Mary K. Campbell
Shawn O. Farrell http://academic.cengage.com/chemistry/campbell
Chapter Two
Water: The Solvent for Biochemical Reactions
Paul D. Adams University of Arkansas

What makes water polar?
What is a polar bond:
• Electrons are unequally shared, more negative charge found closer to one atom.
• Due to difference in _________________________of atoms involved in bond.

Electronegativity
• __________________: a measure of the force of an atom’s attraction for electrons it shares in a chemical bond with another atom
• Oxygen and Nitrogen, ____________ electronegative than carbon and hydrogen
• _______________ is most electronegative (4)

Polar Bonds & Molecules
• Molecules such as CO
2 have polar __________ but, given their geometry, are nonpolar _______; that is, they have a __________ dipole moments

Solvent Properties of H
2
O
• _________ compounds (e.g.,KCl) and low-molecularweight ________ covalent compounds (e.g., C
2
H
5
OH and CH
3
COCH
3
) tend to dissolve in ______________
• The underlying principle is electrostatic ________ of
_____________ charges; the positive dipole of water for the negative dipole of another molecule, etc.
• ____________ interaction: e.g., KCl dissolved in H
2
O
• ____________ interactions: e.g., ethanol or acetone dissolved in H
2
O
• ____________ interactions: weak and generally do not lead to solubility in water

Hydration Shells Surrounding Ions in Water

Ion-dipole and Dipole-dipole Interactions
• ____________ and ____________ interactions help ionic and polar compounds dissolve in water

Solvent Properties of H
2
O
• ____________ : water-loving
• tend to dissolve in water
• ____________ : water-fearing
• tend not to dissolve in water
• ____________ : characteristics of both properties
• molecules that contain one or more ____________ and one or more ____________ regions, e.g., sodium palmitate

Amphipathic molecules
• both ____________ and ____________ character
• Interaction between ____________ molecules is very weak, called van der Waals interactions

Micelle formation by amphipathic molecules
• Micelle: a ____________ arrangement of organic molecules in ___________ solution clustered so that
• their ____________ parts are buried inside the sphere
• their ____________ parts are on the surface of the sphere and in contact with the water environment
• formation depends on the attraction between
____________
____________
____________

Examples of Hydrophobic and Hydrophilic
Substances

Hydrogen Bonds
• Hydrogen bond: the attractive interaction between dipoles when:
• positive end of one dipole is a hydrogen atom bonded to an atom of high electronegativity, most commonly O or N, and
• the negative end of the other dipole is an atom with a lone pair of electrons, most commonly O or N
• Hydrogen bond is ______________________

Interesting and Unique Properties of Water
• Each water molecule can be involved in ___ hydrogen bonds: ___ as donor, and ___ as acceptor
• Due to the ____________ arrangement of the water molecule (Refer to Figure 2.1).

Hydrogen Bonding
• Even though hydrogen bonds are ___________ than covalent bonds, they have a significant effect on the physical properties of hydrogen-bonded compounds

Other Biologically Important Hydrogen bonds
• Hydrogen bonding is important in _______________ of 3-D structures of biological molecules such as:
DNA, RNA, proteins.

Acids, Bases and pH
• Acid: a molecule that behaves as a ____________
____________
• Strong base: a molecule that behaves as a
____________ ____________

Acid Strength
• One can derive a numerical value for the strength of an acid (amount of hydrogen ion released when a given amount of acid is dissolved in water).
• Describe by K a
:
• Written correctly,

Ionization of H
2
O and pH
• Lets quantitatively examine the dissociation of water:
• Molar concentration of water (55M)
• K w is called the ion product constant for water.
• Must define a quantity to express hydrogen ion concentrations…pH

Henderson-Hasselbalch
• Equation to connect K a both acid and base. to pH of solution containing
• We can calculate the ratio of weak acid, HA, to its conjugate base, A , in the following way

HendersonHasselbalch (Cont’d)
• Henderson-Hasselbalch equation pH = log
[Conjugate base] pK a
+
[Weak acid]
• From this equation, we see that
• when the concentrations of weak acid and its conjugate base are equal, the pH of the solution equals the pK a of the weak acid
• when pH < pK a
, the weak acid predominates
• when pH > pK a
, the conjugate base predominates

Titration Curves
• ________________ : an experiment in which measured amounts of acid (or base) are added to measured amounts of base (or acid)
• _______________________ : the point in an acid-base titration at which enough acid has been added to exactly neutralize the base (or vice versa)
• a monoprotic acid releases one H + per mole
• a diprotic acid releases two H + per mole
• a triprotic acid releases three H + per mole

Buffers
• buffer: a solution whose ________ resists change upon addition of either more acid or more base
• consists of a weak acid and its conjugate base
• Examples of acid-base buffers are solutions containing
• CH
3
COOH and CH
3
COONa
• H
2
CO
3 and NaHCO
3
• NaH
2
PO
4 and Na
2
HPO
4

Buffer Range
• A buffer is effective in a range of about +/- 1 pH unit of the pK a of the weak acid

Buffer Capacity
• Buffer capacity is related to the ____________ of the weak acid and its conjugate base
• the greater the concentration of the weak acid and its conjugate base, the greater the buffer capacity

Naturally Occurring Buffers
• ____________ is the principal buffer in cells
• ____________ is an important (but not the only) buffer in blood
• ____________ can result in increased blood pH
• ____________ can result in decreased blood pH
(Biochemical Connections p. 60)

Selecting a Buffer
• The following are typical criteria
• suitable pK a
• no interference with the reaction or detection of the assay
• suitable ionic strength
• suitable solubility
• its non-biological nature

Laboratory Buffers