An Unexpected Hydrogen Bond

Imagine all the things your body does each day. Whether it’s blinking, breathing, stretching or smiling, even the tiniest of tasks requires that your body orchestrate a series of commands and functions to produce the results we expect. So how are all these tasks monitored?

If we trace back the chain of command, two elements at the very core of our bodily functions are proteins and DNA. Molecules of DNA always take a very particular form, discovered and made famous by Watson and Crick: the double helix. This double helix helps DNA function in a very specific way. It can unzip to allow for semi-conservative replication, the mechanism by which DNA duplicates itself to contain one newly formed strand and one strand of the original copy, but also coil very tightly to compact itself for mitosis. Proteins have more variation in their structure, but each protein’s function also follows its form. Membrane proteins contain channels to allow molecules to pass through membranes, antibodies have binding sites with specific shapes to recognize antigens and catalyst proteins have conformations that allow them to help reactions occur by positioning substrates.

The conformations of DNA molecules and proteins are dictated by intermolecular forces, which are the interactions between individual atoms and molecules within the larger chain of nucleotides or amino acids, respectively. These typically fall into one of several categories, including hydrogen bonding, electrostatic interactions and van der Waals forces. Until now, our definition of hydrogen bonds had been an attraction between a positively charged hydrogen atom and a nearby negatively charged atom — usually an oxygen, nitrogen or fluorine atom. But now, a research team at the University of Copenhagen has discovered a new way to bind a positively charged hydrogen atom with another positively charged atom.

The chemistry department at the university found that an intermolecular bond could form between an alcohol, containing an OH group, and a trimethylphosphine molecule. Trimethylphosphine consists of a central phosphorous accommodating three methyl groups around it, making a triangular pyramid shape like ammonia does. Hydrogen bonds typically form between a hydrogen molecule and an oxygen, nitrogen or fluorine because the latter three atoms have high electronegativities, giving them a slightly negative charge that attracts the positively charged hydrogen. However, in the trimethylphosphine molecule, the phosphorus does not have nearly the same electronegativity, giving it a partial positive charge. Nonetheless, a hydrogen bond can form between the hydrogen and phosphorus because the phosphorus has a small region of negative potential within the overall positive charge.

This new discovery has large implications for how we understand intermolecular forces. Having redefined what hydrogen bonding includes, we will now be better able to understand how molecules inside our bodies interact. Moreover, as we engineer new molecules, we will be aware of more ways we can design them to interact with each other. Learning about this type of interaction will give us a foundation for a deeper understanding of the fundamentals of chemistry as well as a springboard for future research and development.

Sources

Anne S. Hansen, Lin Du, Henrik G. Kjaergaard. Positively Charged Phosphorus as a Hydrogen Bond Acceptor. The Journal of Physical Chemistry Letters, 2014; 5 (23): 4225 DOI: 10.1021/jz502150d

Faculty of Science – University of Copenhagen. “Bond and bond alike: Unlikely hydrogen bond discovered.” ScienceDaily. ScienceDaily, 13 March 2015. <www.sciencedaily.com/releases/2015/03/150313101838.htm>.

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