Detecting segmental anisotropic diffusion in disordered proteins by cross-correlated spin-relaxation

Abstract

Abstract. Among the numerous contributions of Geoffrey Bodenhausen to NMR spectroscopy his developments in the field of spin-relaxation methodology and theory will definitely have a long lasting impact. Starting with his seminal contributions to the excitation of multiple-quantum coherences he and his group thoroughly investigated the intricate relaxation properties of these “forbidden fruits” and developed experimental techniques to reveal the relevance of previously largely ignored cross-correlated relaxation (CCR) effects, as “the essential is invisible to the eyes”. Here we want to discuss CCR within the challenging context of intrinsically disordered proteins (IDPs) and emphasize its potential and relevance for the studies of structural dynamics of IDPs in the future years to come. Conventionally, dynamics of globularly folded proteins are modeled and understood as deviations from otherwise rigid structures tumbling in solution. However, with increasing protein flexibility, as observed for IDPs, this apparent dichotomy between structure and dynamics becomes blurred. Although complex dynamics and ensemble averaging might impair the extraction of mechanistic details even further, spin-relaxation uniquely encodes a protein’s structural memory, i.e. the temporal persistence of concerted motions and structural arrangements. Due to significant methodological developments, such as high-dimensional non-uniform sampling techniques, spin-relaxation in IDPs can now be monitored in unprecedented resolution. Not embedded within a rigid globular fold, conventional 15N spin probes might not suffice to capture the inherently local nature of IDP dynamics. To better describe and understand possible segmental motions of IDPs, we propose an experimental approach to detect the signature of diffusion anisotropy by quantifying cross-correlated spin relaxation of individual 15N1HN and 13C'13Cα spin pairs. By adapting Geoffrey Bodenhausen’s symmetrical reconversion principle to obtain zero frequency spectral density values we can define and demonstrate more sensitive means to characterize segmental anisotropic diffusion in IDPs.