Crystal structures of potassium and cesium salts of adenine: the role of alkali cations

This study reports the crystal structures of potassium and cesium salts of adenine (K-adenine and Cs-adenine) from the perspective of the interaction of alkali cations with purine nucleobases. Unlike previously-known guanine salts, both K-adenine and Cs-adenine are anhydrous, with the counter ions (K+ and Cs+) directly coordinating to the ring nitrogens of adenine anions. In both structures, the crystal packing is predominantly determined by cation-anion interactions, with additional stabilization through hydrogen-bonding of neighbouring adenines. Attempts to crystallise either the cesium salt of guanine or the sodium salt of adenine were unsuccessful. To explain this trend, quantum-chemical calculations were performed to rationalise the preferences of sodium, potassium, and cesium cations to coordinate either with water or adenylate/guanylate anions. The exchange energies of cation-anion complexes reveal that sodium cations exhibit a preference for water or guanylate coordination via oxygen, while cesium cations prefer adenylate coordination via nitrogen functions, avoiding water interaction. Potassium exhibits an intermediate trend. Overall, this research offers insights into interactions between alkali-cations and organic anions, aiding the development of new crystalline compounds and co-crystals.