Direct Comparison of α- and γ-Glycine Growth Rates in Acidic and Basic Solutions: New Insights into Glycine Polymorphism

The growth rates of α- and γ-glycine in basic and acidic aqueous glycine solutions were investigated systematically using in situ microscopic observations on single crystal seeds. The most unexpected observation was that inorganic bases (e.g., NaOH and KOH) and acids (e.g., H2SO4) can significantly accelerate the growth of the metastable α-glycine. This finding is contrary to the recently postulated self-poisoning mechanism, which highlights that glycine ions, generated in a basic or acidic solution, inhibit α-glycine so as to permit γ-glycine to appear. Furthermore, it was observed that, in basic solutions, γ-glycine growth remains dominated by α-glycine growth, yet γ-glycine is crystallized preferentially. This strongly implies that the relative nucleation rates, not the relative growth kinetics of the two glycine polymorphs play a dominant role in determining the polymorphic outcome, providing new insights into the riddle of glycine polymorphism. The observed effects of the base and acid additives on the face growth rates of α-glycine and γ-glycine were rationalized at the molecular level.