Effect of counterions on the physical properties of l-arginine in frozen solutions and freeze-dried solids

The objective of this study was to elucidate the physical properties of l-arginine and various counterion combinations in frozen aqueous solutions and in freeze-dried solids. l-Arginine remains amorphous in the highly concentrated non-ice phase in frozen solutions with a T′g (glass transition temperature of maximally freeze-concentrated solutes) of −41.4 °C. Some acids and salts (e.g., H3PO4, H2SO4, HNO3, and NaH2PO4) raised the T′g, whereas others (e.g., HCl, CH3COOH, HCOOH, Na2HPO4, and NaCl) had little effect or lowered the l-arginine T′g. Co-lyophilization with phosphoric acid also raised the glass transition temperature (Tg) of amorphous freeze-dried l-arginine solids. Arginine-H3PO4 combinations exhibited properties that led to either the stabilization or destabilization of a model protein (lactate dehydrogenase: LDH) during freeze-drying, depending on their concentration ratios. Fourier-transform infrared (FT-IR) and diffusion reflectance near-infrared (NIR) spectra indicated the presence of interactions between the amino and/or guanidyl groups of l-arginine and phosphate ions in the amorphous freeze-dried cakes. It was postulated that the interaction between l-arginine and the multivalent counterions, as well as an increase in hydrogen bonding network, reduced the mobility of molecules in the frozen solutions and freeze-dried solids.