1. 27.1 Classification of Amino Acids

2. Classification of Amino Acids amino acids are classified as , etc. to indicate where the nitrogen atom is relative to the carboxylic acid:

3. Selected Amino Acids 1-Aminocyclopropanecarboxylic Acid an  -amino acid that is an intermediate in the biosynthesis of ethylene 3-Aminopropanoic Acid a  -amino acid that is one of the structural units present in coenzyme A 4-Aminobutanoic Acid a  -amino acid involved in the transmission of nerve impulses

4.  Amino Acids are Most Ubiquitous in Nature

5. Revision: Amines are Brønsted Bases Amines are electron-rich and have a reactive lone pair, which can form a covalent bond to a proton to form an ammonium ion.

6. Amino Acids Exist as Zwitterions While their name implies that amino acids are compounds that contain an amine (— NH 2 ) and a carboxylic acid (-CO 2 H), these groups are actually present as their conjugate acid (—NH 3 + ) and conjugate base (—CO 2 – ), respectively.

7. Amino Acids Exist as Zwitterions Resonance-Stabilized Zwitterion

8. Zwitterions Defined Zwitterionic Compounds/Zwitterions neutral compounds having formal unit electrical charges of opposite sign. Some chemists restrict the term to compounds with the charges on non- adjacent atoms. Sometimes referred to as inner salts, dipolar ions (a misnomer), e.g. + H 3 N-CH 2 CO 2 - ammonioacetate (glycine). IUPAC Compendium of Chemical Terminology

9. Amino Acids Exist as Zwitterions What evidence do we have for this behavior………

10. Properties of Glycine Reflect its Zwitterionic Structure High Water Solubility glycine is soluble in water but not in non- polar solvents. High Melting Point when heated to 233°C it decomposes before it melts.

11. Properties of Glycine Reflect its Zwitterionic Structure The physical properties of glycine are consistent with this structure

12. 27.2 Stereochemistry of  -Amino Acids

13. Non-Polar Side Chains: Alanine Alanine, valine, leucine, and isoleucine have alkyl groups as side chains, which are non-polar and hydrophobic Alanine (Ala, A)

14. Non-Polar Side Chains: Valine Valine (Val, V)

15. Non-Polar Side Chains: Phenylalanine The side chain in phenylalanine (a non- polar amino acid) is a benzyl group. Phenylalanine (Phe, F)