Sensitivity enhancement and heteronuclear distance measurements in biological 17O solid-state NMR

In this contribution we present a comprehensive approach to study hydrogen bonding in biological and biomimetic systems through O-17 and O-17-H-1 solid-state NMR combined with density functional theory calculations of O-17 and H-1 NMR parameters. We explore the signal enhancement of O-17 in L-tyrosine center dot HCl using repetitive double-frequency swept radio frequency pulses in solid-state NMR. The technique is compatible with high magnetic fields and fast magic-angle spinning of the sample. A maximum enhancement by a factor of 4.3 is obtained in the signal-to-noise ratio of the selectively excited O-17 central transition in a powdered sample of O-17(eta)-L-tyrosine, HCl at an external field of 14.1 T and a spinning frequency of 25 kHz. As little as 128 transients lead to meaningful O-17 spectra of the same sample at an external field of 18.8 T and a spinning frequency of 50 kHz. Furthermore we employed supercycled symmetry-based pulse sequences on the protons to achieve heteronuclear longitudinal two-spin-order (IzSz) recoupling to determine O-17-1H distances. These sequences recouple the heteronuclear dipolar O-17-H-1 couplings, where dipolar truncation is absent, while decoupling the homonuclear proton dipolar interactions. They can be applied at fast magic-angle-spinning frequencies up and beyond 50 kHz and are very robust with respect to O-17 quadrupolar couplings and both O-17 and H-1 chemical shift anisotropies, which makes them suitable for the use at high external magnetic fields. The method is demonstrated by determining the O-17(eta)-H-1 distance in L-tyrosine, HCl at a spinning frequency of 50 kHz and an external field of 18.8 T.