Over
the decades, X-ray crystallography has become fundamental in the
development of many scientific fields. The method has revealed the
structure and function of many biological molecules, including vitamins,
drugs, proteins, and nucleic acids such as DNA. However, in order to
obtain good data, large single crystals are required. These are often
nearly impossible to grow. There also is the problem that X-rays damage
delicate biological samples.
“From
the beginning, the resolution of images recorded by biologists has been
limited by damage due to the radiation used,” said physicist John C. H.
Spence, a Regents’ Professor in physics at Arizona State University.
“But what happens if a pulse of imaging radiation is used that
terminates before damage begins, yet contains sufficient photons to
generate a useful scattering pattern?”
Indeed,
results of such a method are being reported by Spence at the American
Association for the Advancement of Science annual meeting in Vancouver,
Canada. Spence presented his findings today recently during a special
session on “Imaging and Controlling Molecular Dynamics with Ultrashort
Laser Pulses.”
Many in the scientific community didn’t believe such a method could work.
“The
experiments of Henry Chapman’s (University of California, Davis) group
using lithographed structures and soft (i.e. long wavelength) X-rays had
shown that if we could ‘out-run’ the damage, this might indeed be a
useful path to damage-free imaging at atomic resolution,” said Spence.
“In my lab we were thinking about the data analysis, and building the
hydrated protein-beam injector device, a bit like an ink-jet printer, to
spray the molecules across an X-ray laser. This snap-shot method should
eventually allow us to make movies of molecular machines at work.”
Spence
joined forces with Chapman and many collaborators to recently
demonstrate serial snapshot femtosecond (10-15 of a second) diffraction
(SFX) from nanocrystals using the world’s first hard X-ray laser. The
photosystem I (PSI) nanocrystals came from Professor Petra Fromme’s lab
in ASU’s Department of Chemistry and Biochemistry.
“These
are early days for femtosecond diffractive imaging,” noted Spence, who
provided the theory and much of the data analysis. “But first
indications are that high-resolution data can now be obtained at the
nanoscale by this method. If we can indeed ‘outrun’ the many
radiation-damage processes in this way, it will open the way to future
experiments on laser-excited samples, 3-D image reconstruction and a
host of other experiments on fast imaging, all directed to the grand
challenge of obtaining movies showing molecules at work.”
