Hydrogen bonding gets new definition: International Union of Pure and Applied Chemistry accepts Indian definition of hydrogen bond; could lead to discovery of new drugs

By Syed Akbar
Hyderabad, July 12: Proving a 100-year-old scientific definition inadequate, Indian scientists have given a new meaning to hydrogen bonding, a unique chemical phenomenon that discreetly governs the world, both living and non-living.
The new definition to hydrogen bonding will help in developing new drugs for a plethora  of diseases and better understanding of the chemistry that makes things like water and DNA so unique in nature  and property. If  you are able to boil water at 100 degrees C to kill bacteria and other pathogens for a healthy 
living, and if the DNA has a double helical structure transmitting genetic information from parents to offspring, it's  thanks to the power of this simple and yet complex hydrogen bonding.
Incidentally, this is the first major contribution to the world of chemistry in this  International Year of Chemistry. The International Union of Pure and Applied Chemistry on Friday last accepted the new  
definition of hydrogen bonding proposed by Indian scientists Prof E Arunan and Prof Gautam R Desiraju of the Indian  Institute of Science, Bengaluru. Prof Arunan has chaired an international task force on hydrogen bonding  appointed by the IUPAC.
Hydrogen bonding is an important area of study for researchers ranging from physicists to  biologists and chemists to biotechnologists, as the very nature of life lies hidden inside it. Hydrogen bonding is  central to chemistry and biology and involves things both living and non-living. It's importance can be gauged from the  fact that more than one research work is published every hour in some or other international scientific journals.
"Hydrogen bonding is not string theory or gravitons, which no one has ever seen yet.  There are plenty of
experimental and theoretical results available," Prof Arunan pointed out. 
If there were no hydrogen bonding there would not have been water for drinking, and thus  no life - either plant or animal. Water would not have got its unique property of boiling at 100 degrees and  killing harmful pathogens had there been no hydrogen bonding. Moreover, water would not have turned into ice or snow. 
There would be no bonding of DNA molecules that carry the secret of life and history of individual 
creatures. Even synthetic things like nylon, and natural materials like petrol would not have existed but for hydrogen bonding.
"It is not simple electrostatic interaction as thought earlier. As recognised in the old  definition it is not limited to N (nitrogen), O (oxygen) or F (fluorine).  The energy limit given is restrictive. How do we 
know this? Thanks to experimental and theoretical methods that have become available today. We have built a unique microwave spectrometer which gave us some results to challenge the current definition," Prof Arunan  told this correspondent.
According to Prof Arunan, the new definition is a fundamental advance leading to better  understanding of the phenomenon. Science is full of examples, where such understanding has helped progress.  For example, it was well known that water owes it unique properties to hydrogen bonding and it is covered in the old definition.
"Now we know several examples for C-H  (carbon-hydrogen) and S-H (sulphur-hydrogen)  groups forming hydrogen bonding. This realisation has helped chemists identify these interactions in proteins and 
DNA. Naturally, this understanding can help in designing drugs, for example," he added.

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IUPAC's current definition of hydrogen bonding
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A form of association between an electronegative atom and a hydrogen atom attached to a second, relatively electronegative atom. It is best considered as an electrostatic interaction, heightened  by the small size of hydrogen, which permits proximity of the interacting dipoles or charges. Both electronegative atoms 
are usually (but not necessarily) from the first row of the Periodic Table, i.e. N, O or F. Hydrogen bonds may  be intermolecular or intramolecular. With a few exceptions, usually involving fluorine, the associated energies are less than 20-25 kJ mol-1 (5-6 kcal mol-1).

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IUPAC's new definition recommended by Dr E Arunan's team

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The hydrogen bond is an attractive interaction between a hydrogen atom from a molecule or a molecular fragment X-H in which X is more electronegative than H, and an atom or a group of atoms in the 
same or a different molecule, in which there is evidence of bond formation.