ABSTRACT
Helical motifs are common in nature, for example, the DNA double or the collagen triple helix. In the latter proteins, the helical motif originates from glycine, the smallest amino acid, whose molecular confirmation is closely related to acetic acid. The combination of acetic acid with calcium and water, which are also omnipresent in nature, materializing as calcium acetate hemihydrate, was now revealed to exhibit a collagen-like triple helix structure. This calcium salt is observed as efflorescence phase on calcareous heritage objects, like historic Mollusca shells, pottery or marble reliefs. In a model experiment pure calcium acetate hemihydrate was crystallized on the surface of a terracotta vessel. Calcium acetate hemihydrate crystallizes in a surprisingly large unit cell with a volume of 11,794.5(3)â Å3 at ambient conditions. Acetate ions bridge neighboring calcium cations forming spiral chains, which are arranged in a triple helix motif.
ABSTRACT
As one of the most abundant materials in the world, calcium carbonate, CaCO3, is the main constituent of the skeletons and shells of various marine organisms. It is used in the cement industry and plays a crucial role in the global carbon cycle and formation of sedimentary rocks. For more than a century, only three polymorphs of pure CaCO3-calcite, aragonite, and vaterite-were known to exist at ambient conditions, as well as two hydrated crystal phases, monohydrocalcite (CaCO3·1H2O) and ikaite (CaCO3·6H2O). While investigating the role of magnesium ions in crystallization pathways of amorphous calcium carbonate, we unexpectedly discovered an unknown crystalline phase, hemihydrate CaCO3·½H2O, with monoclinic structure. This discovery may have important implications in biomineralization, geology, and industrial processes based on hydration of CaCO3.
ABSTRACT
Palladium nanoparticles (PdNPs) are used in catalysis, hydrogen storage, biomedicine, and so on. Arranging the self-assembly of PdNPs within colloidal aggregates is desirable for improving their consumer properties. Stable widely dispersed colloidal aggregates of larch arabinogalactan (LARB) containing nanosized (5-nm) PdNPs were obtained by reducing Pd ions in alkaline solutions of LARB. Centrifugation resulted in a set of LARB-PdNP colloids ranging from 60 to 240 nm. The colloids were studied by static light scattering (SLS) and dynamic light scattering (DLS). The SLS data presented as Kratki plots correspond to a particle scattering factor of linear rather than branched chains. The fractal dimension of the LARB-PdNP colloids was found by SLS to be d = 1.96, which is between the values for diffusion- and reaction-limited aggregation. This result is ascribed to the aggregate's internal motion, which is evident from the power-law exponent of the dependence of the DLS relaxation rate on the scattering vector, <Γ> ~ q(α) with α = 2.24. The structure-sensitive ratio of the radius of gyration to the hydrodynamic radius was found to vary within the interval of 0.8 ≤ R(g)/R(h) ≤ 1.2 corresponding, to the spherical form of LARB-PdNP colloids. A spiderweblike PdNP distribution pattern was observed by transmission electron microscopy. Insertion of PdNPs did not affect the fractal dimension, the power-law exponent α, or the architecture of the pristine LARB aggregates in water. The red shift of the surface plasmon extinction observed with increasing LARB-PdNP colloidal size indicates the collective optical response of the PdNP ensemble in the colloid.
