3. INTRODUCTION
X-RAY CRYSTALLOGRAPHY IS A POWER TECHNIQUE FOR VISUALIZING THE
STRUCTURE OF PROTEIN.
IT IS A TOOL USED FOR IDENTIFYING ATOMIC AND MOLECULAR STRUCTURE OF A
CRYSTAL.
IN X-RAY CRYSTALLOGRAPHY THE CRYSTAL CAUSES BEAM OF INCIDENT X-RAY
TO DIFFRACT INTO MANY SPECIFIC DIRECTIONS.
CRYSTALLOGRAPHER CAN PRODUCE A 3D PICTURE OF THE DENSITY OF
ELECTRONS WITHIN THE CRYSTAL.
FROM THE DENSITY THE MEAN POSITION OF THE ATOMS IN THE CRYSTAL CAN BE
DETERMINED.
X-RAY CRYSTALLOGRAPHY CAN LOCATE EVERY ATOM IN A ZEOLITE, AN
ALUMINOSILICATE. 3
4. X-RAY DIFFRACTION
• X-ray crystallography uses uniformity of light diffraction of crystals
to determine the structure of molecule or atom.
• X-ray beam is used to hit the crystallised molecule.
• Electron surrounding the molecule diffract as the x-rays hit them.
• This forms a pattern. This type of pattern is known as x-ray
diffraction pattern.
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5. BRAGG’S LAW
In 1913, W.L. Bragg and W.H. Bragg worked out a mathematical
relation to determine interatomic distance from X-ray
diffraction patterns.
This relation is called Bragg’s equation and is given by,
2dsin θ=nλ
Here, d is spacing between the diffracting planes, θ is incident
angle, n is any integar and λ is wavelength of beam.
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6. This equation showed that
i. X-ray diffracted from atoms in crystal planes obey laws of
reflection.
ii. Two rays reflected by successive planes will be in phase if
extra distance travelled by second ray is integral number of
wavelengths.
Bragg’s equation is used chiefly for determination of spacing
between crystal planes.
Interplanar distance can be calculated with the help of Bragg’s
equation.
For X-rays of specific λ, angle θ can be measured with the help
of Bragg X-ray spectrometer.
• Space between diffracting planes of atoms
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8. SOURCE
Two major sources used to obtain x-rays are as follows:
1. X-RAY TUBE-
It is also called as coolidge tube and is most commonly used source.
It consists of evacuated tube made of glass, fitted with a cathode and an
anode.
Cathode is tungsten filament heated with high voltage.
Anode is heavy block of copper coated with target material(which emits
x-rays) eg tungsten, copper, iron, cobalt etc.
Cathode in form of heated tungsten filament emits electrons which are
accelerasted towards anode. Accelerated electrons hit the metallic target.
X-rays are emitted.
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9. ADVANTAGES:
Electron bombardment on some material produces continuous spectrum
in the x-ray region.
Disadvantages:
• Hardly 5% electric power is converted to radiant power.
• May cause unnecessary heating of anode.
• X-rays produced by high energy electrons on a material have poor output
efficiency.
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10. 2. Using radioisotopes-
• Certain radioactive isotopes produce X-rays as a result of their radioactive
decay process and act as a source.
• Elements like cobalt, iron produce x-rays by electron capture.
• Tritium, Lead produce X-rays by beta emission process.
WAVELENGTH SELECTOR
1. Filters-
• Instruments which make use of filters are called X-ray photometers.
• They are made of specific materials of definite thickness or are thin
metallic strips.
• A number of filters can be used against specific targets.
2. Monochromators-
• Those using monochromators are called spectrophotometers.
• Monochromators are nothing but the crystals that can diffract x-rays
according to their wavelengths.
• Works on the principle of Bragg’s equation.
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11. SAMPLE HOLDER
• Sample holder is nothing but a rotating table called crystal mount.
• A sample i.e. a crystal is placed at centre of a crystal mount, which is kept
rotating at a particular speed.
X-RAY DETECTOR
1. Gas filled detectors-
• Consists of metal tube filled with an Inert gas such
as argon, kypton or Xenon.
• Tube has two transparent windows on opposite sides
and a centrally placed anode in the form of wire.
• Lower side of tube is negatively charged and acts
as cathode.
• Interaction of x-rays with atoms of inert gas results in
their excitation and the loss of one of the outer electrons.
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12. 2. Scintillation counters-
• Scintillation counters consists of transparent cylindrical crystal of sodium
iodide, 3-4 inches in length and breadth.
• when X rays strike the crystal, the electron in the crystal are excited.
• When the electrons come back to their original position, they give photons
and produce scintillations on the fluorescent materials.
• Large no. of electros are produced in the photo multiplier tube which result
in large no. of photons and are collected at one electrode.
• Amount of energy released is called as scintillation energy, which is
converted to electric energy to get equivalent current, the value of which
depends on intensity of X-rays striking the surface.
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13. X- RAY DIFFRACTION METHODS
1.Single crystal x-ray diffraction-
Step 1:
• Obtain an adequate crystal of material under study crystal should be larger
than 0.1 mm in all dimensions, pure in composition and regular in structure.
Step 2:
• Crystal is placed in intense beam of x-rays usually a monochromatic x-ray,
producing regular pattern of reflections.
• Angles and intensities of diffracted x-rays are measured.
• As the crystal rotates, previous reflections disappear and new ones appear.
• Intensity of every spot is recorded at every orientation of crystal.
• Multiple data sets are collected.
Step 3:
• These data are combined to produce a model of the arrangement of atoms
within the crystals.
• The final model of atomic rearrangement is called a crystal structure and is stored in
public database.
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14. 2. Powder diffraction method
• Powder diffraction method is the only analytical method which is capable of furnishing
both qualitative and quantitative information about the compound present in a solid
sample.
• Powdered sample contains small crystals arranged in all orientations some of these will
reflect x-ray.
• Reflected x-rays will make and angle 2θ with the original direction.
• Hence on the photo obtained lines are of constant θ.
• From geometry of the camera, θ can be calculated.
Applications
Identification of unknown crystalline materials.
Eg. Minerals
Material science
Environmental science
Measurement of sample purity
Determination of unit cell dimensions
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15. 3. Rotating crystal
A beam of x-rays of known wavelength
falls on a face of crystal mounted on
graduated turn table
Diffracted rays pass into the ionization
chamber of the recorder.
Here the ionize the air and a current
flow between the chamber wall and
electrode inserted in it which is
connected to an electrometer.
The electrometer reading is proportional
to intensity of x-rays.
As a recorder along with crystal is
rotated, angles maximum intensity is
noted on the scale. 15
16. APPLICATIONS
1. HIV
• Scientists determined the x-ray crystallographic structure of HIV protease, a viral enzyme critical in
HIV life cycle.
• By blocking this enzyme, spreading of virus in the body could be prevented.
2. Dairy Science
• X-ray crystallography technique has been widely used tool for elucidation of compounds present in
milk by information obtained through structure function relationship.
3. In case of new materials
• X-ray crystallography is still the chief method of characterising the atomic structure of new
materials and in differentiating the materials that appear similar
4. Polymer characterization
• Powder method can be used to determine degree of crystallinity of polymer.
• Non-crystalline portion scatters x-ray beam whereas crystalline portion causes diffraction lines.16
17. 5. Structure of crystals
• The method is non-destructive and gives information on molecular structure of sample.
• Also measures the size of crystal planes.
• The patterns obtained are characteristics of the particular compound.
6. Miscellaneous applications
• Soil classification based on crystallinity.
• Examination of tooth enamel and dentine.
• Analysis of industrial dust.
• Study of corrosion products.
• Assessment of degradation of natural and synthetic materials.
• To investigate effects of temperature, humidity, sunlight or corrosive gases on polymers.
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