1 Reactions of amino acids

6.2.2 Amino Acids and Chirality General structure of an α amino acid NH2 CH CO2H R The R group can be a variety of different things depending on what amino acid it is. The simplest amino acid is glycine, where the R is an H NH2 CH2 CO2H Some amino acids have an extra carboxylic acid or an amine group on the R group. These are classed as acidic or basic (respectively) amino acids NH2 C CO2H CH2 H CO2H Aspartic acid Naming amino acids You do not need to know any common names for the 20 essential amino acids. We should, however, be able to name given amino acids using IUPAC organic naming 2-aminobutanedioic acid NH2 CH2 CO2H (2-)aminoethanoic acid NH2 C CO2H CH2 H OH 2-amino-3- hydroxypropanoic acid H2N C CO2H (CH2 ) H NH 4 2 Lycine (basic) 2,6-diaminohexanoic acid NH2 C CO2H CH2 H CO2H Zwitterions The no charge form of an amino acid never occurs. The amino acid exists as a dipolar zwitterion. H2N C CO2H H R H3N C + CO2 – H R Amino acids are often solids The ionic interaction between zwitterions explains the relatively high melting points of amino acids as opposed to the weaker hydrogen bonding that would occur in the no charge form. Zwitterion Acidity and Basicity The amine group is basic and the carboxylic acid group is acidic. H3N C + CO2 – H R H2N C CO2 – H R H3N C + CO2H H OH R – H+ OHH+ +NH3 -CH2 -CO2 – + HCl
Cl- NH3 + -CH2 -CO2H +NH3 -CH2 -CO2 – + NaOH
NH2 -CH2 -CO2 -Na+ +H2O Amino acids act as weak buffers and will only gradually change pH if small amounts of acid or alkali are added to the amino acids. Species in alkaline solution High pH Species in neutral solution Species in acidic solution Low pH N Goalby chemrevise.org The alpha in ‘α’ amino acid means both NH2 and COOH groups are joined to the same C. An amino acid exists as a zwitterion at a pH value called the isoelectric point If the side R group of an amino acid contains an acidic of basic group then pH value of the isoelectric point will be different An amine group on the R group may make the isoelectric point be pH > 10 An carboxylic acid group on the R group may make the isoelectric point be pH < 3 The extra carboxylic acid or amine groups on the R group will also react and change form in alkaline and acid conditions NH2 C COOCH2 H COOAspartic acid in high pH 2 Other reactions of amino acids The carboxylic acid group and amine group in amino acids can undergo the usual reactions of these functional groups met in earlier topics. H2N C CO2H H CH3 + CH3OH H+ e.g. Esterification reaction H3N C + C H CH3O O CH3 + H2O