Pathway control in the self-construction of complex precipitation forms in a Cu(II)-oxalate system
© Toth et al; licensee Chemistry Central Ltd. 2012
Received: 4 April 2012
Accepted: 17 July 2012
Published: 2 August 2012
Many biological systems contain complex precipitation patterns. These structures are considered to be the result of finely tuned and genetically encoded developmental pathways. The amount of encoded information needed to generate and manipulate these structures is poorly understood. Investigating the dynamics of spontaneous pattern formation in non-biological systems provides insights to the physio-chemical phenomena that biological systems must have harnessed for living systems and that modern scientists need to understand for complex nano-technological applications.
Here we show that highly complex, precipitation patterns similar to those found in biological systems can be formed in simple Cu(II)-oxalate systems. In these Cu(II)-oxalate systems, structures are constructed by a hierarchy of multiple processes that are precisely self-organized in space and time to form interconnected causal networks that generate complex and diverse structures dependent on construction trajectories that can be controlled by minor variations of initial conditions.
Highly complex precipitation patterns similar to those found in biological systems can be generated without a correspondingly complex set of instructions. Our result has implications for understanding early biotic systems that existed prior to the evolution of sophisticated genetic machinery. From an applications perspective, processes and structures that occur spontaneously are the building blocks for novel system chemistry based technologies where products are self-constructed. We also provide a simple model of chemical system that generates biomimetic structures for the study of fundamental processes involved in chemical self-construction.
The formation of complex precipitation structures in biological systems such as those found in diatoms, coccolithophorids, corals and seashell shave been the subject of intensive investigations [1–3]. Such precipitation structures are hierarchical, being constructed of many different substructures on nano, meso, micro and higher scales. Molecular biology is useful for understanding which genes are important for the generation of these structures, however, the physical and chemical processes that generate complex structures must also be studied in order to understand how biological systems evolved to manipulate them as well as for technological applications. Until recently, most chemistry-centered studies have focused on elucidating the chemical compositions of structures’ subunit crystals and methods for controlling crystal morphologies [3–5]. Here we show that highly complex precipitation structures can be formed in very simple Cu(II)-oxalate systems. Our results suggest that the formation of many highly complex structures seen in nature may occur by nucleating and regulating spontaneous construction processes.
Complex precipitation patterns are naturally found in both biological and non-biological systems. Biological precipitation patterns are precisely organized and are the products of multiple processes which, despite some progress made in the field of pattern formations and in self-assembly [6–10], are still beyond the technical capabilities of modern chemists. In contrast, complex non-biological precipitation formations such as stalagmites and stalactites , Liesegang Rings , geological structures , snowflakes and Silicate Gardens [14–22] have been synthesized in vitro. These structures may share some similarities to biological structures and are considered important in the study of the origin of life [23, 24]. However, these non-biological structures are usually the products of one process and one “building element” such as the crystallization of water molecules in the formation of snowflakes. The Cu(II)-oxalate systems we describe are more similar to the biological precipitation formations with multiple different “building blocks” and numerous processes that are precisely organized in space and time yet form spontaneously.
Systems Chemistry [25–27], similar to Systems Biology is concentrated mostly on the structures and properties of complex systems. Our understanding of how such systems can emerge is less developed and a general theory of self-construction does not yet exist. In this study we concentrate almost exclusively on how complex structures construct themselves.
Results and discussion
In the present system, a pellet of copper sulfate was immersed in solutions with different concentrations of oxalate and the processes that lead to complex structures were observed. In each case, the copper dissolved into the surrounding solution . Due to density differences, the more concentrated solution sinks to the bottom of the reaction dish and spreads across it to form a layer on the bottom. The bottom solution then moves outward from the pellet, while the upper solution moves towards the pellet.
As is the case in biological systems, the formation of these complex structures involves many processes. Physical processes include the diffusion of copper ions from the pellet to the oxalate solution, establishment of a density gradient resulting in the solute convection of concentrated solution downwards and away from the pellet across the bottom of the dish while less dense solution moves toward the pellet. Chemical processes include CuSO4 dissolution, formation of CuC2O4 crystals with two different morphologies, formation of crystals of Na2Cu(C2O4)2 × 2H2O with two different morphologies and a gel. The CuC2O4(s) precipitation disk serves to organize crystal growth. For oxalate concentrations in the range of 0.15-0.25 M, we observe metamorphosis i.e., the transition of one form into another (see Figure 2B, 2D). Usually, in hierarchical constructions, one structure is built from others. In the presented experiments, however, there is a full metamorphosis to a different structure without elements of the first structure remaining.
The sophistication of processes and complexity of structures that are generated by this simple system has ramifications for our understanding of primitive life and evolutionary processes. Some complex structures do not need to have evolved in a slow, step by step fashion and could exist in the absence of large amounts of genetic information if organisms could nucleate spontaneous, complex processes. Small changes in genes that control nucleation conditions could also lead to drastic new structural forms.
We have shown that highly complex, precipitation patterns similar to those found in biological systems can be formed in the Cu(II)-oxalate reaction (see Figure 5). This simple system presents an excellent model for investigating the principles of self-construction. Future enquires will explore the construction of more complex networks of chemical and physical processes and investigate their control via catalysts and inhibitors as seen in biological systems. Ultimately, further understanding of self-construction will enable us to comprehend the formation of complex structures in biology and develop technologies that lead to sophisticated self-constructed materials.
Understanding and mastering the process of formation of complex structures in chemical systems will lead to an understanding of formation of complex structures in nature. Mastering processes of controlling of networks of chemical processes will lead to new technologies, applied especially on the nano level, where the process of piece by piece assembly is not possible.
A 1.0 g pellet of copper sulfate (VWR) was prepared by grinding copper sulfate crystals in an electric grinder for 5 minutes and then casting it into a pellet using a pellet maker commonly utilized in IR studies. Each pellet was standardized to 1.0 cm in diameter and 4 mm in height. A 22 cm diameter Petri dish was leveled and sodium oxalate was poured into the dish. Pellets were then placed into the center of the reaction dish. Concentration of oxalate (VWR) was varied between 0.0 to 0.25 M. To determine the crystal composition, the copper contents were analyzed spectrophotometrically after dissolving crystals in a solution of ammonia. The oxalate concentration was determined by titration with permanganate after dissolving the crystals in sulfuric acid. The experiments were carried out at a temperature of 20 ± 1°C. Scanning electron microscope images were taken of the dried precipitate by a Hitachi S-4700 high-resolution scanning electron microscope.
This work was supported by the National Science Foundation (Grant No. CHE-0608631 and CHE-1011656).
We would like to thank our undergraduate students, from the University of Alaska Anchorage, that have conducted preliminary experiments: Leon RoseFigura, Ester Heo, Sheila Elano, Ashley Holombo.
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