C-configuration of DNA

"The X-ray studies show that DNA molecules are remarkable in that they adopt a large number of different conformations, most of which can exist in several crystal forms (Wilkins, p139 Nobel Prize Lecture)"

One of these conformations referred to as the C-configuration was first discovered here at King’s College London by the DNA Biophysics research group. Discovered in 1958 by the then PhD student, Donald Marvin, this configuration was the first since Rosalind Franklin had identified A and B configurations in 1951. We are fortunate to have within our papers the research notes, x-ray patterns and manuscript articles charting this discovery and which will be part of the digitised papers of the Maurice Wilkins for inclusion in the Wellcome Digital Library. 

Title page of typescript showcasing new conformation of DNA

Molecules can have the same chemical structure but can be arranged with a different structure. The different three-dimensional structure affects how the molecule behaves. DNA is no exception: In the later stages of the DNA work, 1952-1953,  A and B DNA configuration showed similar properties through the respective x-ray diffraction patterns, however the B-DNA conformation always showed a clearer helical configuration. These observed variations moved the research work away from focusing on a purely helical structure.

The different conformations of the DNA molecules were due to the various crystal forms produced using various different salts of DNA. In the case of the C-configuration of DNA the conformation was obtained using a lithium DNA salt. Whilst, the B-configuration of DNA represented the DNA Double Helix that is present within living cells these new configurations were still important for the structural research undertaken on DNA as Wilkins explained in his Nobel Prize speech:

C-form DNA

‘Comparison of the forms provides further confirmation of the correctness of the structures. In a way, the problem is like trying to deduce the structure of a folding chair by observing its shadow: if the conformation of the chair is altered slightly, its structure becomes more evident.’ (Wilkins, p139)

The C-configuration has a 9 ⅓ nucleotide pair per turn compared to the B-configuration with 10 ½ nucleotides per turn. Both are a semi-crystalline structure showing clear helical packing and therefore the two are very similar compared to the crystalline A-configuration. However, whilst B-configuration is found in nature the C-configuration was an artefact formed by the drying process.

C-form DNA