bonding (-S-S-)
Where does hydrogen bonding exist in proteins?
Exists between polar groups e.g. -OH and -NH2
Where does the hydrogen bonding in proteins do?
Stabilise both secondary and tertiary structure of the protein
Amino acid cysteine contains ___ group (-SH)
thiol
Desribe disulfide (or sulfur-sulfur) bonding (-S-S-) occurs in proteins
Thiol groups on different cysteine residues can lose their H atoms and join together by forming disulfide (or sulfur-sulfur) bond (-S-S-)
What does disulfide (or sulfur-sulfur) bonding (-S-S-) do in proteins
Disulfide bonds link together in different parts of protein chain & help to stabilise tertiary structure
Name 2 factors affect shape of proteins?
Temperature and pH can affect hydrogen bonding and formation of disulfide bonds
Describe the structure of cisplatin
Complex of platinum(II) with 2 chloride ion ligands and 2 ammonia ligands in square planar shape
Draw cisplatin
What is cisplatin used as?
Anti-cancer drug
Describe what cisplatin does
- Cisplatin binds to DNA, causing kinks in DNA helix which stops proteins that replicate DNA from copying it properly
- Stops tumour cells reproducing
Describe in detail how cisplatin causes kinks in DNA
- N atom on guanine base in DNA forms co-ordinate bond with cisplatin’s platinum ion, replacing on of chloride ion ligands
- (Ligand substitution reaction)
- 2nd N atom from nearby guanine can bond to platinum and replace the second chloride ion
- Presence of cisplatin complex bound to DNA strands causes strands to kink
- Means DNA strands can’t unwind and be copied properly = ∴ cell can’t replicate
Describe the disadvantages of using cisplatin as anti-cancer drug
- Cisplatin can bind to DNA in normal cells as well as cancer cells
- Problem for healthy cells that replicate frequently e.g. hair and blood cells ∵ cisplatin stops them from replicating
- Means cisplatin causes hair loss and suppress immune system
- Can also cause kidney damage
Describe how the side effects of cisplatin can be lessened
- By giving patients very low dosages of cisplatin
- Or target cisplatin to tumour
- Means using method that delivers drug only to cancer cells so it cant attack healthy cells
Why is it that despite the side effects of cisplatin, it is still used as a chemotherapy drug?
∵ balance of long-term positive effects outweigh negative short-term effects
Explain how intereactions in the secondary structure of a protein are formed (3)
- N and O both very electronegative or C=O and N–H are polar
- Lone pair on O attracted to the δ+H of N-H
- Hydrogen bond formed
Explain why different amino acids have different Rf values (2)
- Amino acids have different polarities
- So they have different affinities for mobile and stationary phases
Draw the structure of the species formed when serine reacts with an excess of ethanoyl chloride (2)
Explain why alanine has a high melting point (1)
Strong electrostatic attraction between opposite charged ions
