27.1 Classification of Amino Acids

Classification of Amino Acids

amino acids are classified as , etc. to indicate where the nitrogen atom is relative to the carboxylic acid:

O

ON

H

H

H

-Amino Acid

O

ONH

-Amino Acid

H

N

O

OH

-Amino Acid

N

H

H

Selected Amino Acids

1-Aminocyclopropanecarboxylic Acidan -amino acid that is anintermediate in the biosynthesisof ethylene3-Aminopropanoic Acida -amino acid that is one ofthe structural units present incoenzyme A4-Aminobutanoic Acid

a -amino acid involved inthe transmission of nerveimpulses

O

ONHH

N

O

OHN

H

H

NH2

O

OH

Amino Acids are Most Ubiquitous in Nature

O

ON

H

H

H

-Amino Acid

O

ONH

-Amino Acid

H

N

O

OH

-Amino Acid

N

H

H

Revision: Amines are Brønsted Bases

N H X N X

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

Amino Acids Exist as Zwitterions

While their name implies that amino acids are compounds that contain an amine (—NH2) and a carboxylic acid (-CO2H), these groups are actually present as their conjugate acid (—NH3

+) and conjugate base (—CO2–),

respectively.

Amino Acids Exist as Zwitterions

BasicAmine

AcidicCarboxylic

Acid

O

ON

H

H

H

O

ON

H

H

H

O

ON

H

H

H

Resonance-Stabilized Zwitterion

Zwitterions Defined

Zwitterionic Compounds/Zwitterionsneutral 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. +H3N-CH2CO2

- ammonioacetate (glycine).

IUPAC Compendium of Chemical Terminology

Amino Acids Exist as Zwitterions

What evidence do we have for this behavior………

O

ON

H

HH

O

ON

H

HH

Properties of Glycine Reflectits Zwitterionic Structure

High Water Solubilityglycine is soluble in water but not in non-polar solvents.

High Melting Pointwhen heated to 233°C it decomposes before it melts.

Properties of Glycine Reflectits Zwitterionic Structure

O

ON

H

HH

O

ON

H

H

H

Polar Zwitterion

The physical properties of glycine are consistent with this structure

27.2Stereochemistry of -Amino

Acids

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)

O

OH3N

HMe

CO2

HH3N

Me

Non-Polar Side Chains: Valine

Valine(Val, V)

O

OH3N

H

CO2

HH3NMe

MeMeMe

Non-Polar Side Chains: Phenylalanine

The side chain in phenylalanine (a non-polar amino acid) is a benzyl group.

Phenylalanine(Phe, F)

O

OH3N

H

CO2

HH3N