Mas NMR Dipolar Correlation Spectroscopy of Partially Deuterated ¹³C-Labelled Chlorophyll a

Abstract

Chlorophyll (Fig. 1) is a major constituent of photosynthetic complexes. ¹H MAS NMR can provide important information about the electronic and spatial structure of chlorophyll in photosynthetic studies. However, the observation of the ¹H response in solid-type samples is difficult. The proton resolution in solid state NMR is usually insufficient for direct observation, due the combination of strong homonuclear couplings between protons and a small chemical shift dispersion. To circumvent this problem, 2D and 3D heteronuclear spectroscopy has been applied, and the signals from up to ~ 50 protons in a moderately sized multispin cluster have been assigned [1, 21. In these assignment studies, a high field wideline separation (WISE) and a combination of high field and homonuclear frequency-switched Lee-Goldburg (FSLG) ¹H decoupling were applied to resolve the proton response. Here we investigate if additional resolution improvement can be obtained by a suppression of ¹H homonuclear dipolar couplings by dilution of the protons with deuterons. It can be stated a priori that dilution has the disadvantage that the overall ¹H response is weakened, which will inevitably affect the range of the ¹H MAS NMR. The dilution increases the transverse relaxation time and blocks the spin diffusion. Despite of the reduction of the sensitivity, it is important to asses the effect of ²H dilution in high magnetic field, with and without FSLG decoupling. We have chosen to use a moderate dilution level of 75% ²H, reducing the ¹H signal strength by a factor of four.