Biochemistry: A Short CourseSecond Edition

Biochemistry: A Short CourseSecond Edition

Tymoczko • Berg • Stryer

© 2013 W. H. Freeman and Company

CHAPTER 2Water, Weak Bonds, and the

Generation of Order Out of Chaos

2

Water Properties

• Polar

• H-bonding

• Dissolve many molecules

2.2 Biochemical Interactions Take Place in an Aqueous Solution

5

Weak Interactions

• Electrostatic

• H-bonds

• Van der Waals

• Hydrophobic

H-Bonds Form Between an Electronegative Atom and Hydrogen

Water disrupts H-bonds between 2 molecules by competing for the hydrogen bonding capability.

“Stability in numbers”

Phospholipids have hydrophilic & hydrophobic properties. When exposed to water, phospholipids form membranes.

Membrane Formation is Powered by the Hydrophobic Effect

14

Functional Groups

• What are they?

• Key groups in biochemistry?

17

pH

• What is it?

• How is it quantitatively expressed?

• What is the effect of acids & bases?

• How cells maintain a constant pH?

The equilibrium constant Keq for the dissociation of water is:

Kw, the ionization constant of water, is given by:

This can be simplified to:

Water Ionizes to a Small Extent

The pH of any solution is defined as:

Acids ionize to form a proton and a base.

An Acid Is a Proton Donor, Whereas a Base Is a Proton Acceptor

The ionization equilibrium of a weak acid is given by:

The larger Ka, the stronger the acid.

Acids Have Differing Tendencies to Ionize

Taking the logarithm of both sides gives:

When [A-] = [HA]

log ([A-]/[HA]) equals 0, and pH =pKa.

For any acid,

at pH > pKa, A- predominates.

At pH < pKa, HA predominates.

27

Buffers

• What are they?

• Titration curves

• How do buffers work?

• Examples

The titration curve for acetic acid

Buffer action: adding a strong acid, 1 M HCl

The titration curves of three important weak acids