Terbium ions are used as fluorescent probes to characterize binding sites in biological macromolecules (Vazquez-Ibar et al, 2002). Terbium has no known essential biological function, but in the absence of Ca²⁺, Tb³⁺ restored 76% of Bacillus subtilis α-amylase activity (Smolka et al, 1971).

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