Identifying molecules as biosignatures with assembly theory and mass spectrometry

The search for alien life is hard because we do not know what signatures are unique to life. We show why complex molecules found in high abundance are universal biosignatures and demonstrate the first intrinsic experimentally tractable measure of molecular complexity, called the molecular assembly index (MA). To do this we calculate the complexity of several million molecules and validate that their complexity can be experimentally determined by mass spectrometry. This approach allows us to identify molecular biosignatures from a set of diverse samples from around the world, outer space, and the laboratory, demonstrating it is possible to build a life detection experiment based on MA that could be deployed to extraterrestrial locations, and used as a complexity scale to quantify constraints needed to direct prebiotically plausible processes in the laboratory. Such an approach is vital for finding life elsewhere in the universe or creating de-novo life in the lab

Exploration of the accessible chemical space of acyclic alkanes.

Saturated acyclic alkanes show steric strain if they are highly branched and, in extreme cases, fall apart rapidly at room temperature. Consequently, attempts to count the number of isomeric forms for a given molecular formula that neglect this physical consideration will inevitably overestimate the size of the available chemical space. Here we derive iterative equations to enumerate the number of isomers (both structural and optical are considered separately) for the alkane series that take into account the inherent instability of certain carbon skeletons. These function by filtering out certain substructures from the graph representation of the molecule which have been found to be thermodynamically unstable. We use these relations to report new estimates of the size of physically accessible chemical space for acyclic alkanes and show that for large molecules there are more isomers that are structurally disallowed than there are allowed