STATIC METHODS 79 



from the front and rear can produce appreciable, and measurable, inter- 

 ference effects whose magnitudes depend on the angle A from which the 

 scattered light is viewed. By measuring the interference for all angles of 

 viewing, it is possible to make significant deductions about the size and 

 shape of the particles even though they may be too small to be visible, 

 even in the light microscope. 



A question that has been studied by light scattering is whether heating 

 of DNA separates the two Watson-Crick strands from each other. In 

 one set of measurements it was found that the volume of the scattering 

 molecules was unaltered by heating to temperatures previously believed 

 to be high enough to separate the strands completely. If the separation 

 had occurred, the volume of the individual scattering molecules would 

 have been halved. 



Light absorption. Chemical bonds, such as C — H, C — O, C=0, etc., 

 have specific absorption regions in the infrared. Series of atoms with 

 alternating double and single bonds, such as — C=C — C=, absorb at 

 wavelengths which increase progressively with the length of the series. 

 The shorter series absorb in the ultraviolet. Among the materials which 

 absorb in that region of the spectrum, the biologically important ones 

 are proteins and nucleic acids, which contain molecules with short series 

 of these alternating bonds. 



Light of the infrared and ultraviolet regions mentioned above, when 

 absorbed by a solution of molecules, will be absorbed in proportion to 

 the concentration of the molecules. Thus instruments designed to measure 

 this light absorption, called colorimeters or spectrophotometers, can give 

 information as to concentrations, but not as to size and shape. 



X-ray absorption. In contradistinction to light absorption, the ab- 

 sorption of x-rays can be used for studies of size and shape. When an 

 x-ray photon passes through a molecule, the properties are such that 

 there is little likelihood that it will actually interact with any molecules. 

 Therefore the chance of interacting is strictly proportional to the number 

 of opportunities for interaction, and therefore to the thickness of the 

 molecule. In addition, the greater the cross-sectional area of the mole- 

 cule, the greater the probability of the photon's passing through it. 

 Therefore the total probability P of interaction to produce a biological 

 effect is proportional to both factors, i.e., to the thickness t and the cross 

 section A of the molecule: 



P a t A = V, 



where V = tA = the volume of the molecule. 



Thus, by measurement of the rate at which biological entities are af- 

 fected by x-rays, it has been possible to make an estimate of the volume 

 of the molecules involved. 



