1. 20 mL of a 0.1 M solution of a weak acid is mixed with 8.0 mL of a 0.1 M solution of NaOH. The final pH of the solution (28 mL) is 5.12. What is the pK a of the weak acid?

2. Henderson-Hasselbalch limitations You have to assume that acid’s dissociation and base’s hydrolysis are negligible

3. Water as a reactant Condensation: water is lost –H 2 O is a product Hydrolysis: water is gained –H 2 O is a reactant

4. Condensation of two amino acids to form a peptide bond -COOH +NH 2 - → C ║ O N H H2OH2O + Amide bond

5.  -carbon amino group (basic) carboxylic group (acidic) R group (gives the amino acid its identity) Generic  -amino acid

6. (Most) amino acids have a stereocenter “L” isomer: L for life

7. 20 ‘common’ amino acids Make up vast majority of amino acids in natural proteins Coded for in the genetic code Other amino acids: –Posttranslational modifications –Intermediates in metabolic pathways

8. Two main groups of side chains plus subgroups Nonpolar (hydrophobic) –Aliphatic (non-aromatic, mostly straight chains) –Aromatic (conjugated ring structures) Polar (hydrophilic) –Uncharged Hydrogen bonds –Positively charged “basic” –Negatively charged “acidic” At physiologic pH (~7)

9. Things to know about amino acids 1.Name 2.R-group structure 3.R-group classification 4.Three-letter abbreviations 5.One-letter abbreviations

12. Two cysteine residues oxidize to form a disulfide bond -Covalent bond: stronger than a hydrogen bond -Reversible: readily reduced back to free sulfhydryls