Enhancing Electron Diffraction Structures Using Maximum Entropy

Investigating the structure of a membrane protein using electron diffraction can be very difficult, particularly because of the potential lack of accuracy and the limited amount of data collected because of limitations due to tilting.When these issues become a problem a method which can still be used for solving or enhancing the potential density map is the technique of maximum entropy, an implementation of which is included in the program MICE [1].The advantage of this method over other similar ones is its ability to incorporate prior information, such as reflection phases from electron microscopy, the molecular envelope, any known partial structure and to employ entropy optimization techniques.Incorporation of a molecular envelope into the maximum entropy calculation can be vital to impose values on the unit cell where it is most likely to find new places of density.The envelope can easily be produced using the CCP4 program suite [2], however only if a small fragment of the structure is known.To give a starting point for maximum entropy a basis set of known reflection phases is required.These are normally gained through the permutation of the observed reflections, but here they can be found either directly from electron microscopy or the most accurate phases from the Fourier Transform of the partial structure can be used.Using this information maximum entropy can be shown to be a very powerful tool even at relatively low resolutions, enhancing partially known structures and even calculating unmeasured reflections from the missing cone.