Faculty of Chemistry, VUT Brno, 2nd March 2006 Presentation about Amino acids by František SURMAN
Amino acids are derivatives of carboxylic acids formed by substitution of -hydrogen for amino functional group. Description – general properties
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 dissociation. • in acidic environment AA accept proton to form amonium cation –NH3+. Acidic environment defends –COOH against dissociation.
Zwitterionic structure is neutral and its value of pH is called isoelectric point.
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 moleculesproduces 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 producepure stereoisomers.Using this criteria, examination of aminoacids in practice always shows racemicmixture.
Arginine Valine • Physical propeties – AA are colourless crystalline substances soluble in water and insoluble in organic solvents with high melting point. Alanine Lysine
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 Essential AA • Valin • Leucin • Isoleucine • Threonine • Lysin • Methionin • Phenylalanine • Tryptophan • Arginine • Histidine
Nonessential AA • Alanine • Asparagine • Aspartate • Cysteine • Glutamate • Glutamine • Glycine • Proline • Serine • Tyrosine
Besides basic 20 proteinaceous AA more AA exist found only in some organisms. • Essential AA are different for variety of animal species and plants are able to synthesize all 20 AA.
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 bond. • Peptides • 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 cross-link.
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)