The hidden treasure in your data: Phasing with unexpected weak anomalous scatterers from routine datasets

Single wavelength anomalous dispersion (SAD) utilizing anomalous signal from native sulfurs, or other atoms with Z[≤]20, generally requires highly redundant data collected with relatively long wavelength X-rays. Here we present experiences with two proteins, where anomalous signal from serendipitously acquired surface bound calcium atoms with data redundancy as low as 10 was utilized to drive de novo structure determination. In both the cases, the calcium atoms were acquired from the crystallization solution, and data collection strategy was not optimized for exploiting the anomalous signal from these scatterers. The X-ray data were collected at 0.98[A] in one case and at 1.74[A] in the ot