BioPHYSICAL chemistry

surface of the structure is Glu-134. The mutation Glu to Lys at 134

had been found previously in a patient with spinal muscular atrophy

and the structure of the protein with this mutation was also studied.

The structure with this mutation was found to not disrupt the wild-

type structure but replacement of the negatively charged Glu with the

positively charged Lys alters the charge distribution on the surface.

This suggests that the SMN protein interacts with other proteins

through electrostatic interactions involving surface residues.

MRI

The technical developments resulting from NMR have proven to be

useful for medical applications. For both NMR and MRI, information

is obtained about the presence of nuclear spins. In the NMR experi-

ment, a very homogeneous external magnetic field is applied and the

response of the nuclear spins to radiofrequency pulses is measured.

Normally, one measures a molecule in which the resonance energy

is slightly shifted for different spins of the molecule, due to the

features near each spin (such as electron density), which slightly alter

the local magnetic field as measured by the chemical shift. MRI makes

use of the same fundamental ideas to map out the composition of whole

organisms.

First, consider a case where a homogene-

ous sample, water in a small glass tube, is

placed inside the magnetic field in an NMR

spectrometer (Figure 16.15). Because the

sample is homogeneous, the protons respond

equally to the magnetic field and there is a

single resonant peak in the NMR spectrum.

Second, consider a situation where two glass

tubes are placed side by side in the spec-

trometer. Normally, this is not done as the

systems are designed for only one sample,

but it can be performed as a gedenken (or

thought) experiment. All of the spins in the

two tubes experience the same magnetic

field, so the NMR spectrum remains a single

peak. Third, keep the same two tubes of

water in the spectrometer but now replace

the uniform magnetic field with a field that

slowly increases with increasing distance

from the left pole face of the magnet. In

this case, the spins in each tube experience

the same environment but not the same

360 PART 2 QUANTUM MECHANICS AND SPECTROSCOPY

Figure 16.14The
three-dimensional
structure of the Tudor
domain of SMN showing
the extensive βsheet.

B0 B1 Frequency proportional to B

Uniform B

Uniform B

Gradient B Two peaks

One peak

One peak

Figure 16.15Expected spectra of capillaries
containing water for different numbers of
capillaries and magnetic-field configurations.