Fluorescence anisotropy decay studies upon hemoglobin A and its subunits

Fluorescent conjugates of hemoglobin A, its isolated β‐chain, and the apo‐derivative of the β‐chain have been prepared in which the β‐93 sulfhydryl was conjugated with 1,5‐AEDANS. Radiationless enery transfer to the heme group results in a major decrease in fluorescence intensity and decay time. Measurements of the time decay of fluorescence anisotropy, employing single‐photon counting, indicate that the apparent rotational correlation time is, in each case, substantially reduced from the value expected for a rigid molecule of the same molecular weight. This observation raises the possibility that internal degrees of rotational freedom exist.