Chirality is ubiquitous in biology, including in biomineralization, where it is found in many hardened structures of invertebrate marine and terrestrial organisms (for example, spiralling gastropod shells). Here we show that chiral, hierarchically organized architectures for calcium carbonate (vaterite) can be controlled simply by adding chiral acidic amino acids (Asp and Glu). Chiral, vaterite toroidal suprastructure having a ‘right-handed' (counterclockwise) spiralling morphology is induced by L-enantiomers of Asp and Glu, whereas ‘left-handed' (clockwise) morphology is induced by D-enantiomers, and sequentially switching between amino-acid enantiomers causes a switch in chirality. Nanoparticle tilting after binding of chiral amino acids is proposed as a chiral growth mechanism, where a ‘mother' subunit nanoparticle spawns a slightly tilted, consequential ‘daughter' nanoparticle, which by amplification over various length scales creates oriented mineral platelets and chiral vaterite suprastructures. These findings suggest a molecular mechanism for how biomineralization-related enantiomers might exert hierarchical control to form extended chiral suprastructures.
Since Pasteur first successfully separated right-handed and left-handed tartrate crystals in 1848, the understanding of how homochirality is achieved from enantiomeric mixtures has long been incomplete. Here, we report on a chirality dominance effect where organized, three-dimensional homochiral suprastructures of the biomineral calcium carbonate (vaterite) can be induced from a mixed nonracemic amino acid system. Right-handed (counterclockwise) homochiral vaterite helicoids are induced when the amino acid
l
-Asp is in the majority, whereas left-handed (clockwise) homochiral morphology is induced when
d
-Asp is in the majority. Unexpectedly, the Asp that incorporates into the homochiral vaterite helicoids maintains the same enantiomer ratio as that of the initial growth solution, thus showing chirality transfer without chirality amplification. Changes in the degree of chirality of the vaterite helicoids are postulated to result from the extent of majority enantiomer assembly on the mineral surface. These mechanistic insights potentially have major implications for high-level advanced materials synthesis.
The
guiding principle for mineralized tissue formation is that
mineral growth occurs through the interaction of Ca2+ and
phosphate ions with extracellular matrix (ECM) proteins. Recently,
nanoengineered DNA structures have been proposed as mimics to ECM
scaffolds. However, these principles have not been applied to mineralized
tissues. Here, we describe DNA nanostructures, namely, a DNA nanotube
and a DNA origami rectangle that are site specifically functionalized
with a mineral-promoting “SSEE” peptide derived from
ECM proteins present in mineralized tissues. In the presence of Ca2+ and phosphate ions (mineralizing conditions), site-specific
calcium phosphate formation occurred on the DNA nanostructures. Amorphous
calcium phosphate or hydroxyapatite was formed depending on the incubation
time, shape of the DNA nanostructure, and amount of Ca2+ and phosphate ions present. The ability to design and control the
growth of hydroxyapatite through nanoengineered scaffolds provides
insights into the mechanisms that may occur during crystal nucleation
and growth of mineralized tissues and can inspire mineralized tissue
regeneration strategies.
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