Solid-state NMR spectroscopic studies of 13C,15N,29Si-enriched biosilica from the marine diatom Cyclotella cryptica

Felicitas Kolbe,Helena Leona Ehren,Eike Brunner,Daniel Butscher,Simon Kohrs,Marc Baldus,Lukas Reiß

doi:10.1007/s43939-020-00003-7

Abstract

Diatoms are algae producing micro- and nano-structured cell walls mainly containing amorphous silica. The shape and patterning of these cell walls is species-specific. Herein, the biosilica of Cyclotella cryptica, a centric marine diatom with a massive organic matrix, is studied. Solid-state NMR spectroscopy is applied to gain deeper insight into the interactions at the organic–inorganic interface of the cell walls. The various organic compounds like polysaccharides as well as proteins and long-chain polyamines (LCPAs) are detected by observation of heteronuclei like 13C and 15N whereas the silica phase is studied using 29Si NMR spectroscopy. The sensitivity of the NMR experiments is strongly enhanced by isotope-labeling of the diatoms during cultivation with 13C, 15N and 29Si. The presence of two different chitin species in the biosilica is demonstrated. This observation is supported by a monosaccharide analysis of the silica-associated organic matrix where a high amount of glucosamine is found. Moreover, the Rotational Echo Double Resonance (REDOR) experiment provides distance information for heteronuclear spins. 13C{29Si} REDOR experiments reveal proximities between different organic compounds and the silica phase. The closest contacts between silica and organic compounds appear for different signals in the 13C-chemical shift range of 40–60 ppm, the typical range for LCPAs.

Highlights

Diatoms are unicellular algae and frequently studied model organisms in biomineralization research
The silica phase of the diatom cell walls can be characterized using 29Si solid-state NMR spectroscopy whereas the organic compounds are investigated by 13C and 15N solid-state NMR spectroscopy [17,18,19,20,21]
(ii) The presence of two different types of chitin in biosilica extracted from C. cryptica is shown based on 13C solid-state NMR spectroscopy

Summary

Introduction

Diatoms are unicellular algae and frequently studied model organisms in biomineralization research. The application of distance-sensitive NMR techniques like the Rotational Echo Double Resonance (REDOR) experiment [22] help to gain a deeper insight into the organic–inorganic interface of the biosilica. This solid-state NMR experiment (cf Figure S6) is used to determine the distance between hetero-spins I and S by the re-introduction of the dipolar coupling, which is usually averaged by the MAS (magic angle spinning) technique [22, 23]. The heteronuclear dipolar second moment M2 can be extracted directly from the experimentally determined REDOR curve using the first order approximation, which is especially interesting for materials of unknown spin system geometry like glasses or the amorphous biosilica studied here. This first order approximation can only be used for the initial REDOR curve up to a REDOR fraction of 0.3: [23,24,25]

Methods

Results

Conclusion