BioPHYSICAL chemistry

19.7 The modification involves cyclization, oxygenation, and proton

transfer; the GFP becomes colored only after this process takes

place.

19.8 The absorption band has contributions from different states of the

cofactor, the relative populations of which change with pH.

19.9 There is a protonation change of the chromophore.

19.10 GFP can be tagged onto any gene in a genome and the locations

of the protein product can be located in the different parts of the

cell by fluorescence measurements.

19.11 One protein is labeled with GFP and the second is labeled with an

energy acceptor. If the proteins bind, excitation of GFP is followed

by energy transfer to the acceptor and fluorescence is measured

from the acceptor. If the proteins do not bind, excitation of GFP

is followed by fluorescence from GFP because the acceptor is too

far away for energy transfer.

19.12

where the parameter r 0 represents the distance at which the effici-

ency of energy transfer is 50%.

19.13There are several factors, such as the spectral overlap, that influence

the efficiency.

19.14 In DsRed, the chromophore undergoes an additional dehydro-

genation that results in a more red color.

19.15 If GFP is fused to a gene of interest, the encoded protein can be

tracked by monitoring the GFP fluorescence in the cell.

19.16 The decay particles can be α, β, or γradiation, depending upon

the isotope.

19.17 For PET, the subject is injected with a molecule containing a label

with an isotope that will emit positrons.

19.18 The positron is not stable and will combine with an electron to

produce energy that has value equal to twice the electron mass,

2 mec^2 , or twice 511 meV. The energy takes the form of two

photons that, to conserve momentum, are directed along equal

and opposite directions from the point where the positron and

electron came together, with each photon having an energy of

511 meV.

19.19The probe 2-[^18 F]fluoro-2-deoxy-D-glucose is used to locate glycolysis

processes in the body.

Ef ciency of transfer 1 =

+

r

rrDA

0

6

0

66

ANSWERS TO PROBLEMS 485

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