Posted on behalf of Leonie Mueck, Associate Editor, Nature.
In Jaws up close, I highlighted an article from a group of researchers led by Zhongchang Wang at Tohoku University on a new electron microscopy technique that achieved atomic resolution of the fluoroapatite structure in shark teeth without damaging the sample. A technical comment has now been published, which refutes the proposed bonding situation in fluoroapatite. On the basis of density functional theory calculations, Wang et al. had claimed that the Ca-F bond in fluoroapatite is covalent, which could be crucial in understanding why shark teeth are so healthy.
Based on further calculations and bonding analysis, Antti Karttunen from Aalto University, Finland, and Florian Kraus, from Technische Universitaet Muenchen, Germany, now show in their comment that this conclusion is unsubstantiated. Rather, the Ca-F bond should be seen as ionic which rules out its strengthening function in shark teeth.