High-resolution magic angle spinning NMR analysis of whole cells of Chaetoceros muelleri (Bacillariophyceae) and comparison with 13C-NMR and distortionless enhancement by polarization transfer 13C-NMR analysis of lipophilic extracts

Lipid composition in extracted samples of Chaetoceros muelleri Lemmermann was studied with C-13-NMR and distortionless enhancement by polarization transfer (DEPT) C-13-NMR, resulting in well-resolved C-13-NMR spectra with characteristic resonance signals from carboxylic, olefinic, glyceryl, methylene, and methyl groups. The application of a DEPT pulse sequence aided in the assignment of methylene and methine groups. Resonance signals were compared with literature references, and signal assignment included important unsaturated fatty acids such as eicosapentaenoic and docosahexaenoic and also phospholipids and glycerols. Results from the extracted samples were used to assign resonance signals in a high-resolution magic angle spinning (HR MAS) DEPT C-13 spectrum from whole cells of C. muelleri. The NMR analysis on whole cells yielded equally good information on fatty acids and also revealed signals from carbohydrates and amino acids. Broad resonance signals and peak overlapping can be a problem in whole cell analysis, but we found that application of HR MAS gave a well-resolved spectrum. The chemical shift of metabolites in an NMR spectrum depends on the actual environment of nuclei during analysis, and some differences could therefore be expected between extracted and whole cell samples. The shift differences were small, and assignment from analysis of lipophilic extract could be used to identify peaks in the whole cell spectrum. HR MAS C-13-NMR therefore offers a possibility for broad-range metabolic profiling directly on whole cells, simultaneously detecting metabolites that are otherwise not detected in the same analytical set up and avoiding tedious extraction procedures.