BC351 Lecture 2 Amino Acids and pKa Terms The dissociation constant Ka pKa Amino Acid and residue R group or side chain Chiral center Stereoisomer Acids bases Le Chatelier s principle Principles 1 The structure and the NATURE of amino acid side chains 2 Illustrate Le Chatelier s principle 3 To equip you with the tools necessary to be able to predict the charge of an amino acid at any given pH I Amino Acids pgs 71 77 a The building blocks of proteins i Definition of Amino Acids and a residue 1 2 b General Structure of an Amino Acid i The side chain or R group R 1 Definition of R groups a LN02 1 c Amino Acid Stereochemistry i The carbon is a chiral center in 19 out of the 20 amino acids we will discuss 1 Definition of a chiral center a 2 Chiral centers have two unique spatial arrangements a Stereoisomer i Definition of stereoisomer 1 b D and L amino acids ii Remarkably all amino acids found in proteins are L stereoisomers d The Twenty amino acids ii Designations Table 3 1 pg 73 1 Abbreviations and symbols iii Their nature LN02 2 1 Amino acids are normally broken down in broad categories based upon the nature of their side chain specifically its polarity 2 Nonpolar aliphatic R groups 3 Aromatic R groups a These are in general hydrophobic b All contain a 6 membered ring LN02 3 4 Polar Uncharged R groups a Hydrogen bonding b Covalent interaction i Disulfide bond 5 Positively charged basic R groups a Ionic interactions and hydrogen bonding LN02 4 6 Negatively charged acidic R groups a Ionic interactions and hydrogen bonding II Acid base chemistry of Amino acids pgs 57 59 78 81 a Acid and base definitions i Acids are ii Bases are b General acid and base reactions i A general equation HA H A H A Ka HA ii Ka The dissociation constant 1 Definition of Ka a b Definition of pKa i LN02 5 2 Affinity of a chemical group for a proton i High affinity Base Ka small pKa large ii Low affinity Acid Ka large pKa small c Protonation and deprotonation of amino acid groups i Carboxyl group R COOH R COO H ii Amino group R N H3 R NH2 H d Titration of amino acids i Le Chatelier s principle 1 Definition of Le Chatelier s principle a HA H2 O H A4 0 H OH16 0 pKa pKa LN02 6 Titration of Acetic acid CH3COOH H CH3COO pKa 4 76 pH OH added ii What is the predominate charge of glycine at pH 7 1 First I would need a table Table 3 1 pg 73 LN02 7 iii For you to do 1 What is the predominate charge of His at a pH of 1 0 4 5 and 8 2 LN02 8