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, Indianscientists have given a new meaning to hydrogen bonding, a unique chemical phenomenon that discreetly governs theworld, both living and non-living. The new definition to hydrogen bonding will help in developing new drugs for a plethoraof diseases and better understanding of the chemistry that makes things like water and DNA so unique in natureand 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'sthanks to the power of this simple and yet complex hydrogen bonding. Incidentally, this is the first major contribution to the world of chemistry in thisInternational 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 IndianInstitute of Science, Bengaluru. Prof Arunan has chaired an international task force on hydrogen bondingappointed by the IUPAC. Hydrogen bonding is an important area of study for researchers ranging from physicists tobiologists and chemists to biotechnologists, as the very nature of life lies hidden inside it. Hydrogen bonding iscentral to chemistry and biology and involves things both living and non-living. It's importance can be gauged from thefact 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 thusno life - either plant or animal. Water would not have got its unique property of boiling at 100 degrees andkilling 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 olddefinition 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 aunique microwave spectrometer which gave us some results to challenge the current definition," Prof Arunantold this correspondent. According to Prof Arunan, the new definition is a fundamental advance leading to betterunderstanding 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.
======== IUPAC's current definition of hydrogen bonding ---------------
A form of association between an electronegative atom and a hydrogen atom attached to asecond, relatively electronegative atom. It is best considered as an electrostatic interaction, heightenedby 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 maybe 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). ======= IUPAC's new definition recommended by Dr E Arunan's team
The hydrogen bond is an attractive interaction between a hydrogen atom from a molecule ora 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.