7/27/2019 Biochem -Lec 02

1/3

1

Biochemistry I Fall Term, 2004 September 1, 2004

Lecture 2: Water & Hydrogen Bonds

Assigned reading in Campbell: Chapter 2.1-2.2.Key Terms:

Molecular orbitalsLone pair electronsHydrophobic and nonpolar

Hydrophilic and polarAmphipathic (or amphiphilic)

Dipolevan der Waals interactionHydrogen bond

Electronegativity

Links:

(I) Review Quiz on Lecture 2 concepts(S) Chime display of a Water Cluster.

2.1 Structure and Polar Nature of Water

1. Electronic Structure of Watera. Oxygen has the following electronic configuration: 1s22s22p4.b. The 2s and 2p orbitals form four sp3 hybrid orbitals.c. These orbitals are tetrahedral in shape (ideal bond angle of 109, distorted to

104.5).

d. The orbitals are populated such that two orbitals are filled and two contain oneelectron. This distribution is a consequence of Hund's rule: when two electronsare populating two orbitals of equal energy then each orbital will contain oneelectron.

e. The filled orbitals cannot form bonds and are called lone pairs of electronsf.

The half-filled orbitals participate in the formation of a sigma bond betweenoxygen and hydrogen.

7/27/2019 Biochem -Lec 02

2/3

2

2. Solvationa. Hydrophobic (apolar) compounds (e.g. hexane)b. Hydrophilic (polar) compounds (e.g. sodium ion, ethanol)c. Amphipathic (or amphiphilic) compounds are both polar and nonpolar (e.g. fatty

acids)

d. Solvation of ions: forces between two charged particles:

The force depends on the distance (r) between the two charges (q) and thedielectric constant (D) of the media.

Compound Dielectric Constant Dipole Moment ()

Formamide 110 3.37

Water 79 1.85

Methanol 32 1.66

Benzene 2 0.00

The dipole moment reflects the charge distribution of a molecule. It is defined by the

following equation:

A larger dipole moment means that the solvent molecules can interact favorably with

charged solute molecules, thus screening their charges. Consequently, a high dipolemoment usually implies a high dielectric constant. A high dielectric constant, such as thatfound in water, is important because the forces between charges are attenuated.

2.2 Hydrogen Bonds

1. Characteristics of H-Bondsa. Formation of H-bonds is primarily an electrostatic attraction between:

Electropositive hydrogen bond donor (i.e. hydrogen)Electronegative hydrogen bond acceptor (e.g. the lone pair electrons of oxygen inthe case of water)

7/27/2019 Biochem -Lec 02

3/3

3

b. Typical length: 1.8 (from hydrogen to oxygen, 2.7 from nitrogen to oxygen)c. Typical angle: 180 20d. Typical energy: 20 kJ/mole (in a vacuum)e. Facilitates rapid proton diffusion in aqueous media

2. Significance of hydrogen bondsa. Solvent properties of waterb. Responsible for the low density of ice.c. Ice-like structure is present up to the boiling point.

(Hsublimation = 47 kJ/mol; Hfusion = 7 kJ/mol). Radial density functions show that

tetrahedral geometry is present over a wide temperature range.

d. Responsible for the high boiling point of water.e. Heat capacity (Cp = dQ/dT = dH/dT): This is important for temperature regulation

in organisms.

f. Play an important role in recognition at the molecular level- most (all) hydrogenbonds must be satisfied in these interactions.

Examples of Weak Noncovalent Bonding in Water

1. Water, itself2. Ethanol3. Hexane4. Soap (micelles)5. Cations

8.24.04