Amino Acids amine H 2 N O C

Amino Acids amine H 2 N O C OH C H carboxylic acid H R R varies with amino acid R = H glycine

Amino acids C HH 32 NN+ glycine non-chiral O O OH C H H R all other -amino acids in proteins at neutral p. H (p. H = 7. 0) zwitterion very high b. p. (> 200 o. C) very soluble in water L-enantiomers

acid-base chemistry C H 3 N+ O OH C H R C H 32 N+ O O OH C H R C H 2 N O OC H R low p. H neutral p. H high p. H amine and c. a. amine protonated amine and c. a. deprotonated negative charge positive charge no net charge isoelectric point p. H = p. HI amino acids diprotic acids 2 p. Ka

Titration of an amino acid alanine R = CH 3 p. Ka 2 = 9. 69 H 3 N+ Ka 1 = [H+][A-] [HA] C OH C H CH R 3 0. 1 M 4. 57 x 10 -3 = x 2 0. 1 - x X = 2. 14 x 10 -2 O +] [H = p. Ka 1 = 2. 34 Ka 1 = 10 -2. 34 = 4. 57 x 10 -3 [H+] [A-] [HA] initial 0 change +x equil. +x 0 +x +x p. H = 1. 67 0. 1 -x 0. 1 -x

+1 p. H 9. 0 8. 0 7. 0 6. 0 5. 0 4. 0 3. 0 2. 0 1. 0 net charge × 0 equivalents of OH-

net charge +1 p. H 9. 0 8. 0 7. 0 6. 0 5. 0 4. 0 3. 0 2. 0 1. 0 +1/2 0. 05 M p. Ka 1 = 2. 34 0. 05 M × 0 × 1/2 equivalents of OH- 0. 05 M p. H = p. Ka + log [A-] [HA] 0. 05 M p. H = p. Ka = 2. 34

net charge +1 p. H 9. 0 8. 0 7. 0 6. 0 5. 0 4. 0 3. 0 2. 0 1. 0 +1/2 0 × × 0 × 1/2 1 equivalents of OH- at equivalence point: p. Ka 2 = 9. 69 p. Ka 1 = 2. 34 isoelectric point p. H = p. Ka 1 + p. Ka 2 2 p. H = (2. 34 + 9. 69)/2 p. H = p. HI = 6. 02

net charge +1 p. H 9. 0 8. 0 7. 0 6. 0 5. 0 4. 0 3. 0 2. 0 1. 0 +1/2 0 at 2 nd half-way point: -1/2 -1 × × × 0 p. Ka 2 = 9. 69 × 1/2 1 3/2 equivalents of OH- p. H = p. Ka 2 = 9. 69

glutamic acid R = - CH 2 COOH C H 3 N+ O OH C H p. Ka 1 = 3. 20 ( -COOH) O CH CH 2 C = p. K = 4. 25 (R-COOH) R 2 a 2 p. Ka 3 = 9. 67 OH ( -NH 3) It will take ___ 3 equivalents to titrate glutamic acid 1 st group 2 nd group 3 rd group

12 +1 0 -2 -1 10 OH H 3 N+ C H × 8 p. H × p. HI = 3. 7 4 × × × 2 1 C O 2 equivalents OH- O = CH CH C 2 2 OH p. Ka 1 = 3. 20 p. Ka 2 = 4. 25 p. Ka 3 = 9. 67 3. 2 + 4. 25 = 3. 7 2 4. 25 + 9. 67 = 7. 0 2 3

p. HI every amino acid has characteristic p. HI every protein also has characteristic p. HI p. H < p. HI positive net charge p. H = p. HI neutral protein molecules attract each other repel each other p. H > p. HI negative protein molecules repel each other precipitate p. H of milk = 6. 3 p. HI of casein = 4. 7

Electrophoresis + - - - + + migration depends on charge and size

Nonpolar R-groups glycine -H alanine -CH 3 valine -CH-CH 3 proline 2 o amine leucine -CH 2 -CH-CH 3 phenylalanine isoleucine -CH-CH 2 -CH 3 methionine -CH 2 -S-CH 3

Polar R-groups serine -CH 2 -OH threonine -CH-CH 3 OH tyrosine asparagine -CH 2 -C-NH 2 O glutamine -CH 2 -C-NH 2 O tryptophan cysteine -CH 2 -SH

Acidic R-groups glutamic acid O -CH 2 -C = OH O aspartic acid -CH 2 -C = OH

Basic R-groups lysine -CH 2 -CH 2 -NH 2 = arginine -CH 2 -CH 2 -NH-C-NH 2 NH histadine