. Elementary biophysics: selected topics . DNA solution heated to some temperature lower than denaturation temperature DNA solution heated above denaturation temperature to produce strand separation. This is called melted DNA Melted DNA after sudden chilling Fig. 23. A schematic representation of the partial and complete separation of DNA strands by breaking some or all of the hydrogen bonds. If the com- pletely separated strands are suddenly chilled, the strands form intra-strand bonds before any inter-strand bonds can be formed. The opposite would have been the case if the solution had been
. Elementary biophysics: selected topics . DNA solution heated to some temperature lower than denaturation temperature DNA solution heated above denaturation temperature to produce strand separation. This is called melted DNA Melted DNA after sudden chilling Fig. 23. A schematic representation of the partial and complete separation of DNA strands by breaking some or all of the hydrogen bonds. If the com- pletely separated strands are suddenly chilled, the strands form intra-strand bonds before any inter-strand bonds can be formed. The opposite would have been the case if the solution had been chilled slowly. It is worth pointing out that machines which measure light absorption —colorimeters, spectrophotometers, etc.—express the result in terms of what is called the optical density, which is defined as = log I0/Ix, since this equals Emcx and is therefore directly proportional to the con- centration of molecules which absorb, which is the quantity usually being sought experimentally. This expression has been obtained for a single wavelength; the optical density will vary with wavelength if E or Em varies with wavelength. By measuring the at various wavelengths for a given setup (keeping c and x constant), we can determine the dependence of E on wavelength. The plot of E versus wavelength is the intrinsic absorption spectrum of the molecule. The absorption and scattering of light by a substance can depend very strongly on its state of chemical combination and on the light-scattering properties of the solvent in which it is suspended. We can see this from an examination of two experimental situations which have proved of interest and significance to biology. (a) The hyperchromia effect. DNA is a long double-stranded molecule, held together by hydrogen bonds along the length of the molecule. As a solution of DNA is heated, the thermal agitation becomes great enough to break more and more of these hydrogen bonds. At any temperature, there will be equilibrium
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