A SMALL-ANGLE X-RAY-SCATTERING APPARATUS FOR STUDYING BIOLOGICAL MACROMOLECULES IN SOLUTION

This paper describes the development of a simple laboratory-based smal l-angle X-ray scattering apparatus for the study of biological macromo lecules in solution. The instrument is based on a two-circular-apertur e collimation design combined with a conventional rotating-anode Cu K alpha X-ray source, a graphite monochromator and a multiwire area dete ctor. The geometry of the collimator, the beam-stop-to-detector distan ce and the thickness of the platinum foil of the defining aperture hav e been optimized to reduce background scattering. The effective Q rang e is from 0.01 to 0.33 Angstrom(-1), where Q = (4 pi sin theta)/lambda is the magnitude of the scattering vector, 2 theta is the scattering angle and lambda is the wavelength of the X-rays. The length of the co llimator, the pinhole sizes and the helium-flushed sample-to-detector path can be easily changed depending on the resolution and intensity r equirements of an experiment. The diffraction pattern of a polycrystal line pellet of ammonium sulfate mounted about 2.5 cm in front of the b eam stop and 40 cm in Gent of the detector is used to monitor changes in the incident-beam intensity as well as the differences in absorptio n of X-rays by the sample solutions and the solvents, to ensure correc t background subtractions. Data collection is controlled by a computer through a parallel DMA (direct memory access) I/O module. Data collec tion and reduction software has been developed. The typical data colle ction time is about 2 h for a 5 mg ml(-1) 10 kDa protein dissolved in an aqueous solution. Examples of applications of this small-angle X-ra y scattering instrument to studying protein size and conformation chan ges are presented.