Intermolecular correlations are necessary to explain diffuse scattering from protein crystals

TL;DR

Diffuse scattering in protein crystals is primarily caused by intermolecular correlations, and understanding these long-range motions is crucial for interpreting biological dynamics and improving structural models.

Contribution

This study demonstrates that intermolecular correlations are essential to accurately explain diffuse scattering, challenging prior models that focused on intramolecular motions alone.

Findings

Models including intermolecular disorder better reproduce experimental diffuse maps.

Rigid body and short-range liquid-like models are insufficient to explain the observed scattering.

Intermolecular correlations significantly contribute to diffuse scattering signals.

Abstract

Conformational changes drive protein function, including catalysis, allostery, and signaling. X-ray diffuse scattering from protein crystals has frequently been cited as a probe of these correlated motions, with significant potential to advance our understanding of biological dynamics. However, recent work challenged this prevailing view, suggesting instead that diffuse scattering primarily originates from rigid body motions and could therefore be applied to improve structure determination. To investigate the nature of the disorder giving rise to diffuse scattering, and thus the potential applications of this signal, a diverse repertoire of disorder models was assessed for its ability to reproduce the diffuse signal reconstructed from three protein crystals. This comparison revealed that multiple models of intramolecular conformational dynamics, including ensemble models inferred from the Bragg data, could not explain the signal. Models of rigid body or short-range liquid-like motions, in which dynamics are confined to the biological unit, showed modest agreement with the diffuse maps, but were unable to reproduce experimental features indicative of long-range correlations. Extending a model of liquid-like motions to include disorder across neighboring proteins in the crystal significantly improved agreement with all three systems and highlighted the contribution of intermolecular correlations to the observed signal. These findings anticipate a need to account for intermolecular disorder in order to advance the interpretation of diffuse scattering to either extract biological motions or aid structural inference.