On 'globbicity' of low-resolution protein structures

Using Harker's [Harker (1953). Acta Cryst. 6, 731-736] idea of spherically averaged polyatomic groups or 'globs' as the units of structure suitable fo r analyzing low-resolution diffraction data from protein crystals, 'globbic ' scattering factors have been calculated for main-chain peptide units and amino-acid side-chain groups to 3 Angstrom resolution via Debye's [Debye (1 915). Ann. Phys. (Leipzig), 46, 809-823] scattering formula. It is shown th at the scattering factors are insensitive to intra-globbic conformational v ariation and can be approximated fairly well by a single-Gaussian formula, i.e. f(g)(s) = Z(g) exp(-1.7Z(g)s(2)), where s = (sin theta)/lambda and Z(g ) is the total electron count for the atoms of the glob, Phase errors due t o the globbic approximation and their effect on electron-density maps at 3. 5 Angstrom resolution have been assessed via calculations for the crambin s tructure; this analysis indicates that the globbic scattering factors will be useful in efforts to develop procedures for direct-methods phasing of di ffraction data to similar to 3.5 Angstrom resolution from protein crystals.