The document discusses amino acids, which are the building blocks of proteins. It describes that amino acids contain both an amino group and a carboxyl group and exist in different ionized forms depending on pH. There are 20 standard amino acids that make up proteins in humans. Amino acids can undergo various chemical reactions due to these functional groups and are also classified based on the polarity of their side chains.

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2. Amino Acids, Peptides and Proteins

3. The Proteins speak: “ We are the basis of structure and function of life; Composed of twenty amino acids, the building blocks; Organized into primary, secondary, tertiary and quaternary structure; Classified as simple, conjugated and derived proteins.”

4. AMINO ACIDS group of organic compounds containing two functional groups: amino group (-NH 2 )  basic carboxyl group (-COOH)  acidic

5. General Structure of Amino Acids H H R C COOH R C COOH NH 2 NH 3 General Structure Exists as ion

6. ⍺ - amino acids amino groups – attached to the carboxyl same carbon Atom ⍺ - carbon atom  binds to a side chain  represented by R (different for each of the 20 amino acids found in proteins) Ionized forms  how they exist

7. Classification of Amino Acids based on polarity of the R group 4 groups Polarity  reflects the functional role of AA in protein structure

8. Non-polar AA hydrophobic (water hating) No charge on the ‘R’ group Examples are: Alanine Methionine Leucine Phenylalanine Isoleucine Tryptophan Valine Proline

9. 3. Polar AA with (+) ‘R’ group carries (+) charge Examples: Histidine Arginine Lysine 4. Polar AA with (-) ‘R’ group carries (-) charge Examples: Glutamic Acid Aspartic Acid

10. 2. Polar AA with no charge on ‘R’ group no charge on the ‘R’ group possess groups  hydroxyl sulfhydryl amide participate in hydrogen bonding of protein structure Examples: Asparagine Glycine Cysteine Tyrosine Serine Threonine Glutamine

11. Physical Properties 1. Solubility - soluble in water and insoluble in organic solvents 2. Melting Points - melt at higher temperatures often 200 °C 3. Taste sweet (Gly, Ala, Val) tasteless (Leu) bitter (Arg, Ile) Sodium Glutamate – salt of Glutamic Acid – flavoring agent

12. 4. Optical Properties - Assymetric  a carbon atom is attached to 4 different groups exhibiting optical isomerism 4 distinct groups R H - held by an COOH ⍺-carbon NH 3 ໋

13. All AA except Glycine possess optical isomers due to asymmetric ⍺-carbon atom Some AA (Isoleucine, Threonine)  2 nd asymmetric carbon

14. D- and L- forms of AA based on the structure of glyceraldehyde CHO CHO H C OH OH C H CH 2 OH CH 2 OH D-Glyceraldehyde L-Glyceraldehyde

15. R R H C NH 2 H 2 N C H COOH COOH D-Amino Acid L-Amino Acid The proteins are composed of L- ⍺ amino acids

16. 5. Amino acids as ampholytes can donate a proton or accept a proton AA contain both acidic (-COOH) and basic (-NH 2 ) groups

17. Zwitterion or dipolar ion: Zwitter  from German word – means “hybrid” Zwitter ion (or dipolar ion)  a hybrid molecule containing ( + ) and ( - ) ionic groups

18. AA rarely exist in a neutral form with free carboxylic ( - COOH) and free Amino ( - NH 2 ) groups Strongly acidic pH (low pH)  AA ( + ) charged (cation) Strongly alkaline pH (high pH)  AA ( - ) charged (anion) Each AA has a characteristic pH (e.g. Leucine, pH – 6.0), at which it carries both ( + ) and ( - ) charges and exist as zwitterion

19. Existence of an amino acid as Cation, Anion and Zwitterion H H ໋ R C COOH H ໋ NH 2 H Amino Acid H R C COOH R C COO ¯ NH 3 ໋ H NH 2 Cation H ໋ H ໋ Anion (low pH) R C COO ¯ (high pH) NH 3 ໋ Zwitterion (Isoelectric pH)

20. B. Chemical Properties General Reactions  mostly due to the 2 functional groups Reactions due to - COOH group 1. AA from salts (-COONa) with bases and esters (-COOR’) with alcohols 2. Decarboxylation - AA undergo decarboxylation to produce corresponding amines

21. 3. Reaction with Ammonia - the carboxyl group of dicarboxylic AA reacts with NH 3 to form amide Asparatic Acid + NH 3  Asparagine Glutamic Acid + NH 3  Glutamine

22. Reactions due to -NH 2 group 4. The Amino groups behave as bases and combine with acids (e.g. HCl) to form salts (-NH 3 + Cl ¯) 5. Reaction with NINHYDRIN - the ⍺-AMINO ACIDS react with Ninhydrin to form a purple, blue or pink colour complex (Ruhemann’s purple)

23. Amino acid + Ninhydrin  Keto acid + NH 3 + CO 2 + Hydrindantin Hydrindantin + NH 3 + Ninhydrin  Ruhemman’s purple Ninhydrin reaction – quantitative determination of AA and proteins

24. 6. Colour reactions of Amino Acids - AA can be identified by specific colour reactions Color Reactions of proteins / AA Reaction Specific group or AA Buiret Reaction Two peptide linkages Ninhydrin Reaction ⍺ -Amino acids Xanthoproteic Reaction Benzene ring of aromatic AA (Phe, Tyr, Trp) Million’s reaction Phenolic Group (Tyr) Hopkins – Cole Reaction Indole Ring (Trp)

25. 6. Sakaguchi Reaction Guanidino Group (Arg) 7. Nitroprusside Reaction Sulfhydryl groups (Cys) 8. Paulys’ test Imidazole ring (His) 9. Sulfur test Sulfhydryl groups (Cys) 10. Folin – Coicalteau’s Phenolic groups test (Tyr)

26. 7. Transamination - important reaction in AA metabolism - transfer of an amino group from an amino acid to a keto acid to form a new AA 8. Oxidative deamination - AA undergo oxidative deamination to liberate free ammonia

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