Chemistry - A Molecular Science

Chapter 14 Inorganic Chemistry

Consequently, when the shape of the DNA double helix is changed (Figure 14.14), there is a dramatic effect on the processes that occu

r inside the cell. Because the platinum-bonded

DNA strand has a different shape, some of th

e DNA-protein interactions are modified,

thus prohibiting normal function of the protei

n. In the absence of normal protein-DNA

interactions, the cell dies. The exact DNA-protein interactions are not yet fully understood, but the success of cisplatin as an antitumor drug

appears to result from the change in shape

of DNA upon binding of the drug.

The

trans

isomer (Figure 14.8) is much less effective as an antitumor agent. It also

binds to DNA, but the two functional groups end up across from one another. This distorts the shape of DNA, but the distortions

are different from those caused by the

cis

isomer.

Whether this results in less disruption to prot

ein-DNA interactions, or whether this type of

shape change can be more easily

“repaired” in the cell, the

trans

isomer of this complex is

not as effective in killing tumor cells.

14.5

TRANSITION METALS AS CATALYSTS The interaction of ligands with transition me

tals in coordination complexes can also be

exploited to facilitate chemical reactions or to make useful materials. In Section 9.9, we indicated that a catalyst is a material that increases the rate of a reaction by lowering the activation energy. It does this by altering the

path of the reaction to achieve a lower energy

transition state. In many cases, the coordination of a ligand to a transition metal changes the reactivity of the ligand and thus creates

new pathways through which the ligand can

react. The catalyst itself is unchanged by the reaction. A catalyst can be classified as either heterogeneous or homogeneous

, depending on whether it is in a different phase or the

same phase as the reactants.

Heterogeneous catalysts

are solids that catalyze solution or gas phase reactions on

their surfaces. The adsorption of a ligand (reactant) to a metal catalyst weakens a bond in the reactant and makes it susceptible to further reaction. An important example of heterogeneous catalysis is the hydrogenation of alkenes, which has many practical applications, including the hydrogenation of food

oils into solid fats such as margarine.

The activation energy for this reaction is

quite large because both the H-H bond and the

C=C

π

bond of the alkene must be broken to form

the transition state. Consequently, the

reaction is slow unless the temperature is high, but increasing the temperature reduces the yield of the reaction because the reaction is

exothermic (Section 9.11). However, in the

HN^3

Pt

HN^3

HN

N N

N

O

NH

NH^22

HN

N N

N

O

HN^3

Pt

H^3

N

OH

2

OH

2

NH

2

HN

N

N

N

O

NH

2

NH

N

N

N

+ O

+ 2 H

O 2

α

Figure 14.13 Cisplatin bound to two sites of a DNA strand The DNA strand must bend (

α) in order to accommodate the

interaction with cisplatin. Normal DNA

cisplatin

binding site

abnormal bent DNA

Pt

HN^3 HN^3 =Pt

Figure 14.14 Changes in the double helix of DNA due to cisplatin binding The region of cisplatin binding is

outlined. The ribbon-like structure

indicates that the DNA backbone di

storts from the double helix.

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