The proof of the cloning is in the eating

Abstract

Heavy-atom solution of the phase problem. (a) Geometric representation of a single diffracted X ray from a protein crystal The red circle represents the unknown phase angle (0 to 360 degrees). The radius, Fp, is determined from the square root of the X-ray intensity measurement. Location of a heavy atom diffused into the protein crystal allows calculation of the ampfitude and of the phase of its contribution to the X-ray intensity. This is represented by the vector Fh. The heavy atom changes the diffracted X-ray intensity such that the amplitude is now Fph. Drawing a second circle (black outline) of this diameter, centred at the end of Fh, it can be seen that this cuts the first circle at two points. These are the two choices of phase angle. (b) is the same diffracted X ray as in (a), but now the X rays are close to the absorption edge of the heavy atom h. An additional (anomalous) diffraction occurs which advances the phase angle of Fh and changes its amplitude (Fh + Fh ). This can be calculated as before. Now a third (thicker) circle can be drawn with radius Fph and this determines the correct choice of phase. In MAD, the heavy atom h is already part of the structure, so that additional wavelengths are used to provide the information obtained in (a) by the isomorphous derivative. incorporated into soluble CD4 protein in Chinese hamster ovary cells 4, higher eukaryotic cells are more sensitive to the unnatural amino acid. However, it is clear that if a protein is already cloned and expressed in E. coli, going MAD might just get you that first ascent.