General structure of an 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 Optical Activity All amino acids, except glycine, are chiral because there are four different groups around the C They rotate plane polarised light. H2N C CO2H CH3 H NH2 C HO2C CH3 H 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 H (CH2)4 NH2 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. 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. Sometimes questions refer to these. H2N C CO2H H CH3 + CH3OH H+ e.g. Esterification reaction
188.8.131.52 Amino acids (A-level only)
Amino acids have both acidic and basic properties, including the formation of zwitterions.
Students should be able to draw the structures of amino acids as zwitterions and the ions formed from amino acids:
• in acid solution
• in alkaline solution.