Protein Crystallography with Synchrotron Radiation I. General Discussion and High Resolution Data Collection

Abstract

I will discuss in these lectures the present and possible applications of X-synchrotron radiation to protein crystallography. Basic underlying physics can be briefly summarised: when X-radiation impinges on a free electron, the electron is forced into oscillations of the same frequency as the incident wave and scatters radiation. Scattered rays have the same wavelength as the incident radiation and a definite phase relationship to the incident beam; hence, scattering is coherent. The scattering pattern of the electrons in the sample is the Fourier transform of the electron density. If the sample is a single crystal — e.g. a periodic three-dimensional array of scatterers — the Fourier transform is a weighted lattice; the discrete values of the transform are called structure factors F(hk1). Except for centric zones, structure factors are complex numbers because protein structures are not centrosymmetric. Usual detectors record the intensities of diffracted rays, which are proportional to |F(hk1)|² = F(hkl).F*(hk1). In this process, the phases of structure factors are lost; this information has to be recovered by indirect ways from the magnitudes of scattered amplitudes only. Then the data can be back Fourier transformed to obtain the electron density in the crystal; finally, from this map and from the aminoacid sequence, the tertiary structure of the protein is derived.