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Solid-State 29Si VACP/MAS NMR Studies of Silicon-Accumulating Plants: Structural Characterization of Biosilica DepositsRüdiger Bertermann and Reinhold Tacke

Institut für Anorganische Chemie, Universität Würzburg,Am Hubland, D-97074 Würzburg, Germany

Reprint requests to Prof. Dr. R. Tacke. E-mail: r.tacke@mail.uni-wuerzburg.de

Z. Naturforsch. 55 b, 459—461 (2000); received February 21, 2000Biomineralization, Silica, Silicon-Accumulating Plants

A series of silicon-accumulating plants [different Equisetum (horse tail) species, Echium vulgare, and Symphytum officinale] were studied by solid-state 29Si NMR experiments. For this purpose, selected parts of these plants were freeze-dried and then investigated by solid-state ~9Si VACP/MAS NMR spectroscopy. The 29Si NMR spectra of these plants are quite similar and exhibit the typical pattern characteristic of polysilicic acid (amorphous silica).

Introduction

It has been generally accepted that silicon is an essential element for many biological systems, be­ing required for the production of structural m ate­rials and/or metabolic processes (see for example ref. [1], and literature cited therein). Biom ineral­ized silica (biosilica) o f the general form ula type [SiO„/2(OH)4_/,],„ (n = 0 to 4) has been recog­nized to have a structural function in certain marine organisms (such as diatoms, silicoflagellates, ra- diolarians, and sponges) and silicon-accum ulating plants [such as Equisetum (horse tail) species]. As shown by various physical methods (including high- resolution transmission electron microscopy [2], solid-state 29Si NM R spectroscopy [2], and SiK XANES spectroscopy [3]), the structural nature of biosilica is best described as an amorphous m ate­rial. However, some kind of mid-range ordering at a length-scale of 4 - 10 A has been claim ed for silicas isolated from rice husks (Oryza sativa) [3]. Here we report on the application of solid-state 29Si VACP/MAS NM R spectroscopy (VACP = Variable Amplitude Cross Polarization) to the structural in­vestigation of biogenic silica deposits o f plants [4], This particular method allows a direct structural characterization of the biosilica without its isola­tion and enrichment [5]. The plants exam ined in this study were a series of Equisetum species, Echium vulgare, and Symphytum officinale.

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Experimental Section

Different parts of the plants investigated (see Table 1) were separated mechanically and subsequently freeze- dried. The synthetic silica was purchased from Merck (silica gel 60; particle size, 0.015 - 0.040 mm). All sam­ples were studied by solid-state 29Si VACP/MAS NMR spectroscopy. For this purpose, the samples were meas­ured at 23 °C on a Bruker DSX-400 NMR spectrometer with bottom layer rotors (ZrCh; diameter, 7 mm) contain­ing ca. 200 mg of sample [29Si, 79.5 MHz; spinning rate,5 - 7 kHz; contact time, 5 ms; 90° 'H transmitter pulse length, 3.6 ^s; repetition time, 4 s; number of scans, 22000- 150000; external standard, TMS (b = 0)].

Results and Discussion

The biosilica deposits o f the silicon-accu- m ulating plants Equisetum arx’ense, Equisetum giganteum, Equisetum hyemale, Equisetum palus- tre, Equisetum telmateia, Echium vulgare, and Sym­phytum officinale were structurally characterized by solid-state 29Si NM R experiments. For this purpose, certain parts o f these plants (see Table 1) were sep­arated and freeze-dried and then investigated by 29Si VACP/MAS NM R spectroscopy. For reasons o f comparison, synthetic silica was also studied by this method.

The 29Si NM R spectra obtained in these stud­ies look quite similar for all plants. Fig. 1 (leaves

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46 0 R. Bertermann and R. Tacke • Structural Characterization o f B iosilica D eposits

Table 1. Isotropic 29Si chemical shifts (<*> values in ppm) of biosilica deposits of Equisetum ar\’ense, Equisetum giganteum, Equisetum hyemale, Equisetum palustre, Equisetum telmateia, Echium vulgare, Symphytum officinale, and synthetic silica obtained by solid-state 29Si VACP/MAS NMR studies (estimated error ±1 ppm).

Species (part of the plant) <5S7(OSi=)4[Q4] 6Si(O Si=)3OH [Q3] <5S*(OSi=)2(OH)2 [Q2]

Equisetum arvense (stems and blossoms) -111 -101 -92Equisetum giganteum (leaves) -111 -101 -92Equisetum giganteum (stems) -111 -101 -92Equisetum hyemale (blossoms) -111 -101 -92Equisetum hyemale (stems) -111 -101 -91Equisetum palustre (stems and leaves) -110 -101 -92Equisetum telmateia (stems and blossoms) -111 -101 -92Echium vulgare (stem leaves) -110 -101 -91Symphytum officinale (stem leaves) -110 -101 -91Synthetic silica -111 -102 -92

Q3

Q2r J \

-60 -80 -100 -120 -140 ppm

Fig. 1. Solid-state 29Si VACP/MAS NMR spectrum of the leaves of Equisetum giganteum (spinning rate, 5 kHz; number of scans, 150000; for further details, see Experi­mental Section).

Q3

Fig. 2. Solid-state 29Si VACP/MAS NMR spectrum of the stem leaves of Echium vulgare (spinning rate, 5 kHz; number of scans, 80000; for further details, see Experi­mental Section).

of Equisetum giganteum) and Fig. 2 (stem leaves of Echium vulgare) show typical examples of these spectra. The isotropic 29Si chemical shifts listed in Table 1 clearly demonstrate that tetrahedrally co­

Q3

-60 -80 -100 -120 -140 ppm

Fig. 3. Solid-state 29Si VACP/MAS NMR spectrum of synthetic silica (spinning rate, 5 kHz; number of scans, 2072; for further details, see Experimental Section).

ordinated silicon (in S/O4 units) is present in all samples. The spectra show the well known pattern typical o f amorphous silica (Fig. 3) with resolved Q 2, Q 3, and Q4 groups and do not exhibit hints for the presence of penta- and hexacoordinate silicon, although the existence of pentacoordinate silicon (S/O5 units) cannot totally be excluded [6].

It should be m entioned that the ratio of the Q2, Q 3, and Q4 m oieties in the biosilica samples stud­ied does not correspond to the intensity ratio of the respective resonance signals in the spectra, be­cause the VACP/MAS technique does not lead to quantitative spectra [7, 8]. Nevertheless, the results obtained in this study clearly demonstrate that solid- state 29Si VACP/MAS NM R spectroscopy is a valu­able non-destructive method for the structural char­acterization of biosilica in intact biological systems. In contrast to other experimental methods reported, this particular 29Si NM R technique allows the d i­rect investigation of intact (freeze-dried) silicon-

R. Bertermann and R. Tacke • Structural Characterization o f B iosilica D eposits 461

accumulating plants and does not need the isola­tion and enrichment of the biosilica by destruc­tive methods. We are planning to extend our solid- state 29Si NMR studies to biosilica deposits o f m a­rine organisms, such as diatoms, radiolarians, and sponges.

Acknowledgements

This work was supported by the Fonds der Chemischen Industrie. We thank the Botanischer Garten, Julius-von- Sachs-Institut für Biowissenschaften, Universität Würz­burg, for providing us with plant materials.

[1] R. Tacke, Angew. Chem. I l l , 3197 - 3200 (1999); Angew. Chem. Int. Ed. 38, 3015 - 3018 (1999).

[2] S. Mann, C.C. Perry, in D. Evered, M. O’Connor (eds): Silicon Biochemistry, pp. 40 - 58, Wiley, Chichester (1986).

[3] G. Schechner, M. Fröba, B. Pillep, J. Wong, P. Beh­rens, in N. Auner, J. Weis (eds): Organosilicon Chemistry IV - From Molecules to Materials, pp. 17-21, Wiley-VCH, Weinheim (2000).

[4] Preliminary results have already been reported: R. Bertermann, R. Tacke, The 12th International Symposium on Organosilicon Chemistry, May 23- 28, 1999, Sendai/Japan, Abstracts, p. 145.

[5] The solid-state 29Si NMR studies described in ref.[2] were not performed with the VACP/MAS tech­nique and the biosilica samples investigated were isolated from the plants.

[6] For 29Si NMR shifts of pentacoordinate silicon compounds with 5 /0 5 skeletons, see: R. Tacke, B. Pfrommer, M. Pülm, R. Bertermann, Eur. J. Inorg. Chem. 1999, 807 - 816; and references cited therein.

[7] G. Engelhardt, H. Koller, NMR - Basic Principles and Progress 31, 1 - 29 (1994).

[8] Because of the poor signal to noise ratio, contact time depending or quantitative NMR experiments were not performed.