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 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

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

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: 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 Ka: Written correctly,

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

Titration Curves a monoprotic acid releases one H+ per mole ________________ : 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 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