BioPHYSICAL chemistry

Improvements in the resolution of the result-

ing images have led to their application as a tool

for studying physiological responses to certain

stimuli. Functional MRI, or fMRI, can be used to

study changes during cognitive tasks. For example,

the visual and task response needed to sort cards

is associated with transient activation in regions of

the prefrontal cortex that is different in monkeys

and humans (Figure 16.16). These imaging tech-

niques provide a new and promising aid in the

development of therapeutics targeted towards

specific cells.

Electron spin resonance

Electrons have a spin of 1/2 and so have magnetic

dipole moments that can interact with electro-

magnetic radiation. Since electrons normally pair

up, the net electron spin of a molecule is usually zero. Thus, most mater-

ials do not have EPR signals. Unpaired electrons occur when radicals

are present or when metals are present. Many proteins have radicals and

metals that are located at their active site: EPR is an excellent probe of

the characteristics of the active site. The dipole moment is proportional

to the spin according to:

(16.11)

where geis the g factorfor a free electron and is equal to 2.0023. The pro-

portionality factor has a very similar appearance as the dipole moment for

the nuclear spin (eqn 16.12), except that the charge contributes a negative

sign and the mass is now the electron mass. The projection of the electron

spin along the z direction is quantized so the z projection of the magnetic

dipole moment can be written as:

(16.12)

The parameter μBis called theBohr magnetonand has the same form as the

nuclear magneton but is substantially larger by the ratio of the masses:

(16.13)

μ

μ

B

N

e

p

p

e

em

em

m

m

()( )

()( )

===

Z

Z

/

/

2

2

1786

μμμz e

e

seBs B

e

ge

m

mgm

e

m

=− =− =

22

Z1

Z

de ning

GP=−

ge

m

e

(^2) e

362 PART 2 QUANTUM MECHANICS AND SPECTROSCOPY

Figure 16.16Comparison of a shift-activated
response in prefrontal cortex in monkeys
(upper panel) and humans (lower panel).
From Nakahara et al. (2002).