BioPHYSICAL chemistry

Experimental measurement of X-ray diffraction

X-rays can be generated in the laboratory by bombarding a metal with

high-energy electrons (Figure 15.17). As the electrons decelerate they

generate bremsstrahlung radiation, which is approximately independent

of wavelength. Superimposed upon this background are a few high-

intensity sharp peaks that arise from the collision

of the high-energy electrons with the electrons in

the inner shell of the metal. The collision expels

an electron from the inner shell and an electron

from an upper energy level drops into the vacancy.

This transition is associated with the emission of a

photon. For copper, the wavelength of this photon

is 1.54 Å for the L-to-K transition (Kαradiation).

For many protein diffraction experiments, meas-

urements are also performed using synchrotrons.

Synchrotron radiation is generated by electronsthat

circulate in a large storage ring (Figure 15.18). As

the electrons circulate in the ring they generate

bremsstrahlung radiation (Figure 15.17). Whereas

the intensity of the laboratory X-ray beam is restricted

by the efficiency of the cooling system, synchrotron

radiation is limited by the strength of the magnets

that bend the electrons to produce the radiation.

332 PART 2 QUANTUM MECHANICS AND SPECTROSCOPY

Cooling water

Metal

target

Beryllium

window

Electron

beam

X-rays

(a)

Ionization

Electron

beam

L

K

Energy

Ejected

electron

X-ray

(b)

Bremsstrahlung

Kα

Kβ

Intensity

Wavelength

Figure 15.17Experimental

production of X-rays. (a) X-rays

are generated in the laboratory

by striking a target, usually

copper, with a beam of

electrons. (b) This results in a

broad distribution of X-rays,

known as bremsstrahlung

radiation, and two sharp peaks

corresponding to Kαtransitions.