PHYSICS · Multi-part · Fermi University Physics · Mathematical Methods for Physics · Biophysics & Medical Physics · Macromolecules, membranes and cell architecture
Forces, Machines and Signals, Ch Seeing molecules: diffraction, cryo-electron microscopy and fluorescence
Problem
Haemoglobin's T and R states, first read from Perutz's crystal structures as the physical realisation of the MWC allosteric model, differ only slightly in the arrangement of their four subunits.
((a)) Suppose a modern worker instead tries to distinguish the T and R conformations by single-particle cryo-electron microscopy. A single aligned image of the tetramer has signal-to-noise ratio 0.03; distinguishing the two conformations reliably requires a signal-to-noise ratio of 15. Using the counting-statistics rule that random, independent measurements combine so that a relative fluctuation falls as 1/N for N copies (the rule used for copy-number noise), estimate the number of particle images N that must be averaged.
((b)) Perutz's actual 1960s phasing of haemoglobin could not have used molecular replacement. Give the reason, and name the method he used instead.