Faculty of Chemistry, VUT Brno, 2nd March 2006
Description – general properties
• Amino acids are derivatives of carboxylic acids formed by
substitution of -hydrogen for amino functional group.
• majority of amino acids has amphoteric character –
functional group –COOH is the reason of acidity and –NH2
group causes basic properties.
• in basic environment AA dissociate proton to form carboxyl
anion –COO-. Basic surround defends –NH2 against
• in acidic environment AA accept proton to form amonium
cation –NH3+. Acidic environment defends –COOH against
• Zwitterionic structure is neutral and its value of pH is called
• AA are optically active molecules and asymmetry of their
mirror images is not superimposable (except in the case of
glycine where the R-group is hydrogen)
• according new UIPAC
nomenclature L- D- forms
were replaced for (S)- and (R)system
• Ordinary synthesis of asymmetric molecules produces
racemic mixtures. To obtain the naturally ocurring (S-) AA
we must revolve the racemic form. Fortunately we know a
variety of ways.
• Biosynthesis of substances of AA having asymmetric
centers almost produce pure stereoisomers. Using this
criteria, examination of amino acids in practice always
shows racemic mixture.
• Physical propeties – AA are colourless crystalline
substances soluble in water and insoluble in organic
solvents with high melting point.
Separation of amino acids
• Position of amino group − -amino acids exist in two
enantiomeric forms. Only L-(S-) acids are found in nature.
• Proteinaceous AA − proteins consist of 20 AA.
• Essential (unexpendable) − organism is not able to
synthesize these AA but accept from food.
• Nonessential (expendable) − organism produced
from essential AA by transamination.
• According R- functional group
• Nonpolar (hydrophobic)
• Polar (hydrophilic) – better soluble in water
• Basic – contains more atoms of nitrogen
• Acidic – contains more carboxyles
Summary of 20 proteinaceous AA
• Besides basic 20
more AA exist
found only in
• Essential AA are
variety of animal
plants are able to
synthesize all 20
Reactions of amino acids
Polymerization – form peptides, proteins and enzymes
• A condensation reaction between the carboxyl of one
amino acid and the amino group of another forms a peptide
• Oligopeptides − condensation of 2 – 10 AA units
• Polypeptides − condensation of 11 – 100 AA units
• Proteins − more than 100 AA units
• Disulfide linkage – conversion of cysteine to cystine is like a
conversion of thiols to disulfides by mild oxidizing agents.
This conversion can be reversed by mild reducing agents.
• Disulfide bonds stabilize protein structure by providing
Synthesis of -amino acids
• A variety of methods have been developed
• Important in industry due to the commercial relevance
bodybuilding supplements (for big musles)
sources of vitamins not only for human also for animals
The Gabriel synthesis (from potassium phthalimide)
The Strecker synthesis
Enantioselective synthesis produces only or predominantly
pure AA form. (More info Solomons & Fryhle p.1175 - 1177)
Metabolism of -amino acids
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