Impact of molecular speciation on crystal nucleation in polymorphic systems: The conundrum of γ glycine and molecular 'self poisoning'

The polymorphism of the simple amino acid glycine has been known for almost a century. It is also known that in aqueous solutions, at the isoelectric point (p/5.9), the metastable α polymorph crystallizes, while the stable γ form of glycine only nucleates at high and low pH. Despite the importance of understanding the process by which crystals nucleate, the solution and solid-state chemistry underlying this simple observation have never been explored. In this contribution, we have combined solution chemistry, crystallization, and crystallographic data to investigate the mechanisms by which this effect occurs. It is concluded that solution speciation and the consequent interactions between charged species and developing crystal nuclei determine the structural outcome of the crystallization process.