Amino Acid Ionization and Environment contain at least two Ionizable protons each with its own pKa The carboxylic acid has an acidic pKa and will be protonated at an acidic low pH cootie ooo ti The amino group has a basic pKa and will be protonated until basic pH high is achieved no AAAA gpq g gg ANAP NH NH H Environmental Effects A carboxy group is much more acidic than in carboxylic acids A amino group Is slightly less basic than in amines Due to intramolecular interactions The normal pKa for a 7 carboxyl group is about 4 8 10 8 12 COO amino group is about 10 b PKa 2 Methyl substituted groups H c 3 carboxyl and amino groups in glycine a Amino Acid 1919cme CH CH COOH II Acetic acid H H 3 COO I Thetprotonated amino 9 4P withdraws electrons from the carboxyl group lowering Its pKa e my mine H 1MHz H C coo at Am acid glycine t Electronegative oxygen atoms in the carboxyl group withdraw electrons from the amino group lowering it pKa Ionization and Titration curves low pH amino acid exists in a positively charged form cation high pH am 10h acid exists in a negatively charged form anion Between the pKa for each group the amino acid exists in azwitterion form in which a single molecule has both a positive and negative charge cation Zwitterion Anion ZW Itter ON Nit 13 4 2 on Has PK cid 15482 1 13 Glycine 7 7 Pks ae I I predominate at pH values between the pKa values of the amino and carboxyl groups amino acids without ionizable side chains the Isoelectric Point equivalence point pl IS PI pKz 2 Net charge 0 AA is least soluble in water AA does not migrate in electric field 0 g basic 2 OH equivalents group 1 1 I acidic group Han g COOH cid 15482 1 1 111 c coolI amino acid twitter 10h f R MHz I NH NH I H PK pkz I H R COOH R C COO CEoE aOEgoaE6o Ey agyg R g COO H low pH pH high pH Amino Acids Can Act as Buffers Amino acids with uncharged side chains such as glycine have two pKa values The pKa of the a carboxyl group 152 34 The pKa of the a amino group 159 6 As buffers prevent change in pH close 10 the pKa glycine can act as a buffer in two pH ranges Amino Acids With Ionizable R groups Ionizable side chains also have pKa and Act as Buffers Ionizable side chains influence the PI of the amino acid i Nitz 4 doo 2 CHZ CHZ doo Has pk COOH 13 Glycine Buffer Regions t EE 0 l 2 OH equivalents Ionizable side chains can be also titrated Amino Acid PKa PKa Titrated curves are now more complex as each pkahasa buffering Zone of 2 pH units Identify species that carries ane 1 Zero charge COO 400 s 4 H c 11 H N it it f 4 E I JH PK 2 10 Histidine 4 H c 11 P E HIV 4 14 P i H 0 1 PK2 9 17 Pt 59 pkp 6 0 8 6 4 2 0 PK 1 82 I 2 3 OH cequivalent pKa carboxy pKa2 Amino pKa 3 12 group PKa 2 34 2 34 2 32 2 36 2 36 2 28 i gg 1 83 2 83 2 02 2 17 2 21 2 09 2 20 i as 1 88 2 19 2 18 2 1 82 Glycine Alanine Valine Leucine Isoleucine Methionine pro ne Phenylalanine Tryptophan Asparagine Glutamine serine Threonine Tyrosine cysteine Aspartic acid Glutamic acid Lysine Arginine Histidine 9 60 q gg a 62 g 60 9 60 9 21 10 60 9 13 9 39 8 80 9 13 9 15 9 10 9 11 8 18 9 60 9 67 8 95 9 04 g PK I CHZ I NH 3 I ctlz I COO 3 65 4 25 53 12 48 6 pkay 5 97 6 g gg g gg 6 02 5 74 6 30 5 48 5 89 5 41 5 65 5 68 5 60 5 6 2 77 3 22 9 74 10 76 7 59 NH 1 CHZ 1 COO