Untitled Quiz

• Chapter 2 focuses on water, intermolecular interactions, behavior of weak acids and bases, and water's role in biochemical reactions.
• Life evolved in water because it provides protection from UV light.
• Organisms typically contain 70-90% water.
• Chemical reactions occur in an aqueous environment.
• Water determines the structure and function of proteins, nucleic acids, and membranes.

Structure of Water

• The octet rule dictates four electron pairs surround an oxygen atom in water, occupying four sp³ orbitals.
• Two of these pairs link two hydrogen atoms to the central oxygen atom covalently.
• The other two pairs remain nonbonding (lone pairs).
• Water geometry is a distorted tetrahedron (VSEPR).
• Oxygen's electronegativity induces a net dipole moment.
• Because of the dipole moment, water can serve as both a hydrogen bond donor and acceptor and is an amphoteric compound.

Hydrogen Bonds

• Hydrogen bonds are strong dipole-dipole or charge-dipole interactions between a covalently bound hydrogen and a lone pair of electrons.
• They typically involve two electronegative atoms, frequently nitrogen and oxygen.
• Hydrogen bonds are strongest when the bonded molecules allow for linear bonding patterns.
• Ideally, the three atoms involved are aligned.

Ionization of Water

• O-H bonds are polar and can dissociate heterolytically.
• A proton (H+) and hydroxide ion (OH-) are products of ionization.
• Dissociation of water is a rapid, reversible process.
• Most water molecules remain un-ionized in pure water, giving it very low electrical conductivity with a resistance of 18 ΜΩ•cm.
• The equilibrium is strongly to the left, indicated by a low Keq.
• The extent of dissociation depends on the temperature.

Proton Hydration

• Protons (H+) do not exist freely in solution.

• They are immediately hydrated, forming hydronium ions (H3O+).

• A hydronium ion involves a water molecule with a proton associated with one of the nonbonding electron pairs and is solvated by nearby water molecules.

• Covalent and hydrogen bonds are interchangeable, allowing extremely fast proton mobility in water via "proton hopping."

• Proton hopping is exemplified in cytochrome f.

Ice

• Water has many different crystal forms, with the hexagonal ice being the most common.
• Hexagonal ice forms an organized lattice, resulting in low entropy.
• Hexagonal ice contains maximal hydrogen bonds/water molecules, forcing the water molecules into equidistant arrangement.
• Ice has a lower density than liquid water and floats.

Noncovalent bonds

• Noncovalent interactions do not involve electron sharing and can be distinguished by their physical origin

• Ionic (Coulombic) interactions: Electrostatic interactions between permanently charged species, or between an ion and a permanent dipole

• Dipole interactions feature electrostatic interactions between uncharged but polar molecules

• Van der Waals interactions: Weak interactions between all atoms, regardless of polarity, with attractive (dispersion) and repulsive (steric) components

• Hydrophobic Effect: Complex phenomenon that involves ordering of water molecules around nonpolar substances

• Ionic, Metallic and Covalent bonds are intramolecular forces

• Hydrogen, Dipole-Dipole and Van der Waals bonds are intermolecular forces

The Hydrophobic Effect

• It refers to the association or interaction of nonpolar molecules in aqueous solutions.

• It does not arise because of an attractive direct force between nonpolar molecules. It is thermodynamically unfavorable because hydrophobic solutes have low solubility.

• It is a main factor behind protein folding, protein-protein association, the formation of lipid micelles, and steroid hormones binding to their receptors.

• Water surrounding nonpolar solutes has lower entropy.

• Dispersion of lipids in H₂O: Each lipid molecule forces surrounding H₂O molecules to become highly ordered, which creates an unfavorable state.

• Nonpolar portions of amphipathic molecules aggregate so that fewer water molecules are ordered, this increases the entropy.

• When nonpolar groups are sequestered from water, released water molecules increase entropy further and only polar “head groups" are exposed.

• With a high concentration of amphipathic molecules, complete aggregation into vesicles and micelles is possible.

• The hydrophobic effect favors Ligand binding

• Binding sites in enzymes and receptors are often hydrophobic

• Hydrophobic steroid hormones displace water and increase system entropy

• Many Drugs are designed to take advantage of the hydrophobic effect

Osmotic Pressure

• Water moves from areas of high water concentration (low solute concentration) to areas of low water concentration (high solute concentration), or from less salty to more salty areas.
• Osmotic pressure (π) is the force necessary to resist its movement.
• The concentration of each solute in solution influences osmotic pressure.
• Dissociated components of a solute individually influence osmotic pressure.

"Tonics"

• This refers to the movement of water as it depends on the osmolality of non-penetrating solutes.
• A cell in an isotonic solution has no net water movement.
• A cell in a hypertonic solution results in water moving out and shrinking.
• A cell in a hypotonic solution results in water moving in, creating pressure and swelling until eventual cell burst

Henderson-Hasselbalch Equation

• pH = pKa + log([A-]/[HA])
• This equation dictates the pH of the solution
• A buffer with 0.25 M formic acid and 0.80 M sodium formate (pKa of 3.75) has a pH = 3.75 +log (0.25/0.80) = 4.26