Micro-crystal growth for LCLS injection

Crystallization conditions were based on those previously described^{38 (link)}. Micro-crystals were grown using the seeded batch method. Two different conditions were used—10 mg ml⁻¹ protein concentration and 25% (wt/vol) PEG 3350 (Sigma-Aldrich, CAS 25322-68-3) and 15 mg ml⁻¹ and 30% (wt/vol) PEG 3350—and both sets of conditions included 0.1 M Tris (Sigma-Aldrich, CAS 77-86-1) pH 8.5, 0.2 M lithium sulfate (Sigma-Aldrich, CAS 10377-48-7) and 1.5% (vol/vol) seed. Seed stocks were prepared using either condition and subsequently filtered with 50- and 30-μm CellTrics filters (Sysmex Partec). Crystallization was performed in 2-ml round-bottom Eppendorf tubes, and the addition of reagents followed the order ‘buffer, precipitant, protein and seed’ in a final 1-ml volume followed by gentle mixing. Needle-shaped micro-crystals with dimensions of ~3 × 5 × 10–100 μm matured after 24 h at 20 °C. To reduce issues with blockages during crystal injection, especially with the Gas Dynamic Virtual Nozzle (GDNV) during LCLS^{40 (link)} injection, further size optimization was performed by breaking up the micro-crystals using glass beads (Supplementary Section 1 describes the procedure).