Above what relative humidity did Franklin find DNA fibres became long and thin? Above 75% relative humidity.
At King’s College London, Rosalind Franklin and Raymond Gosling studied DNA fibres with X-ray diffraction while carefully controlling the water content in the experimental chamber. Franklin found that humidity produced two recognizable forms: a shorter, more compact form in drier conditions and a long, thin form once the relative humidity rose above about 75%.
This distinction became known as the A and B forms of DNA. The drier A form was more crystalline and produced sharper, more detailed diffraction patterns. The wetter B form was less ordered but better represented DNA under physiological conditions and produced the famous cross-shaped diffraction pattern associated with a helix.
A common mix-up is to choose 90%, because B-DNA is often produced most effectively near that humidity. However, 75% is the important transition threshold in Franklin’s observation, not the optimal humidity for every B-form experiment. Photo 51, taken in 1952, showed the hydrated B form.