how h 2 0 interacts with: itself –hydrogen-bonding ions and charged functional groups...
TRANSCRIPT
How H20 interacts with:
• Itself– Hydrogen-bonding
• Ions and charged functional groups– Solvation, screening, dielectric value
• Non-polar groups– The hydrophobic effect
Hydrogen Bonding
• Atomic requirements– Donor: H attached to electronegative atom
(typically O,N) [the book describes such groups as weak acids]
– Acceptor: Lone pair on electronegative atom (e.g. O,N) [the book describes such groups as weak bases]
• Characteristics– Sort of intermediate between covalent and ionic:
mainly electrostatic but with some directional (bonding) characteristics
– Much weaker than covalent; even weaker than ionic, but ubiquitous
Hydrogen Bonding
• Geometry
ONH
C
Ideal: roughly in-line
Ideal: roughly according to lone pair geometry
Distance between ‘heavy’ atoms: 2.8 to 3.5 Å
The geometric parameters are much looser than for covalent bonding.
Hydrogen Bonding in Water
• In ice (see text, Fig. 2.3)• In liquid water (see text, Fig. 2.4)• Mechanism for proton transfer through chains
of H-bonded water molecules (see text, Fig. 2.15)
Hydrogen Bonding in Water
• In ice– The diamond-like
lattice in crystalline ice is unusually open, giving a lower density than liquid water, with important consequences for aquatic environments
(see text, Fig. 2.3)
Hydrogen Bonding in Water
• In liquid water– rings are sometimes formed (this is
sometimes a point of emphasis, though I don’t think it’s a particularly important point)
– probably most important is the flexibility of the different H-bonding configurations possible; also the number of different partners (2 H-bonds from each O, one from each H, gives 4 partners in the best arrangements)
(see text, Fig. 2.4)
Hydrogen Bonding in Water
• Mechanism for proton transfer through chains of H-bonded water molecules (see text, Fig. 2.15)
How H20 interacts with ions:
• Solvation– Water molecules orient
themselves to make favorable electrostatic interactions with ions
– This stabilizes the ions– Explains the solubility of
ions in water (otherwise they would stay in an ionic solid where they make good ionic interactions)
(see text, Fig. 2.6)
How H20 interacts with ions:
• Screening– Owing to (1) the dipole moment of H20 and (2) its
ability to reorient, water molecules around and between ions screen or dampen the ionic forces between ions
– The ability to screen ionic interactions is embodied in a term called the dielectric value or the dielectric constant, .
– For water near 298K, H20 = 78 (compared to 1 for a vacuum, or ~2 for organic solvents and hydrocarbons)
• i.e. ionic forces are diminished by a factor of 80 in water!
How H20 interacts with non-polar solutes (the hydrophobic effect):
• Unfavorable transfer of non-polar groups from organic phase to water
Non-polar solute
organic organic
aqueous aqueous
G0transfer =
-RTlnK
K=[solute]H20/[solute]org
How H20 interacts with non-polar solutes (the hydrophobic effect):
• Mainly a surface effect– Presence of the non-polar group
requires H20 molecules to H-bond the each other in more ordered fashion than in bulk solution
– The magnitude of the effect depends on the amount of surface area involved
– Mainly an entropic effect (it’s not that the water has bad energetic interactions with the solute, but rather that its entropy is reduced)
(see text, Fig. 2.8)
How H20 interacts with non-polar solutes (the hydrophobic effect):
• Driving force for aggregation/association (minimizes surface area of interaction with water)
Non-polar solutes or functional groups, or non-polar surfaces of larger molecules
How H20 interacts with non-polar solutes (the hydrophobic effect):
• Biological relevance– Binding interactions between
molecules– Protein folding– Formation of membrane lipid
bilayer
though water is described as the ‘universal solvent’, the insolubility of lipid molecules is one of the most vital features for cell structure: it makes the membrane possible
Acid-Base Equilibria in Water
• HA <===> A- + H
+ or (HA
+ <===> A + H
+)
• Ka = [A-][H
+]/[HA]
• [H+] = Ka[HA]/[A
-]
(pH ≡ -log10[H+], pK ≡ -log10K)
• pH = pK + log10 [A-]/[HA] (Henderson-Hasselbalch)
Acid-Base Equilibria in Water
• pH = pK + log10 [A-]/[HA] (Henderson-Hasselbalch)
• Explains:– buffering behavior of acid-base mixtures– distribution of acid vs. base forms of
functional groups in molecules
Acid-Base Equilibria in Water
• pH = pK + log10 [A-]/[HA] (Henderson-Hasselbalch)
– buffering behavior of acid-base mixtures
• when the numerator and denominator of the term in the log function are both similar, moderate changes to the numerator or denominator do not change the value of the log much, meaning the pH doesn’t change much when H+ or OH- are added, or produced by reactions in the cell
• the numerator and denominator are similar when the pH is near the pK, so buffers are only effective in a pH range near their pK value.
Acid-Base Equilibria in Water• pH = pK + log10 [A
-]/[HA] (Henderson-Hasselbalch)
– distribution of acid vs. base forms of functional groups in molecules
• at pH 4, both forms are similarly present• at pH 5, the base form is 10 times more abundant• at pH 3, the acid form is 10 times more abundant• The dependence of protonation state on pH (and environment) is
important for how proteins function, since charged groups often play important roles in binding and catalysis.
O
C
OH
O
C
O-
+ H+
<===>
pK 4