BioPHYSICAL chemistry

The bremsstrahlung radiation is 100 –1000 times more intense than what

is available in the laboratory, with significant improvements planned for

the future.

The structures of proteins are solved using single-crystal X-ray diffrac-

tion techniques. In this technique the intensity of every diffraction point

should be measured. The crystal is positioned between the X-ray source

and a detector (Figure 15.19). Usually, the protein crystal is frozen to

minimize damage from the beam and cooled constantly by a stream of

nitrogen gas. The electronic detector is designed to simultaneously meas-

ure the diffraction over a large area. Once measured, the diffraction is

digitized and displayed on a computer monitor. With a few measurements,

the space group is identified and orientation of the crystal is determined.

The crystal is then rotated in increments with the diffraction measured

for each angle. Electronic detectors are used, allowing the intensity of each

spot to be integrated as the data for the next angle are measured so that,

at the end of data collection, a full data-set is ready for analysis. For a

laboratory source, a full data-set can be measured in 1 or 2 days while

only an hour would be needed for the same crystal at a synchrotron. More

importantly, the much greater intensity of the synchrotron source allows

the accurate measurement of much smaller crystals than is possible using

the laboratory source.

334 PART 2 QUANTUM MECHANICS AND SPECTROSCOPY

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High voltage transformer

Copper

anode

Shutter

Mirrors or

Diffracted monochromator

x-rays

X-ray beam

Beam

stop

Crystal frozen

in loop

Goniometer

Rotation

axis

Nitrogen dewar or

closed nitrogen system

Imaging plate or

CCD detector

Signal

conversion

Cooling flow of

nitrogen gas

Figure 15.19A scheme for the measurement of X-ray diffraction data.