S. Chang et al., Chemical bonding effects in the determination of protein structures by electron crystallography, ACT CRYST A, 55(2), 1999, pp. 305-313
Scattering of electrons is affected by the distribution of valence electron
s that participate in chemical bonding and thus change the electrostatic sh
ielding of the nucleus. This effect is particularly significant for low-ang
le scattering. Thus, while chemical bonding effects are difficult to measur
e with small-unit cell materials, they can be substantial in the study of p
roteins by electron crystallography. This work investigates the magnitude o
f chemical bonding effects for a representative collection of protein fragm
ents and a model ligand for nucleotide-binding proteins within the resoluti
on range generally used in determining protein structures by electron cryst
allography. Electrostatic potentials were calculated by ab initio methods f
or both the test molecules and for superpositions of their free atoms. Diff
erences in scattering amplitudes can be well over 10% in the resolution ran
ge below 5 Angstrom and are especially large in the case of ionized side ch
ains and ligands. We conclude that the use of molecule-based scattering fac
tors can provide a much more accurate representation of the low-resolution
data obtained in electron crystallographic studies. The comparison of neutr
al and ionic structure factors at resolutions below 5 Angstrom, can also pr
ovide a sensitive determination of charge states, important for biological
function, that is not accessible from X-ray crystallographic measurements.