chemical forces & self assembly (part ii)
TRANSCRIPT
Chapter 8 Student Medley
Biological PhysicsNelson
Updated 1st Edition
Slide 1-1
Chemical Forces & Self Assembly (Part II)
Slide 1-2
DISSOCIATION
8.3
Slide 1-3
8.3.1 Ionic and partially ionic bonds
dissociate readily in water
• Electronegativity
– An attraction of a free electron to electrically
neutral atom to have negative charge
– Ion has lower internal energy than neutral
• Ionic bond:
– bond between two ions by transferring
electron
– Stays by electrostatic attraction energy
• E=qV, and
where r is the diameter of an ion
Slide 1-4
8.3.1 Ionic and partially ionic bonds
dissociate readily in water
• But it can dissolve in water
– When an ion pair separates, entropy
increases > free energy cost
• Polar molecules: soluble in water too
– Hydrogen bonding in water
– Dipole-dipole interaction (electrostatic
interactions of permanent dipoles in
molecules), alcohol….etc
Slide 1-5
8.3.2 The strengths of acids and bases
reflect their dissociation equilibrium
constants
• Water
• Dissociation of water does cost more free
energy than that of ionic bond (ex. NaCl)
• In pure water
Slide 1-6
8.3.2 The strengths of acids and bases
reflect their dissociation equilibrium
constants
• Definition:
• If pH = 7, neutral
• If pH > 7, base
• If pH < 7, acid
• Equal amount of HCl and NaOH
• Mix, neutral, saltwater!!!!!!!!!!!!
Slide 1-7
8.3.3 The charge on a proteins varies with
its environment
• Each protein subunit(amino acids(except
proline)) provides the same backbone structure
to the protein, -NH-CH-CO-
• The side chains of the amino acids are different.
• Side chains interact with each other and water to
form folded structure
• This structure define the protein function
• In short proteins are extremely complicated.
Slide 1-8
8.3.3
• Some amino acids liberate H+, while other attract
H+
• The probability for α be protonated depends on
Keq,alpha and on the pH of the surrounding fluid,
this probability is denoted to Palpha
• Protonated and deprotonated subunits will work
on each other not individually
Slide 1-9
8.3.4
• At a certain pH a proteins charge will be neutral,
this is called the isoelectric point
• This can be used to separate proteins from each
other in column chromatography or
electrophoresis
Slide 1-10
Self assembly of amphiphiles
8.4
Slide 1-11
The Invisible Hand
Without any help from the “creation”, appropriate
molecules come together, following chemical
forces to make functioning structures.
Slide 1-12
Recall cell membranes
Organisms undergo metabolism,
maintain homeostasis, possess a
capacity to grow, respond to stimuli,
reproduce and, through natural
selection, adapt to their environment
in successive generations.
Slide 1-13
The building blocks of micelles
Amphiphilic molecules reduce the oil-water
interface tension.
surfactant, emulsifier, detergents
Slide 1-14
Micelles are entropically favored.
A mixture of oil droplets in water is not a self-
assembly structure. It's merely a result of
hydrophobic interactions.
Surfactant molecules however can go with another
option to save their entropic cost.
Slide 1-15
Micelles
Slide 1-16
The “construction” of micelles
Supposed the soap is potassium oleate.
K+ → osmotic pressure
oleate molecule → thermodynamic equilibrium
between
“individual molecule ↔ aggregates of N ions”
Slide 1-17
Osmotic Pressure Contribution
; when half the monomers are free and half are
assembled into micelles.
Slide 1-18
Osmotic Pressure Contribution
Now recall, osmotic pressure:
…
…
…
…
Slide 1-19
Osmotic Pressure Contribution
• At the critical micelle concentration (CMC) – the
ratio of independently moving objects to all ions
dropped sharply
• see the next slide
Slide 1-20
Slide 1-21
Thermodynamic Contributions
Not only the concentration (CMC), but also
temperatures (CMT) contributes the formation,
hence the proof of entropic contribution.
dG = dH-TdS
@ Concentration less than CMC:
monomers ↔ micelles [entropically disfavored]
@ Concentration more than CMC
concentrated micelles ↔ diluted micelles [entropically favored]