Letter from Simon Flexner, Rockefeller Institute for Medical Research to Michael Heidelberger

Flexner, the director of the Rockefeller Institute, here encouraged Heidelberger to pursue the possibility, not realized at the time, of obtaining X-ray images of crystals of hemoglobin, the red, oxygen-carrying pigment protein in the blood. (Such X-ray photographs eventually served to elucidate the three-dimensional structure of hemoglobin in 1953.) Heidelberger had perfected techniques for making large quantities of pure crystalline horse hemoglobin, with its oxygen-carrying property intact and needed for studies of the uptake and release of oxygen in the blood, while working in the biochemical laboratory of Donald D. Van Slyke at the hospital of the Rockefeller Institute in 1921/22.

Flexner also noted a new trend towards the application of chemistry in immunology, a trend which Heidelberger would help to substantially advance over the next two decades.