During evolution, proteins gradually accumulate large numbers of amino acid changes in their sequences. In distantly related proteins, therefore, it is difficult to deduce their evolutionary relationships by sequence comparisons. In such cases, the relationships may be uncovered by comparisons of three-dimensional protein structures, because during evolution the three-dimensional fold of polypeptide chain is better conserved than the constituting amino acids. The change-rates of amino acide residues depend on their locations in the three-dimensional structure; since the internal residues may be more important for protein stability than external ones, the amino acids change more easily on the surface than in the interior of protein molecule. Apart from the divergent evolution, there are two convergent evolutions, convergence to common function and convergence to similar structure. General rules for protein evolution obtained from currently available data are described here. [J. Cryst. Soc. Jpn, 28, 200 (1986) ] .
The effect of surface steps on RHEED is described on the basis of calculated intensities based on multiple scattering theory from periodic arrays of steps. The results show that multiple scattering theory is necessary to interpret intensities even qualitatively. For example, 1) the intensities are quite different from those by single scattering theory, and 2) the intensities strongly depend on the incident beam azimuth to the step edges. In application, the intensity variations from stepped surfaces as a function of the number of atoms in the topmost layer are used to analyse RHEED intensity oscillations during molecular beam epitaxy.
Application of X-ray photoelectron diffraction (XPD) to the study of the structures of submonolayer semiconductor interfaces is described. A simple kinematical formalizm is basically valid for the analysis of XPD data. This is demonstrated for the Ge (111) lattice. Determination of adsorbate geometries of Ge (111) √3 × √3-Sn and Si (111) √3 × √3 -Ag surfaces is explained as examples of XPD analysis. A brief description of future trend and prospect of photoelectron diffraction in general is made. [J. Cryst. Soc. Jpn. 28, 218 (1986) ] .