Using crystal structure prediction to rationalize the hydration propensities of substituted adamantane hydrochloride salts

The crystal energy landscapes of the salts of two rigid pharmaceutically active molecules reveal that the experimental structure of amantadine hydrochloride is the most stable structure with the majority of low-energy structures adopting a chain hydrogen-bond motif and packings that do not have solvent accessible voids. By contrast, memantine hydrochloride which differs in the substitution of two methyl groups on the adamantane ring has a crystal energy landscape where all structures within 10 kJ mol^-1 of the global minimum have solvent-accessible voids ranging from 3 to 14% of the unit-cell volume including the lattice energy minimum that was calculated after removing water from the hydrated memantine hydrochloride salt structure. The success in using crystal structure prediction (CSP) to rationalize the different hydration propensities of these substituted adamantane hydrochloride salts allowed us to extend the model to predict under blind test conditions the experimental crystal structures of the previously uncharacterized 1-(methylamino)adamantane base and its corresponding hydrochloride salt. Although the crystal structure of 1-(methylamino)adamantane was correctly predicted as the second ranked structure on the static lattice energy landscape, the crystallization of a Z′ = 3 structure of 1-(methylamino)adamantane hydrochloride reveals the limits of applying CSP when the contents of the crystallographic asymmetric unit are unknown.The crystal energy landscapes of two model pharmaceutically active chloride salts were used to rationalize the different hydration behaviour of the salts on the basis of the packing of the ions and calculated solvent-accessible volumes in the low-energy structures. The crystal structure of 1-(methylamino)adamantane was characterized and correctly predicted under blind test conditions as the second ranked structure in lattice energy, but the failure to predict the Z′ = 3 structure for the corresponding hydrochloride salt of this relatively simple base illustrates the limits of blind crystal structure prediction.