Information capacity of specific interactions

Significance The past 15 years have seen a proliferation of experimental techniques aimed at engineering self-assembled structures. These bottom–up techniques rely on specific interactions between components that arise from diverse physical mechanisms such as chemical affinities and shape complementarity attraction. Comparisons of specificity across such diverse systems, each with unique physics and constraints, can be difficult. Here we describe an information theoretic measure, capacity, to quantify specificity in a range of recent experimental systems. Capacity quantifies the maximal amount of information that can be encoded and then resolved by a system of specific interactions, as a function of experimentally tunable parameters. Our framework can be applied to specific interactions of diverse origins, from colloidal experiments to protein interactions. Specific interactions are a hallmark feature of self-assembly and signal-processing systems in both synthetic and biological settings. Specificity between components may arise from a wide variety of physical and chemical mechanisms in diverse contexts, from DNA hybridization to shape-sensitive depletion interactions. Despite this diversity, all systems that rely on interaction specificity operate under the constraint that increasing the number of distinct components inevitably increases off-target binding. Here we introduce “capacity,” the maximal information encodable using specific interactions, to compare specificity across diverse experimental systems and to compute how specificity changes with physical parameters. Using this framework, we find that “shape” coding of interactions has higher capacity than chemical (“color”) coding because the strength of off-target binding is strongly sublinear in binding-site size for shapes while being linear for colors. We also find that different specificity mechanisms, such as shape and color, can be combined in a synergistic manner, giving a capacity greater than the sum of the parts.