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Images reproduced with permission from Phillip Gentry, University of Alabama in Huntsville

First look at a crystal's "embryo"—flat, not round


August 3, 2000: The first pictures of individual molecules forming the nucleus of a crystal have been taken by scientists at The University of Alabama in Huntsville (UAH). The unexpected shapes found in these embryonic crystals may reshape the scientific theories used to explain and predict the behavior of materials in settings as varied as storm clouds, medical research labs and heavy industry, says Dr. Peter Vekilov, an assistant professor of chemistry at UAH and a researcher in UAH's Center for Microgravity and Materials Research.

"Nucleation -- the formation of these crystal nuclei -- is an area in which observation and theoretical predictions may vary by 20 orders of magnitude," Vekilov said. "Until now, nobody has seen how the molecules got together."

Instead of the compact geometric shapes assumed by existing theories, Vekilov and Dr. S.T. Yau found the crystal nuclei forming "quasi-planar" sheets.

"The shape we see is very unexpected," Vekilov said. "It's a sheet, a flat thing. We can see clusters of two, four and six molecules. If you have about 16 molecules, they're already arranged like they will be in the crystal, in a sheet that looks as if it could have been chiseled off the crystal.

"When the size of the sheet is maybe six by six molecules, then the second layer starts to build up. When you get to maybe 200 molecules, it looks like the large crystal will look."

Findings from this research are reported in today's edition of the scientific journal Nature (3 August 2000, Volume 406, 494–497).

The formation of planar sheets rather than three-dimensional geometric forms may have significant implications in a wide range of fields, said Vekilov, because it takes more energy for molecules to form planar sheets rather than shapes such as spheres.

"For atmospheric science, for instance, you have to calculate what water vapor is going to do," he said. "It takes a certain amount of energy for water vapor to form rain droplets, and a certain amount of energy to form snow flakes.

"Now we have to think that the kinetics are completely different from what we had expected. To predict these processes, you want to know how many of these nuclei you will have in a certain volume over a certain period of time. The rate of nucleation depends on the shape of the nucleus. And now you should for no process expect the nuclei to be compact and spherical.

"We don't understand why this is happening, but it seems that it is happening. It makes things more complicated."

Vekilov and Yau developed new techniques that allowed them to use an atomic force microscope at UAH to look at the earliest stages of crystal formation. They had to overcome several obstacles. In addition to their microscopic dimensions, these nuclei are also floating freely in solution. That makes it difficult to pinpoint where nuclei are forming.

"We chose a protein which is very convenient, because the molecules are very large, about 13 nanometers in diameter," Vekilov said. (By comparison, a water molecule is about 0.1 nanometers in diameter.) Ferritin, the protein the UAH researchers used, is found in living organisms.

While it stores and transports iron, ferritin also wraps iron in a protein shell which protects the organism from the damage that can be caused by free iron. The UAH team used the apoferritin, or iron depleted, form of the protein. The large spherical apoferritin molecules move slowly in solution, slowly forming organic crystals.

To overcome the challenge of finding the free-floating nuclei, the team put about 50 microliters of solution (perhaps one-tenth of a raindrop) in an atomic force microscope (AFM) cell. The bottom of the AFM cell is scanned with a resultion of about 1.5 nanometers.

"The major difficulty was being able to see these nuclei and to convince ourselves that these are the nuclei and not random agglomorations of molecules," said Vekilov.

The first images of crystal nuclei were obtained in experiments conducted at UAH during the spring and summer of 1999.

Phillip Gentry, gentryp@email.uah.edu



Links

The Center for Microgravity and Materials Research explains protein crystallization
http://www.cmmr.uah.edu/protein/

A short introduction to Atomic Force Microscopy
http://pace.leeds.ac.uk/labs/afm.htm

Protein crystal picture gallery from the Lawrence Livermore National Laboratory
http://www-structure.llnl.gov/crystal_lab/Crys_lab.html

The Molecular Expressions photo galleries from Florida State University
http://micro.magnet.fsu.edu/galleria/index.html



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