Molecular tribology

It’s hard to imagine more intriguing title in Angewandte Chemie International Edition than “Astringent Mouthfeel as a Consequence of Lubrication Failure“. The first impression doesn’t deceive, and the paper is really interesting and fun to read. Somehow manifestations of molecular interactions in the macroscopic world never stop amusing me. And this communication is exactly about such emergent effect.

The core finding of Chinese/Korean team is that tannic acid, a common chemical in plant-derived products, causes aggregation and precipitation of mucoproteins. These proteins are abundant in a variety of physiological fluids, including saliva. According to authors’ model, binding of tannic acid ties up many of mucoproteins’ hydroxylic groups. This ‘hydrophobization’ of proteins leads to their aggregation and eventually precipitation.

Tannic acid

Authors performed a series of tests to show the increase of friction forces when coffee or wine (rich in tannic acid) is applied on a sliding surface. In contrast, milk was a pretty good lubricant. Then atomic force microscopy confirmed tannic-acid promoted aggregation on a surface of a gold chip covered with mucoproteins. Then authors prepared hydrogels containg mucoproteins and found that they are really flexible and slippery, but once treated with tannic acid, become rigid and sticky. So they made a conclusion that astringent feeling from unripe fruits, at least partially, comes from temporary de-lubrication of a tongue. And it really feels like that, isn’t it?

To generalize the principle, the authors hypothesized that tannic acid can bind not only to mucins on the surface of the tongue, but to any other hydrophilic proteins, and will make them more hydrophobic. So they designed the masterpiece of the paper − the experiment of catching a catfish with a tannic acid-releasing glove! Pity that they don’t report the method of making that magic glove in the experimental part and by the way I’m not sure if they performed it in triplicate (hey reviewers, did anybody pay attention?). They noticed though that the skin of tannic acid-captured fish becomes pale compared to uncaught control.

The commentaries of the authors are entertaining as well:

This finding may guide people to change their eating habits. For example, protein-rich and polyphenol-rich foods can’t be eaten together
Well, it seems a little too bold of a statement. But here we have it, a tribological rationale for wine and steak pairing: leaner meat with lighter wine! Science confirms.

Author: Slava Bernat

I did my PhD in medicinal chemistry/chemical biology of G protein-coupled receptors and then explored some chemical biology of non-coding RNA as a postdoc. Currently I'm working in a small biotech company in San-Francisco Bay area as a research chemist. I'm writing about science, which catches my attention in rss feed reader and some random thoughts or tutorials.

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