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August 03, 2010

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Strange crystals found to twist as they grow

July 18, 2010
Courtesy of the American Chemical Society
and World Science staff

Chem­ists have cre­at­ed crys­tals that can twist and un­twist, point­ing to a much more var­ied pro­cess of crys­tal growth than was pre­vi­ously known, they say.

Their work, pub­lished in the lat­est is­sue of the Jour­nal of the Amer­i­can Chem­i­cal So­ci­e­ty, may ex­plain some of the prop­er­ties of high-polymers, used in prod­ucts in­clud­ing cloth­ing and liq­uid crys­tal dis­plays.

Crys­tal growth has tra­di­tion­ally been viewed as a col­lec­tion of in­di­vid­ual atoms, molecules, or small clus­ters adding to a larg­er block that re­mains in a fixed spa­tial rela­t­ion­ship to the rest. But chem­ists from New York Uni­vers­ity and Rus­si­a’s St. Pe­ters­burg State Uni­vers­ity found a crys­tal that con­tin­u­ally changes its shape as it grows.

“This dy­nam­ic has not been ob­served be­fore and points to a much more ac­tive pro­cess of crys­tal growth than we had an­ti­cipat­ed,” said NYU chem­ist Bart Kahr.

The re­search­ers fo­cused on crys­tals from hip­puric ac­id—a de­riv­a­tive of the ami­no ac­id gly­cine, one of the com­po­nents of pro­tein mo­le­cules found in the body. 

As mo­le­cules were added to the end of fi­ne crys­talline nee­dles, stresses built up at the tips of the crys­tals and re­sulted in a hel­i­cal twist, much like DNA’s dou­ble he­lix, the in­ves­ti­ga­tors found. The pro­cess was re­versed when crys­tals thick­ened from the op­po­site end of the grow­ing tip—that is, the crys­tals stiff­ened, there­by un­do­ing the twisted forma­t­ions. This is be­cause the elas­ticity of the crys­tals de­creases as they be­come thicker, thus “squeez­ing out” the de­forma­t­ions that were in­duced at the grow­ing tip, the sci­en­tists said.

“This com­pe­ti­tion be­tween twist­ing and un­twist­ing cre­ates nee­dles with a rain­bow of col­ors, which is a char­ac­ter­is­tic of tightly wound hel­i­ces, as well as rib­bons that have be­come com­pletely un­twist­ed,” said Kahr. “This is a very strange and new per­spec­tive on crys­tal growth.”

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Chemists have created crystals that can twist and untwist, pointing to a much more varied process of crystal growth than previously thought, they say. Their work, published in the latest issue of the Journal of the American Chemical Society, may explain some of the properties of high-polymers, used in products including clothing and liquid crystal displays. Crystal growth has traditionally been viewed as a collection of individual atoms, molecules, or small clusters adding to a larger block that remains in a fixed spatial relationship to the rest. But chemists from New York University and Russia’s St. Petersburg State University found a crystal that continually changes its shape as it grows. “This dynamic has not been observed before and points to a much more active process of crystal growth than we had anticipated,” said NYU chemist Bart Kahr. The researchers focused on crystals from hippuric acid—a derivative of the amino acid glycine, one of the components of protein molecules found in the body. As molecules were added to the end of fine crystalline needles, stresses built up at the tips of the crystals and resulted in a helical twist, much like DNA’s double helix, the investigators found. The process was reversed when crystals thickened from the opposite end of the growing tip—that is, the crystals stiffened, thereby undoing the twisted formations. This is because the elasticity of the crystals decreases as they become thicker, thus “squeezing out” the deformations that were induced at the growing tip, the scientists said. “This competition between twisting and untwisting creates needles with a rainbow of colors, which is a characteristic of tightly wound helices, as well as ribbons that have become completely untwisted,” said Kahr. “This is a very strange and new perspective on crystal growth.”