Distance Distribution between Two Iodine Atoms Derived from Small-Angle X‑ray Scattering Interferometry for Analyzing a Conformational Ensemble of Heavy Atom-Labeled Small Molecules

To
obtain unbiased information about the dynamic conformational
ensemble of a molecule in solution, one promising approach is small-angle
X-ray scattering (SAXS). Conventionally, SAXS data are converted to
a pair distribution function, which describes the distance distribution
between all pairs of atoms within a molecule. If two strong X-ray
scatterers are introduced and the background contributions from the
other atoms are suppressed, then the distance distribution between
the two scatterers provides spatial information about a flexible molecule.
Gold nanocrystals can provide such information for distances of >50
Å. Here, we synthesized a chemical compound containing two iodine
atoms attached to the ends of a flexible polyethylene glycol chain
and used the relevant singly labeled and unlabeled compounds to suppress
the background contribution. This is a feasibility demonstration to
prove that the distance distribution in the range of 10–30
Å can be experimentally accessed by SAXS.