Structure
The structure of metformin was generally represented in a wrong tautomeric form for a number of years. This was corrected in 2005. The energy difference between the correct tautomer and the generally represented tautomer is about 37 kJ/mol (9 kcal/mol). The drug is administered as metformin hydrochloride. The structure of the metformin hydrochloride was also corrected recently. According to these studies, the metformin has different electronic structure compared to its protonated form. The neutral species is a simple conjugated system. Upon protonation, (i) the conjugation breaks down, (ii) the intramolecular hydrogen bond breaks down, (iii) the molecule becomes non-planar (iv) two lone pairs get accumulated at the central nitrogen (v) dynamism increases in the system via C=N rotational process and via N-inversion process. Thus, electronically, metformin hydrochloride should be treated as a simple extension of metformin. The nucleophilicity of metformin hydrochloride is moderate and that is a desired property. Metformin is shown to belong to a new class of compounds called nitreones.
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