The binding of calcium ion to calmodulin, a major biochemical regulator of ion pumps and receptors, occurs on a time scale about a thousand times shorter than that observed for RNA conformational change. This Ca²⁺-calmodulin binding, which can be followed successfully by nuclear magnetic resonance (NMR), occurs in about ten milliseconds.

(A) Structures of calmodulin with and without Ca²⁺ ion binding. Courtesy of Mikael Akke, Lund University, Sweden. (B) Linewidths of ⁴³Ca-NMR spectrum of Ca²⁺/ calmodulin mixture as a function of temperature (circles), together with best fit to the kinetics (solid line). By analyzing the temperature dependence, the activation barrier is determined. Adapted from T. Drakenberg, S. Forsén, H. Lilja, J. Mag. Res. 53, 412 (1983). Reprinted from Journal of Magnetic Resonance, 53, T. Drakenberg et al., 43Ca NMR Studies of Calcium Binding to Proteins: Interpretation of Experimental Data by Bandshape Analysis, 412-422, (1983), with permission from Elsevier Science.

Other chemical changes can be much faster than the RNA conformational changes illustrated here. Photodissociation of sodium iodide (NaI) in the gas phase occurs on the time scale of a few picoseconds (10⁻¹² seconds). To measure this phenomenon, NaI molecules are irradiated by a sub-picosecond ultraviolet pulse of radiation, and the subsequent events are clocked by another short light pulse that detects the newborn