Beamsize

X-ray crystallography relies on the interaction of X-rays with protein crystals, where the number of unit cells contributing to Bragg diffraction directly influences the intensity of diffraction spots and, consequently, the achievable signal-to-noise ratio (I/σ) and maximum resolution. Traditionally, this drove a need for large crystals, a requirement that persists in techniques like neutron crystallography. However, the advent of high-flux X-ray beams has lessened the reliance on very large crystals in modern macromolecular crystallography. Instead, the focus has shifted towards managing crystal inhomogeneity, where variations in mosaicity and lattice deformations across a crystal can lead to differing diffraction quality in different regions. The development of stable, microfocus beams (5-50 μm, compared to older 50-200 μm beams), combined with high-speed detectors (capable of 500 Hz acquisition), and improved beam stability, allows us to selectively probe and exploit the best-diffracting regions within crystals. Furthermore, for small crystals, matching the beam size to the crystal dimensions is crucial to minimize background scatter from the sample holder, mother liquor, or air, which can significantly improve the signal-to-noise ratio (I/σ). This article will discuss the beam size capabilities at the Diamond Light Source I04 beamline and provide practical best practices for users.