© 2019 by the authors. Licensee MDPI, Basel, Switzerland. Living systems employ both covalent chemistry and physical assembly to achieve complex behaviors. The emerging field of systems chemistry, inspired by these biological systems, attempts to construct and analyze systems that are simpler than biology, while still embodying biological design principles. Due to the multiple phenomena at play, it can be difficult to predict which phenomena will dominate and when. Conversely, there may be no single rate-limiting step, but rather a reaction network that is difficult to intuit from a purely experimental approach. Mathematical modeling can help to sort out these issues, although it can be challenging to build such models, especially for assembly kinetics. Numerical and statistical methods can play an important role to facilitate the synergistic and iterative use of modeling and experiment, and should be part of a systems chemistry curriculum. Three case studies are presented here, from our work in peptide-based systems, to illustrate some of the tools available for model construction, model simulation, and experimental design. Examples are provided in which these tools help to evaluate hypotheses, uncover design principles, and design new experiments.

The rhizosphere, the narrow zone of soil around plant roots, is a complex network of interactions between plants, bacteria, and a variety of other organisms. The absolute dependence on host-derived signals, or xenognosins, to regulate critical developmental checkpoints for host commitment in the obligate parasitic plants provides a window into the rhizosphere’s chemical dynamics. These sessile intruders use H2O2 in a process known as semagenesis to chemically modify the mature root surfaces of proximal host plants and generate p-benzoquinones (BQs). The resulting redox-active signaling network regulates the spatial and temporal commitments necessary for host attachment. Recent evidence from non-parasites, including Arabidopsis thaliana, establishes that reactive oxygen species (ROS) production regulates similar redox circuits related to root recognition, broadening xenognosins’ role beyond the parasites. Here we compare responses to the xenognosin dimethoxybenzoquinone (DMBQ) between the parasitic plant Striga asiatica and the non-parasitic A. thaliana. Exposure to DMBQ simulates the proximity of a mature root surface, stimulating an increase in cytoplasmic Ca2+ concentration in both plants, but leads to remarkably different phenotypic responses in the parasite and non-parasite. In S. asiatica, DMBQ induces development of the host attachment organ, the haustorium, and decreases ROS production at the root tip, while in A. thaliana, ROS production increases and further growth of the root tip is arrested. Obstruction of Ca2+ channels and the addition of antioxidants both lead to a decrease in the DMBQ response in both parasitic and non-parasitic plants. These results are consistent with Ca2+ regulating the activity of NADPH oxidases, which in turn sustain the autocatalytic production of ROS via an external quinone/hydroquinone redox cycle. Mechanistically, this chemistry is similar to black and white photography with the emerging dynamic reaction-diffusion network laying the foundation for the precise temporal and spatial control underlying rhizosphere architecture.

Truncated and mutated amyloid-β (Aβ) peptides are models for systematic study-in homogeneous preparations-of the molecular origins of metal ion effects on Aβ aggregation rates, types of aggregate structures formed, and cytotoxicity. The 3D geometry of bis-histidine imidazole coordination of CuII in fibrils of the nonapetide acetyl-Aβ(13-21)H14A has been determined by powder 14N electron spin echo envelope modulation (ESEEM) spectroscopy. The method of simulation of the anisotropic combination modulation is described and benchmarked for a CuII-bis-cis-imidazole complex of known structure. The revealed bis-cis coordination mode, and the mutual orientation of the imidazole rings, for CuII in Ac-Aβ(13-21)H14A fibrils are consistent with the proposed β-sheet structural model and pairwise peptide interaction with CuII, with an alternating [-metal-vacancy-]n pattern, along the N-terminal edge. Metal coordination does not significantly distort the intra-β-strand peptide interactions, which provides a possible explanation for the acceleration of Ac-Aβ(13-21)H14A fibrillization by CuII, through stabilization of the associated state and low-reorganization integration of β-strand peptide pair precursors.

Semagenesis, the process of signal generation, is a novel signaling strategy first uncovered within the parasitic plants. Recent evidence suggests that the parasite's production of reactive oxygen species (ROS) has been focused externally to exploit the host's innate immunity. Here we use the inducer identified from decoding semagenesis, as well as other signaling strategies of the parasitic plants, to synchronize host commitments of Striga asiatica and reveal the molecular events that control plant development.

Vast arrays of structural forms are accessible to simple amyloid peptides and environmental conditions can direct assembly into single phases. These insights are now being applied to the aggregation of the Aβ peptide of Alzheimer's disease and the identification of causative phases. We extend use of the imaging agent Pittsburgh compound B to discriminate among Aβ phases and begin to define conditions of relevance to the disease state. Also, we specifically highlight the development of methods for defining the structures of these more complex phases.

Protein and peptide assembly into amyloid has been implicated in functions that range from beneficial epigenetic controls to pathological etiologies. However, the exact structures of the assemblies that regulate biological activity remain poorly defined. We have previously used Zn2+ to modulate the assembly kinetics and morphology of congeners of the amyloid β peptide (Aβ) associated with Alzheimer's disease. We now reveal a correlation among Aβ-Cu2+ coordination, peptide self-assembly, and neuronal viability. By using the central segment of Aβ, HHQKLVFFA or Aβ(13–21), which contains residues H13 and H14 implicated in Aβ-metal ion binding, we show that Cu2+ forms complexes with Aβ(13–21) and its K16A mutant and that the complexes, which do not self-assemble into fibrils, have structures similar to those found for the human prion protein, PrP. N-terminal acetylation and H14A substitution, Ac-Aβ(13–21)H14A, alters metal coordination, allowing Cu2+ to accelerate assembly into neurotoxic fibrils. These results establish that the N-terminal region of Aβ can access different metal-ion-coordination environments and that different complexes can lead to profound changes in Aβ self-assembly kinetics, morphology, and toxicity. Related metal-ion coordination may be critical to the etiology of other neurodegenerative diseases.

Metal ions (Zn(II)) are demonstrated as probes of amyloid structure in simple segments of the Aβ peptide, Aβ(13−21). By restricting the possible metal binding sites to His13/His14 dyad, we show that Zn2+ can specifically control the rate of self-assembly and dramatically regulate amyloid morphology via distinct coordination environments as characterized by X-ray absorption spectroscopy. The data establish that the single His13 is sufficient to coordinate Zn2+ productively for typical amyloid fiber formation, while a distinct Zn2+ coordination environment can be accessed in the presence of His13/Hi14 dyad to stabilize sheet/sheet associations and the transition to a ribbon/tube morphology.

Significant limitations have emerged in America's science training pipeline, including inaccessibility, inflexibility, financial limitations, and lack of diversity. We present three effective programs that collectively address these challenges. The programs are grounded in rigorous science and integrate through diverse disciplines across undergraduate, graduate, and postdoctoral students, and resonate with the broader community. We discuss these models in the context of current economic constraints on higher education and the urgent need for our institutions to recruit and retain diverse student populations and sustain the successful American record in scientific education and innovation.

Histidine kinases serve as critical environmental sensing modules, and despite their designation as simple two-component modules, their functional roles are remarkably diverse. In Agrobacterium tumefaciens pathogenesis, VirA serves with VirG as the initiating sensor/transcriptional activator for inter-kingdom gene transfer and transformation of higher plants. Through responses to three separate signal inputs, low pH, sugars, and phenols, A. tumefaciens commits to pathogenesis in virtually all flowering plants. However, how these three signals are integrated to regulate the response and why these signals might be diagnostic for susceptible cells across such a broad host-range remains poorly understood. Using a homology model of the VirA linker region, we provide evidence for coordinated long-range transmission of inputs perceived both outside and inside the cell through the creation of targeted VirA truncations. Further, our evidence is consistent with signal inputs weakening associations between VirA domains to position the active site histidine for phosphate transfer. This mechanism requires long-range regulation of inter-domain stability and the transmission of input signals through a common integrating domain for VirA signal transduction.

As the molecular top–down causality emerging through comparative genomics is combined with the bottom–up dynamic chemical networks of biochemistry, the molecular symbiotic relationships driving growth of the tree of life becomes strikingly apparent. These symbioses can be mutualistic or parasitic across many levels, but most foundational is the complex and intricate mutualism of nucleic acids and proteins known as the central dogma of biological information flow. This unification of digital and analog molecular information within a common chemical network enables processing of the vast amounts of information necessary for cellular life. Here we consider the molecular information pathways of these dynamic biopolymer networks from the perspective of their evolution and use that perspective to inform and constrain pathways for the construction of mutualistic polymers.