pH, pKa, Problems

pH, pKa, Problems - C H A P T E R 23 pH, pKa, pROBLEMS...

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241 C H A P T E R 23 pH, p K a , pROBLEMS Proton: H 1 or H 3 O 1 Acid Base Not All Acids and Bases Are Created Equal p K a 52 log( K a ) Weak Acids Make Strong Bases (and Vice Versa) Who Gets the Proton? Don’t Forget Stoichiometry The Sadistic Little p Taking log 10 ( x ) Taking 2 log 10 ( x ) pH log 10 [H 1 ] p K a log 10 ( K a ) Buffers Henderson-Hasselbalch Equation Titration Curves p I —Isoelectric Point The Bicarbonate Buffer Imbalance in Blood pH Acidosis and Alkalosis
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242 Basic Concepts in Biochemistry The usual concerns about acid–base behavior (other than knowing in your soul that it really can’t be very important) are where protons go if they’re given a choice, how buffers work, and logarithms. Your profes- sor’s job is to try to convince you that acid–base behavior has a place in biochemistry. Your job is to learn it—just in case your professor is right. It’s interesting that the simplest molecule in biology has succeeded in terrifying generations of chemistry and biochemistry students. The thing that may be confusing here is that when an acid gives up a proton, it becomes a base, and when a base picks up a proton, it becomes an acid. It’s a constant identity crisis. To make it even worse, some molecules with protons aren’t acids. Every proton could be an acid if the base removing it were strong enough. In water (which limits the strength of acids and bases that can be used) RNH 1 3 is an acid but RNH 2 is not. The difference between the protons in RNH 1 3 and RNH 2 is that RNH 2 is such a weak acid that the strongest base available in water (hydroxide, 2 OH) is too weak a base to remove it (to give RNH 2 ). In contrast, RNH 1 3 is a sufficiently strong acid that 2 OH can easily remove one proton to give RNH 2 . Whether or not something is an acid (for our purposes) comes down to whether or not the strongest base available in water ( 2 OH) can remove a proton from it. The same sort of reasoning can be applied to a base. A base (for our purposes) is something that can be protonated by the strongest acid in water (H 3 O 1 ). There’s no easy, foolproof way to decide whether something is an acid or a base. Fortunately, there are only a few types of acids and bases you will encounter in biochemistry. Notice that all bases will be more negatively charged than the acids they came from. ACID Something that has a proton it’s willing to give up BASE Something that has a place to put a proton PROTON: H 1 OR H 3 O 1 A hydrogen nucleus without electrons
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23 pH, p K a , pRoblems 243 ACID BASE p K a Asp, Glu R—CO—OH 1 H 2 O R—CO—O 2 1 H 3 O 1 3–5 Carboxylic acid Carboxylate Lys, His, Arg ( not NH of peptide bond) R—NH 1 3 1 H 2 O RNH 2 1 H 3 O 1 9–10 (amines) Protonated amine Amine 6–7 (His) Cys R—SH 1 H 2 O R—S 2 1 H 3 O 1 8–9 Thiol Thiolate Tyr ( not Ser or Thr) R—OH 1 H 2 O R—O 2 1 H 3 O 1 9–11 Phenol Phenolate CO 2 1 H 2 O HCO 2 3 1 H 3 O 1 6.1 Carbon dioxide Bicarbonate H 3 PO 4 1 H 2 O H 2 PO 1
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This note was uploaded on 04/09/2008 for the course BIO SCI 98 taught by Professor Goulding during the Spring '08 term at UC Irvine.

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pH, pKa, Problems - C H A P T E R 23 pH, pKa, pROBLEMS...

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