4.7 amino acids, peptides and proteins Flashcards
what is the general formula of an amino acid?
NH2 CHR COOH
- carboxylic acid group
- amine group
- hydrogen atom
- R group (alkyl group)
by changing what do you change the amino acid?
the R group
what is an α-amino acid?
- an organic molecule containing a carboxylic acid group and amine group bonded to the same carbon atom
- an amino acid where the amino and acid groups are attached to the same carbon atom
are all α-amino acids chiral?
no - all apart from glycine
how to name amino acids?
- e.g 2-aminoethanoic acid
- if 2 COOH, then e.g ethanedioic acid
what does amphoteric mean?
- that a substance can act as both an acid and as a base
are amino acids amphoteric?
yes
how are amino acids amphoteric?
- COOH group = can donate a proton and ∴ behave like an acid
- NH3 group = has a lone pair of electrons on the N atom - can accept a proton and behave like a base
what happens to the amino acid’s structure in acidic solutions?
- high H+ conc
- +ve charge on N and NH3+
what happens to the amino acid’s structure in alkaline/basic solutions?
- high OH- conc
- -ve charge on O of COO-
- phenols
- carboxylic acids
- alcohols
- which one is more acidic?
- alcohol = leasts
- phenol = middle
- carboxylic acid = most
do alcohols react with NaOH? Na2CO3?
- NaOH = no
- Na2CO3 = no
do phenols react with NaOH? Na2CO3?
- NaOH = yes
- Na2CO3 = no
do carboxylic acids react with NaOH? Na2CO3?
- NaOH = yes (
- Na2CO3 = yes
what is the isoelectric point of an amino acid?
the pH at which the amino acid had a neutral charge (no net charge overall)
- the pH at which an amino acid exists primarily in its zwitter form
at the isoelectric point, almost all amino acid molecules in a solution (more than 99%) are present in their zwitterion form
if an amino acid is not reported to be in an acidic or alkaline solution, you must assume it is in its zwitterionic form
zwitterions:
- a salt
- ionic bonding between zwitterions
- can be crystallised
- high melting and boiling point
how does a zwitterion form?
- the COOH group donates the proton to the NH2 group which accepts the proton
- this causes a positive charge to form on the amino group (becomes an ammonium ion)
- this causes a negative charge to form on the carboxylic acids group (becomes a carboxylate ion)
when an amino acid is at low pH, all NH2 (even in variable group) becomes NH3+
at the isoelectric point, only the NH2 and OH of the amino acid (NOT in variable group) is changed to NH3+ and O-
what are some properties of zwitterions?
- highly soluble in water
- have high melting points
why are zwitterions highly soluble in water?
- because water molecules are polar
- the δ+ and δ- parts of the water will interact forming hydrogen bonds with the positive and negative parts of the zwitterion
why do zwitterions have high melting points?
- since they have full positive and negative charges within the substance, high temperatures are required to break these string ionic bonds between zwitterions
amino acids combine to form what bonds?
peptide (amide)
two amino acids combine to form what?
a dipeptide + H2O
two amino acids bonding to form a dipeptide is an example of what type of reaction?
condensation
there can be two products of two amino acids bonding to form a dipeptide.
- depending on order of amino acids
what is an example of dipeptides?
- aspartame - the most common artificial sweetener used worldwide
what are tripeptides?
3 amino acid residues bonded together
tyrosine is an α-amino acid that occurs in many foods. the melting temperature of tyrosine is 314°C. explain how the structure of tyrosine contributes to this high melting temperature [2]
- exists as zwitterions (at isoelectric point)
- ionic bonding between zwitterion
atom economy formula:
(desired Mr / reactants Mr) x100
what are polypeptides?
- manny amino acids bonded together
(known as a proteins)
what are enzymes?
biological catalysts that speed up the rate of reactions in the human body
the foldijng of the polypeptide chains into globular structures forms enzymes
what are the different structures of polypeptide chains?
- primary structure
- secondary structure
- tertiary structure
- quaternary structure
what is the primary structure of a polypeptide chain?
- the linear sequence of amino acids in the chain
what is the secondary structure of a polypeptide chain?
- hydrogen bonding between the polypeptide chains means it folds into either an α-helix or β-pleated sheet
polypeptide chains are held together by hydrogen bonding. explain how these hydrogen bonds form [2]
- H bonded to electronegative element
- unequal electron density —> polar bond
what is the tertiary structure of a polypeptide chain?
- the alpha-helices and beta-pleated sheets are folded further
- driven by R-group interactions, and hydrogen bonding, ionic bonding and disulphide bridges
what are the two examples of proteins you need to know?
- haemoglobin
- insulin
haemoglobin:
- 4 tertiary polypeptide chains
- contains haem group - a large iron-containing molecule
- each protein chain is bonded to one haem group
- haem group is responsible for binding the oxygen that haemoglobin transports
- has coordination number of 6 because the central metal ion is forming 6 coordinate honds
how does haemoglobin contain a haem group?
- each of the lone pairs of nitrogen can form a co-ordinate bond with the iron (II) ion
- the iron forms 4 co-ordinate bonds with the haem
insulin:
- proteins can also act as chemical messengers - hormones
- consists of two chains
- linked by disulfide bridges between cysteine residues
which amino acid is the only amino acid that is not chiral?
- glycine
- the R group is hydrogen, so not bonded to 4 different groups
why are all amino acids, apart from glycine chiral?
- the carbon atom is bonded to 4 different groups
what is a zwitterion?
- a dipolar ion which means it has a positive charge in one part of the molecules and a negative charge in another part
what happens when an alkali is added to an amino acid zwitterion?
- the NH3+ group donates a hydrogen ion to the OH- ions of the alkali to form water
- the organic compound is no longer a zwitterion because it only contains a negative charge
what happens when an acid is added to an amino acid zwitterion?
- the COO- group accepts a hydrogen ion from the acid
- the organic compound is no longer a zwitterion as it only contains a positive charge
why do amino acids have relatively high melting points?
- in the solid state, the zwitterion is the usual form that an amino acid exists in
- this means there are strong ionic attractions between neighbouring zwitterions in the solid, so a large amount of energy is required to break the ionic attractions
give examples of how proteins are essential for living systems
- proteins are important for structural functions in living organisms
- e.g nails, feathers, skin and collagen are all made up of protein
- enzymes are proteins that are biological catalysts that speed up chemical reactions in the body
- some hormones are proteins e.g insulin
