Aggregation of nucleobases and metabolites: Adenine-theobromine trimers.

The selection of cytosine, guanine, thymine, and adenine as components of the information biopolymers was a complex process influenced by several factors. Among them, the intermolecular interactions may have played a determinant role. Thus, a deep understanding of the intermolecular interactions between nucleobases and other prebiotic molecules may help understand the first instants of chemical evolution. Following this hypothesis, we present here a combined spectroscopic and computational study of theobromine-adenine and thebromine-adenine trimers. While adenine is a nucleobase, theobromine was probably part of the prebiotic chemistry. The trimers were formed in jets and probed by a combination of UV and IR spectroscopic techniques. The spectra were interpreted in light of the predictions obtained using density-functional methods. The results suggest the existence of a subtle balance between formation of hydrogen bonds and π-π interactions. Thus, while theobromine-adenine tends to form complex in stacked structures, theobromine-adenine prefers formation of planar structures, maximizing the interaction by hydrogen bonds. The small energy difference between planar and stacked structures highlights the importance of accurately modeling the dispersion forces in the functionals to produce reliable predictions.