Mary K. CampbellShawn O. Farrellhttp://academic.cengage.com/chemistry/campbell

Chapter TwoWater: 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 CO2 have polar __________ but, given their geometry, are nonpolar _______; that is, they have a __________ dipole moments

Solvent Properties of H2O

• _________ compounds (e.g.,KCl) and low-molecular- weight ________ covalent compounds (e.g., C2H5OH and CH3COCH3) 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 H2O

• ____________ interactions: e.g., ethanol or acetone dissolved in H2O

• ____________ interactions: weak and generally do not lead to solubility in water

Hydration Shells Surrounding Ions in Water

• ____________ and ____________ interactions help ionic and polar compounds dissolve in water

Ion-dipole and Dipole-dipole Interactions

Solvent Properties of H2O

• ____________ : 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: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: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:Acid: a molecule that behaves as a ____________ ____________

• Strong base:Strong base: a molecule that behaves as a ____________ ____________

• 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 Ka:

• Written correctly,

Acid Strength

Ionization of H2O and pH

• Lets quantitatively examine the dissociation of water:

• Molar concentration of water (55M)

• Kw is called the ion product constant for water.

• Must define a quantity to express hydrogen ion concentrations…pH

Henderson-Hasselbalch

• Equation to connect Ka to pH of solution containing both acid and base.

• We can calculate the ratio of weak acid, HA, to its conjugate base, A-, in the following way

Henderson-Hasselbalch (Cont’d)

• Henderson-Hasselbalch equation

• 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 pKa of the weak acid

• when pH < pKa, the weak acid predominates• when pH > pKa, the conjugate base predominates

[Weak acid]

[Conjugate base]log=pH pKa +

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: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• CH3COOH and CH3COONa

• H2CO3 and NaHCO3

• NaH2PO4 and Na2HPO4

Buffer Range

• A buffer is effective in a range of about +/- 1 pH unit of the pKa 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 pKa

• no interference with the reaction or detection of the assay

• suitable ionic strength • suitable solubility• its non-biological nature

Laboratory Buffers