Emerging Complexity in Supramolecular Systems
Strasbourg Complex Systems Roadmap (January 2012)
Participants
- [Nicolas Giuseppone]
- Mihail Stadler
- Emilie Moulin,
- Antoine Bonnefont,
- Franck Hoonakker,
- [Mario Ruben]
- Jean-Marc Planeix,
- Mourad Elhabiri,
- Stéphane Mery
Keywords
Hierarchical structures, Dynamic systems, Adaptive behaviour, Molecular evolution, Smart functional systems, Information-gaining systems
Introduction
Complexity can be defined as C = M*I*I
M: Multiplicity
- One single molecule can present several properties e.g. Multiplicity of binding sites, number of energy levels....
- Mutiplicity of components: number of components (molecular) /constituents (supramolecular) in the system
I: Interaction
- Complentaries of shapes, of charges, of energy levels (Program writing / reading)
- Thermodynamic and kinetic of the interaction (reversibility, lability), covalent / non-covalent bonds, short-range/long-range
- Interactions of molecules with their environments (possibly in flux of energies far from equilibrium)
I: Integration
- Collective structuring
- In space: From sub-nano, to meso, to macro
- In time: Modulation of structures, oscillations
- Emergence of new properties because of the network topologies (feedback loops)
- Emergence of new functions
Grand Challenges
- Specificity of interactions and integrations
- Reaching emergent properties in complex supramolecular systems
- Applications of complex supramolecular systems and societal implications
- Teaching complex systems in chemistry (Strasbourg Erasmus Mundus)
1. Designing supramolecular systems able to generate complexity
The conception of these complex supramolecular systems require first to understand and engineer specific tools before bringing them all together.
Specificity of interactions and integrations
Synthetic chemistry - often supported by modelling - remains the prerequisite to design any molecular and supramolecular systems. Because of the specificity of supramolecular objects which leads to hierarchical structuring, the information contained in the basic building blocks should be precisely tailored. This remains true for small host-guest complexes with a high degree of organization, but also for softer objects with long range organizations.
Spatio-temporal dynamics at the Multiplicity, Interaction, and Integration levels
Thermodynamics governs (supra)molecular systems at a number of levels and in particular, the transient conformations of individual molecules can affect their interactions with the other constituents of the system. The constitutional dynamic, which refer to objects reversibly connected by one another in larger aggregates, is also under thermodynamic control, although they can be pushed far from equilibrium when crossed by flux of energies, including photochemical and electrochemical triggers, electric and magnetic fields, shearing forces, gradients of chemical species. Dynamics is also a crucial property of chemical networks of reactions, and the relative ratii of concentrations and kinetic rates, depending on the network topologies, provide new properties such as stability, robustness, spatial and temporal organization such as patterns, oscillations, and waves. Together, the dynamics occuring at the molecular, supramolecular and network levels are merged to allow adaptation processes.
Chemical reversibility as a requirement for evolvability
The reversibility of chemical reactions and supramolecular interactions is mandatory for two main aspects. The first one concerns the necessity to explore the whole combinatorial space containing all the potential structures. These species are not necessary expressed by the system, but should be able to emerge when necessary (virtuality). The second aspect, which is a consequence of the first one, is that reversible systems will be able transiently to express an object which will then be destructed to express another one for other environmental conditions.
Cooperativity in the integration processes and as part of modulations
The idea of cooperativity is illustrated by many biological systems such as allosteric effects in enzymes, or ordered mesophases at longer distances with dynamic collective behaviors such as those encountered in cellular membranes. In addition, the neural connections in brain represents also a source of inspiration for network cooperativity, integrating feedback loops and spatial coupling processes for enhanced information processing. Finally, because of the multiplicity of components in complex supramolecular systems, entropy represents an important drawback for the amplification of the adapted species. Thus, fast kinetic amplification processes, in particular in the form of self-replicating loops, are necessary to overcome entropy and disorder. Auto-catalysis and cross-catalysis are also phenomena which can create differential rates of reaction in these systems resulting in gradients effects and thresholds effects.
2. Reaching emergent properties in complex supramolecular systems
Another technical challenge concerning complex supramolecular system will be to combine all the previously described tools to benefit of their interplay in order to permit the emergence of new properties, for both the system structure and resulting properties.
Diversity
The diversity results from the multiplicity of the systems in terms of number of components, of number and nature of interactions they can generate one another, and of reversiblity of these interactions which allow a full exploration of the combinatorial space.
Selection
The selection results from the specificity of the interactions, from the topology of the reaction networks (random, scale free, auto-catalytic pathways), from the kinetics of diffusion, from the overall reversibility of the system, and from the interactions with the environment possibly far from equilibrium in open systems
Evolution
Supramolecular systems will be ultimately able to evolve if they can generate the components necessary to sustain their own structures, if they can perform mutations in relation to their environment (adaptation), and if they can self-replicate to grow their population.
New functions
Complex supramolecular systems are by essence multi-functional systems which are able to produce several functions from a common set of basic constituents. The expression of these functions can vary in space and time, depending on the internal parameters of the systems, and on the external pressure of environment. In addition, emergence of new functions can appear from these systems, although they were not contained within their individual components. This is allowed by numerous synergistic effects in multi-component systems and networks, including allostery, short and long range spatial couplings, compartmentalization, and feedback loops.
Open questions
- Is supramolecular complexity (one of the) the support to produce thinking matter?
- If yes, is this pathway continuous or does it present at one point a strong nonlinearity in evolution? Information/consciousness?
3. Applications of complex supramolecular systems and societal implications
Supramolecular structures and self-assemblies have already proved their importance in the implementation of new applications in various domains. Their systems counterpart will certainly be of even greater interest for the next generation of smart architectures and devices with multifunctional properties.
Medicine
Supramolecular systems can help in the synthesis of new drugs and imaging agents with better efficiency, for instance by using dynamic combinatorial chemistry to optimize the inhibitor for a given biological target. Other recent complex approaches including enzyme directed evolution and DNA templated approaches will also certainly represent important tools for drug design. One might ultimately imagine the synthesis of pro-drugs which will become active only when necessary in vivo. In addition, drug delivery and transfections, which are often supported by large self-assemblies, are typical problems which will benefit from a system approach.
Cellular biology
Because of their intrinsic bottom-up constructions, supramolecular systems are crucial to understand how new properties emerge from a percolation of networks. In that sense, systems chemistry is complementary to systems biology which tackles complex problems by a deconvolution approach. Another long standing problem in life science is related to the prediction of protein folding, also based on multiple dynamics of supramolecular interactions. A better fundamental understanding of these phenomena will contribute to solve this problem and to design new biomimetic systems.
Chemistry and materials
Complex structuring and dynamic features of supramolecular systems will be of increasing interest for many new chemical approaches, in particular at the frontier with materials science, a domain with major societal implications. The new generation of smart materials will be necessary responsive, adaptive, multifonctionnal, and possibly self-healing. Systems-based materials can be of importance in the following fields: materials for catalysis, for CO2 capture and water purification, for organic electronics and solar cells, and for information (possibly parallel) processing.