# Minimized sample consumption for time-resolved serial crystallography applied to the redox cycle of human NQO1

## TL;DR

Researchers developed a method to drastically reduce sample use in X-ray experiments while still capturing enzyme dynamics.

## Contribution

A segmented droplet generation strategy reduces sample consumption by up to 97% in time-resolved serial crystallography.

## Key findings

- Segmented injection achieves 97% less sample use than continuous-flow methods.
- Electron density features confirm low-occupancy NADH binding in NQO1.
- The method is compatible with XFEL and maintains data quality for TR crystallography.

## Abstract

Sample consumption for serial femtosecond crystallography with X-ray free electron lasers remains a major limitation preventing broader use in macromolecular crystallography. This drawback is exacerbated in time-resolved (TR) experiments, where the amount of sample required per reaction time point is multiplied by the number of time points investigated. To reduce this limitation, we demonstrate a segmented droplet generation strategy coupled to a mix-and-inject approach for TR studies at the European XFEL. The injector produces synchronized droplet trains that enable stable and reproducible injection of protein crystal slurries at significantly reduced flow rates. Using the human flavoenzyme NAD(P)H:quinone oxidoreductase 1 (NQO1) as a test system, we collected diffraction data after mixing with NADH at 0.3 s and 1.2 s delays. The segmented injection approach achieved up to 97% reduction in sample consumption compared with continuous-flow injection while maintaining data quality suitable for TR crystallography. Reproducible electron density features consistent with low-occupancy NADH binding illustrate both the feasibility and the current limits of studying dynamic redox enzymes using this approach. This work establishes segmented droplet generation as a sample-efficient and XFEL-compatible method for future time-resolved serial crystallography experiments.

Time-resolved serial femtosecond crystallography at X-ray free-electron lasers is a powerful approach for studying macromolecular dynamics, but its widespread use is limited by high sample consumption. Here, the authors introduce a segmented-droplet mix-and-inject strategy at the European XFEL that reduces sample consumption by up to 97% while preserving the data quality required for time-resolved structural studies of the enzyme NQO1.