CATHODE RAY TUBE IN CELLULAR ANDMOLECULAR BIOLOGY PART 2 X-RAY DIFFRACTION AND CRYSTALLOGRAPHY

CATHODE RAY TUBE THE PATHFINDER OF CELLULAR AND MOLECULAR BIOLOGY

PART 2 X-RAY DIFFRACTION AND X-RAY CRYSTALLOGRAPHY

Benjamin Franklin is credited for discovering electricity in the 1700s with his kite experiment, in which he flew a kite with a metal key tied to it during a thunderstorm. Since then, there has been a steady stream of developments, discoveries and inventions with electricity as the main character.

Wilhelm Konrad Roentgen who was a professor of physics at the Wuerzburg university, Bavaria Germany while working with a cathode ray tube in his laboratory observed a fluorescent glow of crystals on a table near his tube.

The tube Roentgen was working with consisted of a glass tube with positive and negative electrodes encapsulated in it. The air inside the tube was evacuated and when a high voltage was applied and the tube shielded with heavy black paper Roentgen still observed green colored fluorescent light generated by a material kept a few feet away from the tube. The generated rays could penetrate through human muscles and tissues and expose the nature of skeleton on a photographic paper. This was a great success in the field of medicine to see the internal

parts of human body without surgery. Professor Roentgen did not know the nature of the radiation and called the new found rays, X-RAYS. Today we know them as ROENTGEN RAYS also.

For this epoch-making serendipitous discovery Roentgen was awarded the FIRST NOBEL PRIZE IN PHYSICS in 1901

This observation is the fore runner of the modern-day cathode-ray tubes (CRT) used as television screen.

Unlike the cathode ray tube used as electron gun in electron microscope for generating x-rays the cathode rays are made to impinge on a fast-rotating copper target.

Before entering x-ray diffraction, it is better to know about the difference between diffraction and scattering.

Take a look at the diagram

When x rays interact with a single particle it scatters the incident beam uniformly in all directions.

When x-rays interact with solid material the scattered beams can add together in a few directions and reinforce each other to yield diffraction. The regularity of the material is responsible for the diffraction of the beams.

You have likely seen diffraction before in your day-to-day life. For example, if you look at CD or DVD when exposed to white light you can see it diffracted into various wavelengths of colour. The pits or grooves in the CD or DVD are the regularity of the material that causes the diffraction.

Now let us look into the scattering of sunlight

When sun light reaches the atmosphere, it is scattered in all directions by dust particles water molecules present in the atmosphere. This process is known as light scattering which results in the blue colour of the sky. The intensity of the scattered light is inversely proportional to the fourth power of the wavelength of the incident light. This can be expressed mathematically

I ƛ4

This equation is called the Rayleigh’s Scattering equation.

In simpler terms the light of a shorter wavelength is scattered much more than the light of a longer wave length. Smaller particles scatter light of shorter wave length such as blue light, hence resulting in blue sky.

Particles of larger size scatter light of longer wave length

Such as orange and red-light resulting in the orange and red colour of horizon and evening sky. If the size of the scattering particles is very large, then the scattered light appears white.

It is very appropriate to mention here the man who introduced to the scientific world the powerful area called x-ray crystallography also the pioneer and father of x-ray crystallography, Maximillian Theodor Felix von Laue a German Professor of physics who received the Nobel Prize in Physics in 1914 for his discovery of of the diffraction of x-ray by crystals.

A phenomenon known as diffraction pattern occurs when waves pass through small tightly spaced openings (slits). In 1912, Max von Laue came upon with the idea that x-rays passing through crystals might create similar patterns on a screen. That is crystal a structure would correspond to slits. Experiments confirmed von Laue’s idea. This demonstrated that x-rays could be described as waves. The method also made possible to use diffraction patterns to determine crystal structure. Laue was the first to suggest the use of crystal to act as grating for the diffraction of x-rays, showing that if a beam of x-rays passed through a crystal diffraction would take place and a pattern would be formed on a photographic plate at right angles to the direction of the rays. The pattern would mark out the symmetrical arrangements of atoms in the crystal. This was verified experimentally in 1912 by two of Lause’s students working under his direction. This success demonstrated that x-rays are electromagnetic radiation similar to light and also provided experimental proof that the atomic structure of crystals is a regularly repeating arrangement. Laue received the Nobel Prize for physics in 1914 for the discovery of the diffraction of x -rays in crystals. This enabled scientists to study the structure of crystals and hence marked the origin of solid state physics, an important field in the development of modern electronics.

Derivation of Braggs’ Law

Braggs law was discovered by father and son team William Henry Bragg (father) and William Lawrence Bragg (son). Braggs’ law is the fundamental mathematical formalization in the field of x-ray crystallography

Braggs’ Law can easily be derived by considering the conditions necessary to make the phases of the beams coincide when the incident angle equals the reflecting angle. The rays of the incident beams are always in phase and parallel up to the point at which the top beam strikes the top layer at atom Z. The second beam continues to the next layer where it is scattered by atom B. The second beam must travel the extra distance of AB+BC if the two beams are to continue travelling adjacent and parallel. This extra distance must be an integral (n) multiple of the wavelength (ƛ) for the phases of the two beams to be the same

nƛ = AB + BC

Recognizing “d” as the hypotenuse of the right-angled triangle ABZ we can use trigonometry to relate “d” and to the distance (AB + BC). The distance AB is opposite to

So, applying trigonometric ratio

Sine = opposite side/ hypotenuse

That is AB/d

So, sine@ = AB/d

AB = d sine@

Because AB=BC, the equation n= AB+BC becomes

nƛ= 2AB

As AB =d sine@

The equation nL = 2AB

Becomes nL = 2d sine@

THE FAMOUS BRAGGS’ EQUATION

For this brilliant and outstanding contribution William Henry Bragg (father) and William Lawrence Bragg (son) were awarded the Nobel Prize in physics in 1915. When he received the Nobel Prize William Lawrence Bragg was only 25 years old the youngest Nobel Laureate in science till today.

The famous works based on Braggs’ equation are Dorothy Hodgkin (Nobel Prize in 1964) for solving the atomic structure of molecules such as penicillin, insulin, cholesterol vitamin B12.various proteins and pepsin

Rosalind Franklin (Nobel Prize was not given posthumously) structure of DNA

Max Perutz and John Kendrew developed the technique of protein crystallography which uses the way that crystals of proteins cause x-rays to change direction to produce unique patterns from which their structures can be established. This technique is used worldwide to determine the structure of large molecules specially myoglobin and hemoglobin (Nobel Prize in 1962)

To interpret x-ray crystallographic pictures the two-dimensional images taken at different orientation are converted into a three-dimensional model of density of electrons appearing as dark spots in photographs using the mathematical method, FOURIER TRANSFORMATION combined with chemical data known for the sample.

K.A.BALASUBRAMNIAN

Ph.D.(IARI NEW DELHI) Ph.D.(LONDON) DIC(LONDON)