amino acids

• general structure 


  depending on the R group, they are often optically active

  
• acidic and basic properties

  • acidic properties from carboxylic acid group and basic from amino group
  • basic - can gain a H+ ion to form NH3+ protonates NH2 group
  • acidic - can lose a H+ to form COO- deprotonates COOH group

  
• acids donate protons to bases
• bases accept protons from acids
• reaction of alanine with OH- ion

  CH3CH(NH2)COOH + OH- -> CH3CH(NH2)COO- + H2O
• reaction of alanine with H+

  CH3CH(NH2)COOH + H+ -> CH3CH(NH3+)COOH
• naming amino acids 


  H2N = amino-

  COOH = highest priority group - always -oic acid

  = aminoethanoic acid

  
• serine 


  2-amino-3-hydroxypropanoic acid

  
• leucine 


  2-amino-4-methylpentanoic acid

  
• aspartic acid 


  aminobutanedioic acid

  
• threonine


  2-amino-3-hydroxybutanoic acid

  
• lysine


  2,6-diaminohexanoic acid

  
• structure of amino acids

  • pure state -> organic, white crystalline solids with relatively high melting points for their Mr
  • generally soluble in water to produce a solution that conducts electricity
  • mostly insoluble in non-polar solvents
  • high melting point due to their ionic bonding in solid state
  • in pure form, amino acids exist as zwitterions
• zwitterions

  • dipolar ion with both a positive and negative charge on different parts of the molecule
  • solid form -> amino acids exist as ionic zwitterions (high met)
  • solution -> only exist at a pH specific to the amino acid
  • this pH is called the isoelectric point (pI) and is a unique property of each amino acid

  
• isoelectric point 


  the pH at which the overall charge on a amino acid is zero
• amphoteric behaviour of amino acids

  • if the pH decreases (acidic), the COO- group gains a proton
  • as the pH increases (alkaline) the NH3+ group loses a proton

  
• esterification 


  CH3OH / c.H2SO4

  
• nucleophilic addition-elimination 


  conditions: excess CH3COCl
• nucleophilic substitution 


  excess CH3Br