Dissociation Constant (Kd)
Circular dichroism (CD) spectra of LanM were collected using a Jasco J-1500 CD spectrometer, thermostatted at 25 °C, and a 1-mm pathlength quartz CD cuvette (Jasco J/0556). Samples were scanned from 260 to 200 nm, with the following instrument settings: 1 nm bandwidth, 0.5 nm data pitch, 50 nm/min scan rate, 4 s average time. Three scans were acquired and averaged for each condition. For stoichiometric titrations of LanM with all LnIII ions and CaII, the cuvette contained 20 μM LanM in 200 μL Chelex-treated Buffer B, into which 1-8 equiv. each metal ion (from a 1 mM solution in Buffer B) were titrated, and each spectrum was acquired. A buffer blank spectrum was subtracted from the sample spectra, and the spectra were corrected for volume change before plotting.