Thallium is a relatively rare element in our day to day lives, but has been shown to displace other cations in biological contexts such as DNA as well as binding to the sulfur found on cysteine residues in proteins. This makes Thallium salts extremely toxic. Heavy elements are often used to help crystalise biological and structures. Despite Thallium being in a very different part of the periodic table, its similar size and charge to potassium salts means it can fulfil the same role in natural quadruplex structures. These quadruplex structures are implicated in both normal and cancer biology and are of interest to researchers around the globe.
Thallium isn't just present in protein structures like DNA, it can also be found in small molecule structures in the CSD for example CSD Entry WIKQUC which is described in DOI: 10.1021/ic991492n. This compound displays a strong visible luminescence, under UV excitation, which is sensitive to temperature due to the interaction the Thallium ion has with gold.
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