Fast Magic-Angle-Spinning 19F Spin Exchange NMR for Determining Nanometer 19F-19F Distances in Proteins and Pharmaceutical Compounds

Abstract

Internuclear distances measured using NMR provide crucial constraints of three-dimensional structures but are often restricted to about 5 Å due to the weakness of nuclear-spin dipolar couplings. For studying macromolecular assemblies in biology and materials science, distance constraints beyond 1 nm will be extremely valuable. Here we present an extensive and quantitative analysis of the feasibility of [superscript 19]F spin exchange NMR for precise and robust measurements of interatomic distances up to 1.6 nm at a magnetic field of 14.1 T, under 20-40 kHz magic-angle spinning (MAS). The measured distances are comparable to those achievable from paramagnetic relaxation enhancement but have higher precision, which is better than ±1 Å for short distances and ±2 Å for long distances. For [superscript 19]F spins with the same isotropic chemical shift but different anisotropic chemical shifts, intermediate MAS frequencies of 15-25 kHz without 1H irradiation accelerate spin exchange. For spectrally resolved [superscript 19]F-[superscript 19]F spin exchange, [superscript 1]H-[superscript 19]F dipolar recoupling significantly speeds up [superscript 19]F-[superscript 19]F spin exchange. On the basis of data from five fluorinated synthetic, pharmaceutical, and biological compounds, we obtained two general curves for spin exchange between CF groups and between CF[subscript 3] and CF groups. These curves allow [superscript 19]F-[superscript 19]F distances to be extracted from the measured spin exchange rates after taking into account [superscript 19]F chemical shifts. These results demonstrate the robustness of [superscript 19]F spin exchange NMR for distance measurements in a wide range of biological and chemical systems.