THE STATISTICAL LABELING METHOD AND IN-SITU NEUTRON-SCATTERING AND DIFFRACTION MEASUREMENTS ON ORDERED SYSTEMS

Neutron scattering experiments, combined with selective isotopic label ing and contrast matching, allow in situ shape and distance informatio n about selected particles in a macromolecular complex to be obtained. However, the observed intensities can be distorted by intercomplex in terference and by scattering-length-density fluctuations of the (other wise) contrast-matched matrix. Two methods by which such distortion ca n be eliminated have been proposed [Hoppe (1973). J. Mel. Biol. 78, 58 1-585; Pavlov and Serdyuk (1987). J. Appl. Cryst. 20, 105-110] and eff ectively used for globular particles in solution. A third method, a st atistical labeling method, has also been proposed for measuring distan ces between two identical particles in complexes in solution [Kneale, Baldwin and Bradbury (1977). Q. Rev. Biophys. 10, 485-527]. In the pre sent work, it is shown that this method can be generalized. With relia nce in part on this generalization, it is then shown that all of the m ethods can be generalized to apply to ordered systems having any numbe r of labeled particles. For macromolecular complexes having underlying order but no net orientation, the same distance or shape information can be obtained as with systems having no underlying order. The feasib ility of measurements using the statistical labeling method on such an ordered system was demonstrated experimentally. If ordered complexes are oriented, orientational as well as distance or shape information c an be obtained about the labeled particles. Possible applications to X -ray crystallography are also discussed.