BioPHYSICAL chemistry

19.20 Areas that are undergoing high metabolism are shown by high

use of glycolysis and correspondingly high concentrations of 2-

[^18 F]fluoro-2-deoxy-D-glucose. Such profiles of the brain have been

found to be characteristic of certain diseases, such as Alzheimer’s

disease. Alternatively, use of fluorothymidine, is used to located

areas that are undergoing high DNA replication, which is charac-

teristic of cancer cells.

CHAPTER 20

20.1 The different substitutents result in differences in the nature of the

conjugated system of the macrocycle.

20.2 Bacteriochlorophyll a absorbs at 865 nm, corresponding to an energy

of 1.2 eV, while P680 absorbs at 680 nm, corresponding to 1.8 eV.

20.3 The cofactors of the bacterial reaction center are well buried within

the protein whereas for the light-harvesting complex II the protein

forms two concentric rings with the cofactors found in the middle,

also forming rings, with bacteriochlorophylls in close contact despite

being associated with different protein subunits.

20.4 The differences in the optical spectra reflect the different inter-

actions of the bacteriochlorophylls with each other as well as with

the surrounding protein.

20.5 Different answers are accepted. For example, the availability of two

complexes provides the cell with the opportunity to regulate the

amount of one complex, the light-harvesting complex II, without

disturbing the energy transfer into the reaction center.

20.6

20.7 Although the structure is symmetrical, the energetics of the cofactors

may be determined by the local protein environment and hence the

rates of electron transfer may differ due to energetic considerations.

Other answers are possible.

20.8 Both cofactors are electron acceptors but the properties of their elec-

tronic states differ considerably, including their oxidation/reduction

potentials.

20.9 The conversion of light energy involves the sequential formation of

the S states. Each absorption of light gives rise to the transfer of one

electron. After the absorption of four photons the water-oxidation

complex is capable of the four-electron process of water oxidation

into molecular oxygen.

Δ

Δ

H

H

AN

AN

monomer

dimer

monomer

monomer

==

1

2

2

2214 =.

486 ANSWERS TO PROBLEMS

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