Article Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Occluded Organic Nanofibers Template the Hierarchical Organization of Nanosized Particles in Calcium Oxalate Raphides of Musa spp Wenjun Zhang, Jialin Chi, Lijun Ma, and Lijun Wang* College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China S Supporting Information *
ABSTRACT: The formation of needle-shaped calcium oxalate crystals called raphides is unique to plants, in which related matrix proteins control crystallization of raphides at biomacromolecule−mineral interfaces with convoluted internal structure and complex morphology. However, investigations for understanding intermediate structures and the underlying mechanisms of raphide biomineralization have been lacking. We present a more detailed single-raphide composition by nanoscale secondary ion mass spectrometry (NanoSIMS) mapping to reveal the presence of individual organic fibers embedded inside raphides. This is in situ observed by imaging demineralization of individual raphides using atomic force microscopy (AFM) to unveil the template-mediated organized aggregation of calcium oxalate nanoparticles via a nonclassical particle-based pathway; internal structures are analyzed using high-resolution transmission electron microscopy (HRTEM) to demonstrate the multistep phase transformation by beam-induced coarsening through intermediates of nanoparticles from amorphous calcium oxalate (ACO) to calcium oxalate trihydrate (COT)/calcium oxalate dihydrate (COD) to the final product of elongated and tapered hexagonal crystals of calcium oxalate monohydrate (COM). The findings at the single-raphide level may improve the fundamental understanding of the structural and morphological complexity in biominerals evolved for survival and adaptation occurring in most plant taxa.
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INTRODUCTION Calcium oxalate crystals are the primary constituent of human renal stones as pathological calcification.1−3 The monohydrate form (whewellite, Ca(C2O4)·H2O, COM) commonly occurs along with the less stable forms of dihydrate (weddellite, Ca(C2O4)·2H2O, COD) and trihydrate (caoxite, Ca(C2O4)· 3H2O, COT).4 As the dominant minerals, they are also present in all major groups of photosynthetic organisms in terrestrial and aquatic habitats from algae to gymnosperms,4−7 and lichens8 as well as fungi.9 Some recent findings provided their likely functions in regulating calcium and oxalate ions and physical protection against herbivory.10−12 Recently, Tooulakou et al. demonstrated that plant calcium oxalate crystals may act as an internal CO2 source to provide subsidiary carbon for photosynthetic assimilation.13 Korth et al. used Medicago truncatula mutants to demonstrate a potential clue of genes controlling calcium oxalate crystal formation, morphology, and localization in plants.14 Calcium oxalate occurs naturally in a wide range of morphologies, including tetrahedron-like grains and regular prisms, and their twins and clustered aggregates as druse, as well as bundles of needle-shaped crystals called raphides.5 Complex raphides form within the vacuoles of specialized cells known as idioblasts where they compartmentalize the raphide formation that is unique in plants.4,5 In addition to matrix membranes, organic macromolecules regulate the growth of these needle-shaped crystals.15,16 Our previous results showed that self-assembled protein nanofibers are buried within © XXXX American Chemical Society
raphides and template the organized aggregation of calcium oxalate nanoparticles to grow into elongated and tapered hexagonal crystals.17 This suggests a mechanism of crystallization by particle aggregation and subsequent fusion during the formation of raphide crystals in the presence of biomacromolecules. Growth by aggregation of particles is now considered as a nonclassical pathway,18 challenging the simple assumptions of classical crystallization theory19 in explaining the nucleation and growth of calcium oxalate crystals based on the spiral20−22 and two-dimensional (2D) island growth model.19,23 The nonclassical pathway emphasizes the aggregation of small particles nucleated at the liquid−solid and solid−solid interfaces,18,24,25 particularly aggregation and coalescence of amorphous particles that are fundamental processes in the creation of highly complex morphologies of biominerals.26−30 These nanoparticles subsequently transform into crystalline phases.31 In the calcium oxalate system, amorphous precursor phases chemically synthesized or biogenically formed have also been observed.32−35 A recent study showed that calcium oxalate forms after Ca2+ and C2O42− association into polynuclear stable complexes that aggregate into larger assemblies, from which amorphous calcium oxalate nucleates.36 However, there is no direct evidence to demonstrate phase Received: November 11, 2017 Revised: December 19, 2017 Published: December 19, 2017 A
DOI: 10.1021/acs.cgd.7b01578 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
tips for ScanAsyst mode, k = 0.7 N/m and tip radius
