November 1, 2005

NSLS 2005 Annual Users’ Meeting Workshop

Application of SAXS to Biological Structures

The development new synchrotron sources has enabled challenging small-angle x-ray scattering (SAXS) experiments that require extremely small, yet intense x-ray beams. An example is time-resolved scattering from proteins undergoing folding at millisecond or better time resolution. However, the emergence of these new sources does not make SAXS instruments that are based on older second-generation sources, such as the NSLS, obsolete. Instead, these instruments continue to contribute to biology and biomaterials research. This workshop was intended to show the user community how SAXS can be used to facilitate structural studies of biological systems with examples of studies achievable at second-generation synchrotron sources.

The morning session of this workshop focused on solution scattering from proteins and protein complexes. Dmitri Svergun (European Molecular Biology Laboratory) described applications of the low-resolution structural modeling programs developed in his group. These data analysis methods are based on static measurements that do not require very high source brightness. He gave examples for a broad range of sizes, from individual macromolecules to multi-domain proteins and large macromolecular assemblies. Structural modelling of molecular complexes is particularly effective when combined with high-resolution structures of the constituting subunits.

The scattering at higher angles corresponds to structures at smaller length scales. Lee Makowski (Argonne National Laboratory) showed that scattering within this region contains information on the secondary structures and folding motif of the protein. Wide-angle x-ray scattering (WAXS) data can therefore be utilized to quantify the structural difference between protein structures as a distance in the WAXS space. Proteins known to have similar structural motifs have the shortest distances. This method, therefore, may be a sensitive, global method for detecting structural changes in proteins, narrowly categorizing proteins based on their scattering homology to known folds and elucidating the differences between crystal structures and aqueous conformations.

Scattering from protein solutions is not only capable of charactering the structure of proteins, but also the interaction potential between them. This capability is employed by Annette Tardieu (Centre Nationale de la Recherche Scientifique) to study the conditions that are optimal to obtain protein crystals. The major result from her research is that attraction between protein molecules may be tuned with salt and/or with PEG. The optimal condition depends upon the macromolecular size: With small compact proteins the Hofmeister effect may be sufficient to induce an attractive regime and crystallization, whereas the presence of PEG is required with higher molecular weight complexes.

The last speaker of the morning session was Jack Johnson (The Scripps Research Institute), who studied the maturation of HK97 bacteriophage capsid. This dynamical process proceeds at a moderate speed (on the order of seconds) and was monitored with time-resolved SAXS. This study is a great example of how protein crystallography, electron microscopy, small-angle scattering, and fluorescence complement each other to provide a more complete picture of the biological process that may not be elucidated by any of these techniques alone.

The afternoon session moved on to the application of SAXS to more complex structures in biological tissues. Due to the presence of various periodic structures, scattering data from tissues often contain diffraction peaks and there is no uniform method of analyzing the data. Ben Hsiao (Stony Brook University) modeled the shape and position of small-angle diffraction peaks from fish bones. His data analysis quantified the diameter of collagen fibrils, the orientation distribution of fibrils, the coherence length, as well as the mineral (calcium phosphate) dimensions and orientation in the bones. Myosins and actin fibers in muscle also produce diffraction peaks. Leepo Yu (National Institutes of Health) studied the structural change in muscles as the chemical-to-mechanical energy conversion that drives muscle movement takes place. In search for materials for replacing defective heart valves, Jun Liao (University of Pittsburgh) studied candidate tissues under biaxial stretch either with constant force or constant displacement. The response of the tissue gives a good indication of its mechanical integrity.

The workshop also included a brief tour of SAXS beamlines X27C and the newly renovated wiggler-based X21 beamline, and ended with a discussion session during which a number of NSLS SAXS users presented their results.

FOR MORE INFORMATION
Lin Yang
National Synchrotron Light Source
Brookhaven National Laboratory
Upton, NY Email: lyang@bnl.gov