Assembly Theory Provides New Estimate of Chemical Space Size
Researchers applying Assembly Theory to chemical space estimation have calculated that the number of possible drug-like molecules (below 500 Da molecular mass) may reach approximately 10^117, far exceeding the conventional heuristic estimate of 10^60. Assembly Theory quantifies molecular complexity via the minimum number of recursive bond-joining steps needed to construct a molecule, allowing chemical space to be partitioned and bounded by complexity level. The finding suggests that prior estimates dramatically undercount chemical diversity by ignoring structural and synthetic complexity.
A preprint submitted to arXiv by Prof. Leroy Cronin and colleagues proposes a first-principles estimate of the size of chemical space using Assembly Theory, a framework that measures the causal complexity required to form a molecule through its Assembly Index. The Assembly Index is defined as the minimum number of recursive bond-joining operations needed to construct a molecular graph, providing a measurable complexity metric. The authors show that chemical space grows at least super-exponentially and at most double-exponentially with respect to the Assembly Index. Using the GDB-13 molecular database as a reference for growth-rate calibration, they model how chemical space expands with increasing complexity and contracts under structural constraints such as atom and bond types, ring count, ring size, and chemical motifs. Under drug-like constraints comparable to standard estimates — including a molecular mass ceiling of 500 Da — the analysis yields roughly 10^117 possible molecules at Assembly Index 25, compared to the widely cited heuristic of 10^60. The study also explores biologically relevant motifs and identifies structurally significant molecules near the accessible boundaries of assembly-defined spaces. As a preprint, the work has not yet undergone formal peer review.
What's missing
As a preprint, this work has not yet been peer-reviewed, and independent validation of the growth-rate modeling and the choice of GDB-13 as a calibration reference has not been established. Open questions include whether the Assembly Index bounds are tight enough to be practically informative and how the framework handles stereochemistry and conformational diversity.
What different sources said
- arXiv q-bioCenter
Elucidating the Size of Chemical Space with Assembly Theory
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