This is a Hard One – Sildenafil Citrate

What does it look like?

—Image generated with Crystal Maker software http://www.crystalmaker.com/

—Image generated with Crystal Maker software http://www.crystalmaker.com/

What is it?

Most people will have heard of sildenafil citrate by its trademark name – Viagra.  The active part of the drug is the complicated cationic species, which blocks an enzyme that has a detrimental effect on blood-flow to, well, we all know where to.  What most people would not be aware of is that Viagra was also found to help hamsters to recover from jetlag. Seriously.  This work won the Ig Nobel prize for aviation in 2007.

The structure shown above is Viagra monohydrate (with the water not shown in the picture).  From a crystallographic perspective, the structure contains a lot of hydrogen bonding interactions from the carboxylic acids of the citrate counter-anion to the cation, which contains a number of basic nitrogen sites, and to other citrate anions. Polycarboxylic acids are often used in crystal engineering due to their ability to form strong interactions and can be used to help crystallise important drug molecules.

Where did the structure come from?

This structure was reported in 2005.  H.S.Yathirajan, B.Nagaraj, P.Nagaraja, M.Bolte, Acta Crystallogr.,Sect.E, 2005, 61, o489.

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