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Abstract Serial crystallography (SX) has revolutionized structural biology by enabling high-resolution structure determination for important classes of proteins, including the study of relevant biomolecular reaction mechanisms. However, one of the ongoing challenges in this field remains the efficient use of precious macromolecule samples whose availability is often limited. Reducing sample consumption is thus critical in maximizing the potential of SX conducted at powerful X-ray sources such as synchrotrons and X-ray free-electron lasers (XFEL) to expand to a broader range of significant biological samples, gaining insights into unraveled biological reaction mechanisms. This review focuses on three primary sample delivery systems: fixed-targets, liquid injection, and hybrid methods, each with distinct advantages and limitations concerning sample consumption. The progress and challenges associated with these methods, highlighting advancements in reducing sample consumption and thus enabling the study of more diverse biological samples, are summarized. We compare the currently reported sample delivery methods in view of the minimum amount of sample required to obtain a full data set and discuss how the current approaches compare to this theoretical minimum. With this overview, we aim to provide a critical and comprehensive assessment of the current methods and experimental realizations for sample delivery in SX with proteins. 

Abstract Sample consumption for serial femtosecond crystallography (SFX) with X-ray free electron lasers (XFELs) remains a major limitation preventing broader use of this powerful technology in macromolecular crystallography. This drawback is exacerbated in the case of time-resolved (TR)-SFX experiments, where the amount of sample required per reaction time point is multiplied by the number of time points investigated. Thus, in order to reduce the limitation of sample consumption, here we demonstrate the implementation of segmented droplet generation in conjunction with a mix-and-inject approach for TR studies on NAD(P)H:quinone oxidoreductase 1 (NQO1). We present the design and application of mix-and-inject segmented droplet injectors for the Single Particles, Clusters, and Biomolecules & Serial Femtosecond Crystallography (SPB/SFX) instrument at the European XFEL (EuXFEL) with a synchronized droplet injection approach that allows liquid phase protein crystal injection. We carried out TR-crystallography experiments with this approach for a 305 ms and a 1190 ms time point in the reaction of NQO1 with its coenzyme NADH. With this successful TR-SFX approach, up to 97% of the sample has been conserved compared to continuous crystal suspension injection with a gas dynamic virtual nozzle. Furthermore, the obtained structural information for the reaction of NQO1 with NADH is an important part of the future elucidation of the reaction mechanism of this crucial therapeutic enzyme. 

A 3D-printed modular droplet injector successfully delivered microcrystals of human NAD(P)H:quinone oxidoreductase 1 (NQO1) and phycocyanin with electrical stimulation in a serial crystallography experiment at 120 Hz repetition rate.