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Materials

Humidity controls supramolecular self-assembly

Trace water that gets into organic solvents can participate actively when molecules start to self-assemble

by Stu Borman
May 31, 2018 | APPEARED IN VOLUME 96, ISSUE 23

 

The weather can affect picnics and trips to the beach, but supramolecular chemists haven’t usually blamed it for influencing lab experiments. Now, scientists report that humidity can contribute trace levels of water to organic solvents, changing the structures of self-assembled supramolecular aggregates in ways that were previously difficult to understand and control. The findings have implications for designing supramolecular materials for biological, electronic, and catalytic applications.

E. W. Meijer of Eindhoven University of Technology and coworkers investigated this phenomenon when they observed unpredictable changes in helical supramolecular structures formed by self-assembly of biphenyl tetracarboxamide in organic solution. Through spectroscopic, calorimetric, light-scattering, and theoretical techniques, they identified the culprit to be trace water in the solvents added by humidity (Nature 2018, DOI: 10.1038/s41586-018-0169-0).

Self-assembly formed three different helices: one type with no water present; a second in low humidity, in which one water molecule associated with every two building blocks; and a third at higher humidity, in which two waters bound each building block. Humidity also helped explain erratic results the scientists encountered in previous self-assembly studies.

Typical alkane solvents contain less than 0.01% water at room temperature, a seemingly negligible amount that supramolecular chemists have tended to ignore. And supramolecular aggregates are flexible, dynamic materials that are hard to characterize, making it difficult to identify exactly where waters bind in a structure, even when their presence is suspected.

Meijer and coworkers propose that water intercedes in self-assembly because the organic solvent’s hydrophobicity prevents water molecules from hydrogen-bonding with one another. The frustrated water molecules are so determined to hydrogen-bond that they join the self-assembling supramolecular structure instead.

“Even one water molecule, if present in organic solution, will attack a developing assembly by forming strong hydrogen bonds, dramatically altering the properties of the system,” comments Dimitris Vlassopoulos of the University of Crete.

“This is a brilliant paper, groundbreaking in many ways,” says Songi Han of the University of California, Santa Barbara. “It will make ripples through the supramolecular chemistry community. Anybody who has worked with organic self-assembly in organic solvents will find themselves looking back and wondering what the findings mean for their own work.”

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Comments
Rose Winn (June 3, 2018 5:50 AM)
I’m very curious about the statements made in your article. Please forgive me if this seems like a nutty questionnaire. Not knowing anything about supramoleculars, do scientists believe that the humidity is bad or good? In what ways do trace levels of water contribute to organic solvents? Is this dangerous that it is difficult to understand and control?

You wrote:
“Even one water molecule, if present in organic solution, will attack a developing assembly by forming strong hydrogen bonds, dramatically altering the properties of the system.”

Do the scientists think that the fact that just one water molecule can form a strong hydrogen bond, dramatically altering it’s properties of the system, could this also alter the way the molecule performs?

Suppose one is creating a chelate for a metal ion and water molecules are used to bind them together. Could the water molecule form such a strong hydrogen bond with the metal ion that instead of the metal ion being expelled through the body immediately, through urine or sweat, could the hydrogen bond actually keep them bound together unable to be expelled from the body and instead settle in the brain, organs, skin and bones? So curious about your answer!

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