CHEM 481 01-02

Enthalpy

(H). The heat content of a system.

Entropy

(S). The extent of randomness or disorder in a system.

2.4 Water as a Reactant

...

2.5 The Fitness of Aqueous Environment for Living Organisms

...

2.8 Physical meaning of pKa. Which of the following aqueous solutions has the lowest pH: 0.1 M HCl; 0.1 M acetic acid (pKa =4.86); 0.1 M formic acid (pKa = 3.75)?

0.1 M HCl has the lowest pH because HCl is a strong acid and dissociates completely to H+ and Cl-, yielding the highest [H+].

2 Water

2.1 Weak Interactions in Aqueous Systems 2.2 Ionization of Water, Weak Acids, and Weak Bases 2.3 Buffering against pH Changes in Biological Systems 2.4 Water as a Reactant 2.5 The Fitness of Aqueous Environment for Living Organisms

K eq

= [H+][A-]/[HA] = Ka

Endothermic reaction

A chemical reaction that takes up heat, that is, for which delta H is positive.

Write a paragraph describing water's role in the formation of the symptoms of cystic fibrosis. Consider the properties of water and intermolecular forces in your response.

A defect in the CFTR gene causes cystic fibrosis (CF). This gene makes a protein that controls the movement of salt and water in and out of your body's cells. The protein creates a chute that allows salt to enter and leave the cell. In people who have cystic fibrosis, the gene makes a protein that does not work right. When the protein is not the right shape it is not allowed into the membrane and salt is trapped inside the cell. The outside of the cell becomes sticky and is covered with thick mucus.

View Video segment on Cystic fibrosis

Almost always develops in early childhood. Genes make proteins. People are getting into the protein business. The proteome. From the letters we can learn the genes; from there we learn the proteins. We have so many fewer genes than we teach our students. We only have 30,000 or so genes, which isn't that different from a mouse, or twice as many as a fruit fly. You have more proteins, which can make modifications to the genes, which can make much more modifications. Scientists first must make a parts list, and make a list of all three billion letters.

1.7 Identification of Functional Groups. Name the following groups: -NH3+, -OH, -P(OH)O2-, -COO-, -CO-NH-, and -CHO.

Amino, hydroxyl, phosphoryl, carboxyl, amide, and aldehyde.

2.3 Buffering against pH Changes in Biological Systems

Buffers are mixtures of weak acids and their conjugate bases. The Henderson-Hasselbalch equation relates pH, pKa, and buffer concentration. Weak acids or bases buffer cells and tissues against pH changes. Untreated diabetes produces life-threatening acidosis.

1.12 Components of Complex Bimolecules. Figure 1-10 shows the major components of complex biomolecules. For each of the three important biomolecules below (shown in their ionized forms at physiological pH), identify the constituents. (See pg 12) c) Phosphatidylcholine

Choline esterified to a phosphoric acid group, which is esterified to glycerol, which is esterified to two fatty acids, oleic acid and palmitic acid.

After reviewing the chapter, you should be able to:

Describe the properties of water. Know what intermolecular forces and thermodynamic considerations are as they relate to cells. Explain pH and pKa.

2.1 Weak Interactions in Aqueous Systems

Hydrogen bonding gives water its unusual properties. Water forms hydrogen bonds with polar solutes. Water interacts electrostically with charged molecules. Entropy increases as crystalline substances dissolve. Nonpolar gases are poorly soluble in water. Nonpolar compounds force energetically unfavorable changes in the structure of water. Van der Waals interactions are weak interatomic attractions. Weak interactions are crucial to macromolecular structure and function. Solutes affect the colligative properties of aqueous solutions.

Know what intermolecular forces and thermodynamic considerations are as they relate to cells.

Hydrogen bonds are fleeting, primarily electrostatic, and weaker than covalent bonds. Water is a good solvent for polar (hydrophilic) solutes, with which it forms hydrogen bonds, and for charged solutes, with which it interacts electrostatically. Nonpolar (hydrophobic) compounds dissolve poorly in water, the cannot hydrogen-bond with the solvent, and their presence forces an energetically unfavorable ordering of water molecules at their hydrophobic surfaces.

2.2 Ionization of Water, Weak Acids, and Weak Bases

Pure water is slightly ionized. The ionization of water is expressed by equilibrium constant. The pH scale designates the H+ and the OH- concentrations. Weak acids and bases have characteristic acid dissociation constants. Titration curves reveal the pKa of weak acids.

Review Slides

Review Slides

2.14 pH and Drug Absorption. Aspirin is a weak acid with a pKa of 3.5 (see image on pg 68). It is absorbed in to the blood through the cells lining the stomach and the small intestine. Absorption requires passage through the plasma membrane, the rate of which is determined by the polarity of the molecule: charged and highly polar molecules pass slowly, whereas neutral hydrophobic ones pass rapidly. The pH of the stomach contents is about 1.5, and the pH of the contents of the small intestine is about 6. Is more aspirin absorbed into the bloodstream from the stomach or from the small intestine? Justify your choice.

Stomach; the neutral form of aspirin present at the lower pH is less polar and passes through the membrane more easily.

1.8 Drug Activity and Stereochemistry. Why do the two enantiomers have such radically different bioactivity

The bioactivity of a drug is the result of interaction with a biological "receptor," a protein molecule with a binding site that is also chiral and stereospecific. The interaction of the D isomer of a drug with a chiral receptor site will differ from the interaction of the L isomer with that site.

Explain pH

The pH of an aqueous solution reflects, on a logarithmic scale, the concentration of hydrogen ions: pH = log 1/[H+] = -log[H+]. The greater the acidity, the lower the pH.

Explain pKa.

The pKa expresses, on a logarithmic scale, the relative strength of a weak acid or base: pKa = log 1/Ka = -log Ka. The stronger the acid, the lower its pKa; the stronger the base, the higher its pKa.

1.6 Is synthetic vitamin C as good as the natural vitamin C?

The properties of the vitamin-like any other compound-are determined by it chemical structure. Because vitamin molecules from the two sources are structurally identical, their properties are identical, and no organism can distinguish between them. If different vitamin preparations contain different impurities, the biological effects of the mixtures may vary with the source. The ascorbic acid in such preparations, however, is identical.

1.12 Components of Complex Bimolecules. Figure 1-10 shows the major components of complex biomolecules. For each of the three important biomolecules below (shown in their ionized forms at physiological pH), identify the constituents. (See pg 12) a) Guanosine triphosphate.

Three phosphoric acid groups (linked by two anhydride bonds), esterified to an alpha-D-ribose (at the 5' position), which is attached at C-1 to guanine.

1.12 Components of Complex Bimolecules. Figure 1-10 shows the major components of complex biomolecules. For each of the three important biomolecules below (shown in their ionized forms at physiological pH), identify the constituents. (See pg 12) b) Methionine enakphalin.

Tyrosine, two glycine, phenylalanine, and methionine residues, all linked by peptide bonds.

Chapter 2

Water

Describe the properties of water.

Water is a highly polar molecule, capable of forming hydrogen bonds with itself and with solutes.

Memorize the Henderson-Hasselbach equation.

pH = pKa + log([A-]/[HA])