Chemistry - 4.7 amino acids, peptides and proteins

What are amino acids?

molecules containing an amine group and a carboxylic acid group

What is the general formula of naturally occuring a-amino acids?

RCH(NH₂)COOH

What is an a-amino acid?

when the amino group is attached to the 2nd carbon next to the carboxyl group

What is a b-amino acid?

when the amino group connected to the third carbon

Amino acids are amphoteric, what does this mean?

they have both acidic and basic properties - contain 2 functional groups

what are the reactions of amino acids?

• reacting with acids

• reacting with bases

• esterification with alcohols

What is the reaction of amino acids with strong acids?

• exist in cationic (positive ion form)

• NH₂ becomes NH₃⁺

What is the reaction of amino acids with strong bases?

• produce a salt and water

• exists in anionic (negative ion form)

• COOH becomes COO^-

What is the reaction of amino acids with an alcohol?

• easily esterified by heating an alcohol

• in presence of concentrated sulfuric acid

• amino group is protonated

H₂NCH₂COOH  + C₂H₅OH + H⁺   →   H₃N⁺CH₂COOC₂H₅    +   H₂O

What is a zwitterion?

a molecule that has both a positive and negative charge but no overall electrical charge

how does a zwitterion form in amino acids?

a hydrogen ion transfers from the -COOH group to the NH₂ group forming COO^- and NH₃⁺

Why do amino acids have high melting points?

they have a strong electrostatic attraction (ionic bonds) between oppositely charged ions in the solide lattice requiring lots of energy to break

Why are amino acids soluble in water?

their charged ions interact with polar water molecules (forming ion-dipole bonds) helping them dissolve

what is the isoelectric point?

the pH at which amino acid exists as a zwitterion and has no overall charge

why do different amino acids have different isoelectric points?

because their R groups are different and affect how easily they gain or lose protons

How do you form amides?

reacting acyl chloride with ammonia and amines

example of primary amide

propanamide

example of secondary amide

N - methylethanamide

example of tertiary amide

N.N - dimethylmethanamide

How is a dipeptide formed?

• two amino acid molecules reacting together

• condensation reaction

• forms peptide bond/amide link - CONH

• eliminates one molecule of water

How do you form polypeptides?

Multiple amino acids reacting in condensation reactions to form polymers called polypeptides

• called polyamides

How you can break the peptide link in proteins?

heating in a strong acid or strong alkali

describe acid hydrolysis of proteins

• heat under reflux with strong acid

• amino acids produced are in cationic form (NH₂ to NH₃⁺)

describe alkaline hydrolysis of proteins

• heat under reflux in strong alkali

• amino acids are produced in anionic form (COOH to COO-)

What are the structure levels of proteins?

• primary structure

• secondary structure

• tertiary structure

what is the primary structure of proteins?

the sequence of amino acids in the protein chain

what is the secondary structure of proteins?

a primary polypeptide chain folded into an alpha helix or a beta pleated sheet

describe the structure of the alpha helix in the secondary structure of a protein

• polypeptide chain is coiled into a spiral

• shape is maintained by hydrogen bonds between the N-H hydrogen atom of an amide group and the C=O carbonyl oxygen of another amide group

• found in muscle and wool

describe the structure of the beta pleated sheet in the secondary structure of a protein

• maintained by hydrogen bonds but the C=O and N-H groups are in different chains

• van der waals forces are responsible for producing a pleated sheet rather than a flat arrangement

what is the tertiary structure of a protein?

alpha helix structure of a protein bent, twister or folded into a particular shape - e.g. coiled into another spiral or super helix

what are enzymes?

biological catalysts (proteins)

what is the role of enzymes?

to maintain life by speeding up the rate of reactions

• without them reactions would be too slow

what are some things proteins are found in in living organisms?

• nails

• skin

• collagen of cartilage

2 examples of proteins and their importance

Amylase (enzyme)

• found in human saliva

• catalyses the breakdown of starch into sugars during digestion, allowing energy to be released in respiration

Haemoglobin

• found in red blood cells

• transports oxygen around the body by binding to oxygen

• allows oxygen to be carried from the lungs to respiring tissues - essential for aerobic respiration.