An efficient optimization screen was devised using random microseed matrix screening to quickly derivatize crystals using an iodinated phasing ligand.

Tetrabromoterephthalic acid (B4C) provides sufficient anomalous signal to solve the crystal structure of hen egg-white lysozyme using MAD phasing. The use of B4C as an experimental phasing compound offers several advantages over existing compounds, including its compatibility with common crystallization reagents and its amenability to co-crystallization.