X-ray microprobe unveils biostructures.

Proteins play a crucial role in binding a sperm to an egg to begin fertilization. The absence of these proteins in defective sperm may contribute to infertility.

Now, researchers have developed a new form of X-ray microscopy that can locate and map proteins in and on sperm cells. The technique can also probe a variety of biological structures to determine the distribution of DNA and other molecular constituents.

The method involves passing intense X rays, generated by a particle accelerator (SN: 5/4/96, p. 276), through a system of mirrors and lenses that focuses the rays into an extremely narrow beam. Developed by physicist Janos Kirz and his coworkers at the State University of New York at Stony Brook, the system generates a shaft of X rays only 50 nanometers wide at a wavelength between 2.3 and 4.4 nm.

"We think this is the smallest beam of focused electromagnetic radiation of any wavelength [anyone has yet produced]," Kirz says. He described the technique this week at an American Physical Society meeting in Kansas City, Mo.

By tuning the X-ray wavelength, it's possible to map specific molecules. With scientists from the Lawrence Livermore (Calif.) National Laboratory, Kirz and his team used an X-ray microbeam to determine the distribution of protein and DNA in bull, mouse, stallion, and hamster sperm. Though the individual proteins vary from species to species, their arrangement proved remarkably similar.

The researchers hope to develop an X-ray microscope that will enable them to image cellular structures in three dimensions without having to stain or thin-slice samples. Such a tomographic technique would require taking a large number of images, which would normally damage the sample. Recent experiments, however, show that flash-freezing to liquid-nitrogen temperatures lessens dramatically the damage inflicted by X rays, opening the way for developing nanotomography.