How to "see" a Molecule - Part 1

In his book, Gene Machine, Venki Ramakrishnan explains how we “see” molecules and DNA strands. Actually, that’s not entirely true. Here’s why:

“When light passes through a very narrow opening or around the edge of an object, it spreads out… in a process called diffraction… If two very small objects are close together, their images spread out and merge with each other.”

This puts a limit on resolution capabilities of any viewing instrument:

“Someone looking through a microscope would see just one large fuzzy object rather than two distinct ones.”

 

Ernst Abbe calculated that you could see two objects as separate if they were no closer than half the wavelength of the light used to look at them. Therefore, no matter what the magnification power of the microscope, one can’t see anything smaller than half the wavelength used to see, i.e., ½ of 500 nanometers (a nanometer is a billionth of a meter). And since molecules are smaller than that:

“It would be impossible to see individual molecules even with the best light microscopes.”

 

So had humans hit a dead end? Not quite. In 1912, Max von Laue hit zinc sulphide with X-rays. Instead of getting scattered all over, the X-rays were concentrated in spots. Von Laue realized what was happening:

  1.       .  Zinc sulphide must be a crystal (A crystal is a regular, repeated 3D arrangement of molecules);
  2.         X-rays were scattered by the molecules in the crystal. Sometimes, they cancelled each other out, but at others, they reinforced each other (the concentrated spots).

 

Could X-rays then be used to determine the structure of the crystal? Von Laue tried and failed. Lawrence Bragg, however, did find a way. He realized that:

“The atoms in a crystal could be thought of as different sets of planes. These planes could be in different directions and separated by different spacings.”

This combo of directions and spacing would result in the concentrated spots. But Bragg went further. He subjected the crystal to the X-ray beam. He noted the concentrated spots. Next, he rotated the crystal a bit and repeated. The concentrated spots now showed up in different places (because the directions and spacings were now different wrt the beam):

“When you have completely rotated the crystal about the beam, you will have measured all the possible spots of the beam.”

And thus Bragg arrived at the structure of the zinc sulphide crystal. But more importantly, he had found a general method to determine the structure of crystals:

“For the first time, molecules could be ‘seen’.”

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