There is a very basic chemical process called crystallization that even schoolchildren can witness with their own eyes. But scientists had not, until now, been able to observe this process on the molecular level – that is, the instant in which molecules overcome their tendencies to float individually in a liquid solution and take their place in the rigid lattice of a solid crystal structure.
Researchers at the Weizmann Institute of Science in Rehovot have, for the first time, directly observed the process of crystallization on the molecular level, validating some recent theories about crystallization, as well as showing that if one knows how the crystal starts growing, one can predict the end structure.
Prof. Ronny Neumann of the organic chemistry department explained that to bond to one another, the molecules must overcome an energy barrier.
“The prevalent theory had been that chance contact between molecules leads to bonding, eventually creating small clusters that become nuclei for larger crystals to grow. But the molecules, which move randomly in solution, must be aligned properly to crystallize. In recent years researchers have begun to think that this process might present too high an energy barrier.”
Theories proposed in the past few decades suggest that if the molecules were to congregate together in a so-called dense phase, in which they aggregate into a sardine-like state – close together but unorganized – and then crystallize from this state, the energy barrier would be lower.
To test the theories, Neumann and doctoral student Roy Schreiber created large, rigid molecules and froze them in place in solution. They then placed the frozen solution under an electron microscope beam that warmed up the mixture just enough to allow some movement – and thus interaction between the molecules.
Adjusting the makeup of the solution by adding different ions enabled the scientists to produce crystallization with and without dense phases; for the first time, the team was able to observe dense phases forming and subsequently transforming into crystal nuclei.
While both states yielded crystals, the experimental results showed that when dense phases form, the energy barrier to formation of an orderly, crystalline arrangement of molecules is, as the theory predicted, lower.
The scientists also found that the growth arising from dense phases results in larger, more stable crystal nuclei.
In addition, they discovered that the arrangement of molecules in fully grown crystals, which they determined by X-ray crystallography – with the aid of Dr. Gregory Leitus of the chemical research support unit – was in good agreement with that in the small clusters of just a few molecules in the original nuclei.
“This means that the forces and factors that determine the process are constant throughout the growth of the crystal,” said Neumann.
“We have really observed an elementary event in the world of chemistry,” noted Neumann. “The findings are also leading us into new inquiries in this area, looking at the effects and significance of dense phases on chemical reactivity.”
WHY BIG BRAINS ARE RARE
Studies of electric fish support the idea that really big brains can evolve only if constraints on energy intake are eliminated. As a species, we’re so proud of our brains that it doesn’t occur to most of us to ask whether a big brain has disadvantages as well as cognitive benefits.
“We can think of lots of benefits to a larger brain, but the other side of that is brain tissue is incredibly ‘expensive,’ and increasing brain size comes at a heavy cost,” said Kimberley Sukhum, a graduate student in biology in arts & sciences at Washington University in St. Louis.
So evolving a large brain requires either a decrease in other demands for energy or an increase in overall energy consumption, said biology Prof. Bruce Carlson, Sukhum’s advisor. Previous studies in primates, frogs and toads, birds and fish found support for both hypotheses, leaving the evolutionary path to a larger brain unclear.
Carlson’s lab studies mormyrid electric fishes from Africa, which use weak electric discharges to locate prey and to communicate with one another. These have a reputation as large-brained fish, and one has a brain that constitutes 3% of its body size, comparable to human brains, which range from 2% to 2.5%. But it was unclear whether other mormyrids were equally brainy.
Examining 30 out of the more than 200 species in the mormyrid family, the scientists discovered that they have a wide variety of brain sizes.
“We realized this meant the fish presented a great opportunity to study the metabolic costs of braininess,” Sukhum said.
Using oxygen consumption and the ability to tolerate hypoxia as proxies for energy use and energy demand, the scientists put the fish to the test. They found that the largest-brained species had the highest demand for oxygen and the smallest-brained species the lowest.
The results were published in the Proceedings of the Royal Society B.
Having a body that needs to be fed more just to exist is a risky strategy both for mormyrids and people. Carlson and Sukhum point out that the mormyrids’ ability to sense their environment by “electrolocation” helps them forage more efficiently.
People, too, are remarkably vulnerable to any interruption in the food supply because of their big-brain energy budgets. We might mitigate this risk by efficient walking or cooking and sharing food – but we also do it by storing fat. Body fat provides an important buffer against food shortfalls.
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