Collagen-like triple helix structure

Sebastian Bette and Robert E. Dinnebier

Helical structures, in particular double and triple helices are omnipresent in the living nature, e.g. in the DNA, in polysaccharides like amylose or in proteins like collagen. Synthetic organic compounds like helicenes, inorganic materials like [(CH3)2NH2]K4[V10O10(H2O)2(OH)4(PO4)7] ·4H2O or inorganic-organic hybrid materials like Cd3(OH)2(TCBA)H2O) (H4TCBA = 3,4,5-tris(carboxymethoxy)-benzoic acid) also show helical motifs. To our surprise the seemingly simple compound calcium acetate hemihydrate, Ca(CH3COO)2·½H2O, was found to exhibit a triple helical motif analogous to the collagen proteins.

In the DFG-funded project "In search of structure" the research groups for X-ray diffraction from the Max-Planck-Institute for Solid State Research, Stuttgart and for Conservation Science from the State Academy of Art and Design, Stuttgart, investigate corrosion phases found on heritage objects. During decades and centuries of storage in museums and collections, historic objects undergo various corrosion processes leading to a large variety of corrosion phases. Many of them are unknown or poorly characterized.

Excerpt of the marble relief "Adoration of the Shepherds" by G. Torretti(1664–1743) exhibited in the museum for Byzantine Art in Berlin, with detailed view of the efflorescence. Ca(CH3COO)2·½H2O was identified as the corrosion phase. The crystal structure exhibits a collagen-like triple helix motif.

The marble relief "Adoration of the Shepherds" by G. Torretti (1664–1743) exhibited in the Museum for Byzantine Art in Berlin has faced an eventful history. It received severe damage by a fire during World War II. Then it was brought to the Soviet Union were emergency conservation measures including fixing with clue were performed. In the early 90’s the relief was transferred back to Germany were the Glue was removed using ethyl acetate. Some years later, white, needle-like efflorescence crystals appeared on the surface. An investigation by the Rathgen Research lab revealed that the corrosion was caused by an attack of acetic acid originating from hydrolysis of the ethyl acetate. Calcium acetate hemihydrate could be identified tentatively as the efflorescence phase, but the reference data for phase identification were evaluated to be of low precision. In a model experiment pure, well crystalline Ca(CH3COO)2·½H2O was synthesized by migration of a saturated calcium acetate solution through the pores of a terracotta pot at a low relative humidity. An investigation by X-ray powder diffraction (XRPD) revealed the substance to exhibit an unexpectedly large unit cell of 11794.5(3) ų. The crystal structure was solved ab intio from XRPD data and confirmed in a corporation with the Centre for Electron Microscopy Mainz by automated electron diffraction tomography. In the crystal structure, acetate ions bridge neighboring calcium cations forming spiral chains, which are arranged in a triple helix motif. A fourth helix interconnects the single helices. Hence, the structural motif is analogous to collagen triple helix where a fourth helix consisting of water molecules and hydrogen bonds interconnects the single rods.

The discovery of a triple helical motif analogous to the collagen proteins in such a simple chemical compound like Ca(CH3COO)2·½H2O is fascinating. As this phase can be obtained by crystallization on calcareous or porous materials or by simple isothermal decomposition of Ca(CH3COO)2·H2O, it doesn't need a helical precursor for crystallization, but it may serve as a simple and easy accessible precursor for future works. The collagen proteins played an essential role in tissue evolution, in particular in the transition from unicellularity to multicellularity. Collagen consists to one third of glycine, which enables the triple helical structure due to its small size. As the constitution of acetate is similar to glycine, the existence of helical calcium glycinates appears to be very likely, which is opening new perspectives in the file of bio-inorganic chemistry.

Acknowledgement:
Funding by DFG for the project "In search of structure" (grant EG 137/9-1) is gratefully acknowledged.

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