SCH 405 presentation - powerpoint recent.pdf

ElectroChemistry

19. Collision diameter

32. 2
3
2 3
32 3
2
3
2
Encounter

40. Peter Debye (1884 – 1966)

60. O3 M
An inelastic collision
required to provide
energy for breaking
the bond

62. d[O
2
]
dt
Step 2
= 2k
2
[O
3
][O]

74. 1/3 d[O
2
]/dt = k
2
[O
3
]
k
1
[O
3
][M]
k
1
'[O
2
][M] + k
2
[O
3
]
=
k
1
k
2
[O
3
]
2
[M]
k
1
'[O
2
][M] + k
2
[O
3
]

75. 1/3 d[O]/dt = k
2
[O
3
]
k
1
[O
3
][M]
k
1
'[O
2
][M] + k
2
[O
3
]
=
k
1
k
2
[O
3
]
2
[M]
k
1
'[O
2
][M] + k
2
[O
3
]

84. Construct n
steps
Correct
stichiometry
Correct
number of
intermediates

85. Intermediates
Number = n-1
Gaseous = radicals;
Aqueous = ions
Occur on r.h.s then
on l.h.s
Must cancel in
subsequent steps

86. Approximation?
Rate
limiting
step
1
st
step ≈
r.l.s
1
st
step ≠
r.l.s
Steady state
Definite
Orders?
Indefinite
Orders?
No pdct in
Rate law
Pdct; (-ve) orders
In rate law

121. Theodore von Grotthus
1785 - 1822
John William Draper
1811 - 1882

123. Johannes Stark
1874 - 1957
Albert Einstein
1875 - 1955

139. Catalysis
Homogeneous Heterogeneous Enzyme

149. Catalysis in soln, cnt’d
K
1 = [H
3O
2
+]
[H
2O
2][H
3O
+] and
Rate for step 2 = k[H
3O
2
+][Br
-]
Thus d[O
2] = k
eff[H
2O
2][H
3O
+][Br
-]
dt
Where k
eff = k
2K
1

150. Catalysis in soln, cnt’d
This agrees with the experimental
observation of dependence of rate on
[Br
-] and pH of the solution
In acid catalysis the crucial step is
the transfer of proton to the
substrate:
X + HA HX
+ + A
-
HX
+ Products

151. Catalysis in soln, cnt’d
Acid catalysis is the primary process
in the solvolysis of esters and keto-
enol tautomerism.
In base catalysis, a proton is
transferred from the substrate to the
base:
XH + B X
- + BH
+
BH
+ Products

152. Catalysis in soln, cnt’d
Base catalysis is the primary step in
the isomerization and halogenation
of organic compounds, and of the
Claisen and aldol condensation rxns

153. ENZYME CATALYSIS

154. Enzymes
Are homogeneous biological catalysts
Are special proteins or nucleic acids
They contain an active site which is
responsible for binding the substrate
(reactants), transforming them into
products
The active site returns to the original
state after the products are released

155. Enzyme specificity
Enzymes generally exhibit specificity, i.e.
catalyze only specific reactions.
Three types of specificity:
Specificity
Absolute group Stereochemical

156. Enzyme Specificity
Absolute specificity – catalyzes the rxn
of only one substance – Urease only
catalyzes urea
Group specificity – catalyzes any of a
group of reactions – protease catalyzes
the hydrolysis of proteins
Stereochemical specificity – catalyzes
the reaction of one optical isomer but not
its enantiomorph – protease only
catalyzes hydrolysis of polypeptides made
of L- amino acids

157. Binding of enzyme onto substrate
The substrate can bind to the active
site in a lock and key model.
Here, the enzyme and the substrate
have complementary 3-d structures
and dock perfectly without need for
major atomic rearrangements.

158. Binding of substrate onto Enzyme
Experimental evidence favours the
induced fit model.
Here, the binding of the substrate
induces a conformational change in
the active site.
After the change, the substrate fits
snugly in the active site.

159. Docking of substrates onto
Enzymes
+
Products
Induced fit
or
Or
Lock
and
Key

162. Enzymes, cnt’d
Enzyme-catalyzed reactions are prone to
inhibition by molecules that interfere with
formation of product
Many drugs for the treatment of disease
function by inhibiting enzymes
E.g. Important strategy in the treatment
of AIDS = steady administration of a
specially designed protease inhibitor

163. Enzymes, cnt’d
Will inhibit an enzyme that is key to
the formation of the protein envelope
surrounding the genetic material of
the HIV.
Without a proper envelope, HIV
cannot replicate in the host organism

164. The Michaelis Menten
Mechanism of Enzyme Catalysis
Experimental studies of enzyme kinetics
are conducted by monitoring the initial
rate of product formation in a solution in
which the enzyme is present at very low
concentration
Principal features:
1. For a given initial conc. of substrate,
[R]
o, the initial rate of product formation is
proportional to the total conc. of the
enzyme

165. Principal features, cnt’d
2. For a given [E]
o, rate of product
formation is proportional to the total
conc. of the enzyme, [E]
T.
3. For a given [E]
o and high values of
[R]
o, the rate of product formation
becomes independent of [R]
o,
reaching a maximum value, known
as the maximum velocity, ν
max.

166. Michaelis-Menten Mechanism
Michaelis and Menten proposed a
mechanism for enzyme catalysis.
For the case of a single reactant, R
and single product, P, the
mechanism is:
1. E + R ER
2. ER E + P
ER is the enzyme-reactant complex

167. Michaelis-Menten Mechanism,
cnt’d
Invoking the steady state approximation:
d[ER]=k
1[E][R]–k
1’[ER]–k
2[ER]≈0 (1)
dt
d[P]/dt = k
2[ER]……………..(2)
[E]
total = [E] + [ER] ………(3)
Enzyme conc. in ER is unknown; but
significant.
[R]
total >>>>>[ER]; therefore
[R] = [R]
total – [ER] ≈ [R]
total 3(a)

168. Michaelis-Menten Mechanism,
cnt’d
[R] = [R]
total – [ER] ≈ [R]
total 3(a)
Substituting [E]
total – [ER] in eqn 1:
d[ER]/dt = k
1([E]
total – [ER])[R] –
k
1’[ER]–k
2[ER]…….3(b)
Solving for [ER] we get:
[ER] = k
1[E]
total[R]
k
1’ + k
2 + k
1[R]…………..(4)
Substituting eqn 4 into eqn 2 we get

169. Michaelis-Menten Mechanism,
cnt’d
Fwd rate = d[P] = k
2[E]
total[R]
dt K
m + [R] (5)
This is the Michaelis - Menten
equation
Where Km = k
1’ + k
2 (6)
k
1
Km is the Michaelis-Menten constant;
is dependent on temperature

170. The Michaelis-Menten
Mechanism, cnt’d
Equation 5 is not easily integrated,
thus the method of initial rates is
used
A number of experiments are
conducted with the same [E] but
different [R] and the initial rate
determined for each

171. Michaelis –Menten
mechanism, cnt’d
k2[E]total
Initial
rate
[R]
0 [R] = Km
Initial rate as a function of Reactant concentration for the Michaelis - Menten mechanism

172. Michaelis-Menten Mechanism,
cnt’d
For small values of [R] the initial rate
is proportional to [R]
For large [R] values, the rate levels
off and approaches the value k[E]
total
asymptotically
Km is determined by locating the
asymptote and equating K
m to the
value of [R] at which the initial rate
= ½ asymptotic value

173. Michaelis-Menten Mechanism,
cnt’d
The number of reactant molecules
that react per enzyme molecule per
second is called the turnover
number.
Its maximum value is k
2, and can
range up to 10
6s
-1

174. The Lineweaver-Burke
Equation
If there is considerable experimental
error, the asymptote may be difficult
to locate
Thus we get the reciprocal of the
initial rate, r
i from eqn 5:
1 = K
m + 1 ..…….(7)
r
i k
2[E]
total[R] k
2[E]
total
This is the Lineweaver-Burke eqn.

175. The Lineweaver-Burke
Equation, cnt’d
When 1/ri is plotted against 1/[R] a
straight line is obtained with:
Slope = K
m/k
2[E]
total
y- intercept = 1/k
2[E]
total
x –intercept = -1/K
m
Exercise;
Show that the intercept on the
Lineweaver-Burk plot on the x-axis =
-1/K
m

176. The Lineweaver-Burke slope
1/[R]
0
1/ri
-1/K
m
1/k
2
[E]
total
Slope
= K
m
/k
2
[E]
total

177. Example
The following data was gathered for
the myosin-catalyzed hydrolysis of
ATP at 25
oC and pH 7.0:
Determine the value of the Michaelis-
Menten constant

178. 1/[ATP] 1/ri
0.133333 14.9254
0.08 10.5263
0.05 8.4034
0.022989 6.7114
0.016 5.4054
0.006452 5.2356
0.003125 5.1282
y = 75.846x + 4.6685
-0.1000
1.9000
3.9000
5.9000
7.9000
9.9000
11.9000
13.9000
15.9000
-0.15 -0.05 0.05 0.15
Plot of 1/ri vs [ATP]

179. y = 75.846x + 4.6685
-0.1000
1.9000
3.9000
5.9000
7.9000
9.9000
11.9000
13.9000
15.9000
-0.15 -0.05 0.05 0.15
1/ri

180. Solution, cnt’d
Slope = 76.58s
Y- intercept = 4.547L/µmol s
X- intercept = -0.0595L/µmol
Thus the Michaelis- Menten constant
= 16.8µmol/L

181. Exercise
An alternative linear plot is the Eadie plot
for which eqn 5 is put in the form:
r
i = r
i + k
2[E]
total
[R] K
m K
m
Using the previous data, make a linear
least-squares fit of r
i/[R] as a function of
r
i. Find the value of the Michaelis Menten
const. and compare with the one obtained
in the previous example

182. [S] (M) 10
6
x initial rate (Ms
-1
)
0.020 0.585
0.004 0.495
0.002 0.392
0.001 0.312
0.00066 0.250

183. POLYMERIZATION
KINETICS

184. Polymerization kinetics
Poly = many
Meros = parts
Thus a polymer molecule is made
from small molecules (monomers)
They react to form covalently-
bonded chains or networks

185. Polymers, cnt’d
Examples: polyethylene, nylon and
polyesters (synthetic) and rubber,
proteins, starches, celluloses and
nucleic acids (natural)
Some have chain-like molecules eg.
Polyethylene, polystyrene and
polypropylene

186. Polymers, cnt’d
These sometimes soften when
heated, thus the name
thermoplastic polymers
Other polymers are made up of
networks instead of chains eg.
Bakelite.
These are sometimes called
thermosetting because they are
formed at high temperatures

187. Polymers, cnt’d
Synthetic
polymers
Condensation
polymers
Addition
polymers

188. Polymers, cnt’d
Condensation polymers: When a
monomer is added, there is a small
molecule (often water) produced in
addition to lengthening the chain
The process is usually referred to as
Stepwise polymerization
Examples: Nylon and polyester.

189. Polymers, cntd
Addition polymers: no other
product is added, besides the chain;
usually C-C double bonds opening up
to bond with other monomers.
The process is usually referred to as
chain polymerization
Addition monomers examples:
polyethylene and polystyrene

190. Polymerization kinetics
Consider the formation of a polyester
from a monomer diacid HOOC-X-
COOH, and a monomer dialcohol HO-
Y-OH, where X and Y represent two
hydrocarbon chains.
The first step in the polymerization
is:
HOOC-X-COOH+ HO-Y-OH → HOOC-
X-COO-Y-OH + H
2O