1-423-08WaterStudent

1-423-08WaterStudent - Intro to the Course and Water OBJECTIVES Be able to describe the chemical basis for each of the following non-covalent

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Intro to the Course and Water OBJECTIVES Be able to describe the chemical basis for each of the following non-covalent chemical interactions, and know the relative strengths of each: -Hydrogen bonds -Electrostatic interactions -van der Waals interactions -Hydrophobic interactions Be able to: -describe the properties of water that make it a unique solvent for biological reactions -define pH, pKa, conjugate acid/base pair -draw a titration curve for a weak acid and identify the pKa for that acid from the titration curve. -use the Henderson-Hasselbalch equation to calculate pH, pKa, and the concentrations of acid and conjugate base in a solution. OUTLINE I. Constraints on chemical reactions in living systems II. Biochemistry is chemistry in water Water is a polar molecule, which leads to its special qualities: H-bonds, Ice III. Biomolecules interact through weak noncovalent interactions A. H-bonds B. Electrostatic interactions C. Hydrophobic interactions Definitions: -Hydrophilic -Hydrophobic -Amphipathic D. van der Waals interactions IV. Water as a biological solvent (Colligative Properties) Thermal Properties (boiling point, freezing point, vapor pressure) Osmotic Properties
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V. Ionization of water, pH, Weak Acids/bases A. Ion product of water Kw B. pH pH scale C. Weak acids and bases An acid (HA) is a proton (H+) donor A base (A-) is a proton acceptor An acid will dissociate into a proton and its conjugate base. The tendency for an acid to dissociate HA -- > H+ + A- is described by the dissociation constant, Ka. Ka = ([H+] [A-]) / [HA] pKa = -log (Ka) Reality check: pH, pKa D. Titration curves 1. Additional definition: pKa is the pH at which an acid is half dissociated, i.e. [HA] = [A-] 2. Henderson-Hasselbalch equation Describes the shape of the titration curve of any weak acid pH = pKa + log [proton acceptor] / [proton donor]. VI. Summing up: Why care about water, acids, bases, pH, pKa? Problems 1. The pH of a sample of blood is 7.4; the pH of a sample of gastric juice is 1.4. The blood sample has: A. 5.29 times lower [H+] than the gastric juice B. A million times higher [H+] than the gastric juice C. 6000 times lower [H+] than the gastric juice D. A million times lower [H+] than the gastric juice E. 0.189 times the [H+] as the gastric juice 2. Hydrogen bonds between adjacent water molecules allow water to have the cohesiveness which gives it such properties as surface tension. However, hydrogen bonds are relatively weak forces (bond energy = 20 kJ/mol) especially when compared to covalent bonds (bond energy of covalent O-H bond in water = 460 kJ/mol). Hydrogen bonds also only last less than
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1 x 10 -10 s. If hydrogen bonds are so weak, and last such a short time, why does water still have such great internal cohesion? 3. When you are very warm, because of high environmental temperature, or when you are
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This note was uploaded on 04/05/2008 for the course BIOCHEM 423 taught by Professor Osgood during the Spring '08 term at New Mexico.

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1-423-08WaterStudent - Intro to the Course and Water OBJECTIVES Be able to describe the chemical basis for each of the following non-covalent

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