Wednesday, April 27, 2011
From: Newsfeed to Email Gateway <email@example.com>
Date: Tue, Apr 26, 2011 at 5:01 PM
Subject: Crystallizing molecular assemblies that don't exist (Metamodern)
Leroy Cronin's group has pioneered the development of large, atomically precise metal oxide clusters (the polyoxometalates) with an eye to applications in nanotechnology. A recent publication shows why crystal engineering is a natural complement to this work — and to the engineering of self-assembling systems in general.
Here's an excerpt from the abstract (emphasis added):
The synthetic engineering of anionic polyoxometalate (POM) clusters with predefined properties tailored to specific applications is a great challenge using routine "one-pot" POM syntheses. Under such conditions, difficulties often arise from the multitude of complex reaction pathways and self-assembly processes occurring in solution….the role of the cation facilitating the selective crystallization of a particular cluster type cannot be divorced from the reaction process since the crystallization process itself can help pull "virtual" building blocks into being….Consequently, this indicates that the process of crystallisation can have a profound effect on self-assembly at the molecular level. We therefore propose that the crystallization process itself may define the molecular structure of the cluster leading to the conundrum, which came first, the cluster or the crystal of the cluster?
There's a take-away message here for research groups working on self assembly (and likewise foldamers) based on a wide range of molecular systems:
The cooperativity of crystallization can select and stabilize structures that might be disfavored or unstable in solution, hence it may be easier to make crystalline arrays of self assembled structures than it is to make the structures themselves.
Rather than thinking in terms of two challenges in sequence — first, self assembly (or analogously, folding), then crystallization — it may be advantageous to reformulate these objectives as a single design problem leading to a one-step process that merges crystallization and assembly. Paradoxically, the combined challenge can be less than the sum of its parts — indeed, less than the challenge of the first step alone in the traditional methodology.
It goes without saying that a design-and-test cycle centered on crystalline products directly supports x-ray crystallography, the gold standard for determining structures with atomic resolution and geometric precision.
It's also well known that discrete units arrayed in porous crystals can be highly functional in themselves. The virtues of cross-linked enzyme crystals, for example, include stability and activity at elevated temperatures, in organic solvents, and under stringent industrial conditions.