Ice Recrystallization Inhibition Using l-Alanine/l-Lysine Copolymers

Recently, poly(l-alanine-co-l-lysine) (PAK) was reported to exhibit an ice recrystallization inhibition (IRI) activity comparable to that of poly(vinyl alcohol), which has the highest IRI activity among synthetic polymers (ACS Macro Lett. 2021, 10, 1436–1442). As a continuation of the study, we synthesized a series of PAK copolymers to investigate the structure–property relationship of PAK on the IRI activity and its mechanism by varying the (1) composition of the l-alanine/l-lysine (A/K) ratio among 0/100, 50/50, 60/40, and 70/30 and (2) molecular weight of PAK among 4, 9, and 15 kDa at a fixed 50/50 ratio of A/K. As the hydrophobic l-alanine composition of PAK increased from 0 to 70%, the mean largest grain size (MLGS) relative to phosphate-buffered saline decreased from 74 to 11% at 1.0 mg/mL. As the molecular weight of PAK increased from 4 to 15 kDa, the MLGS decreased from 32 to 15% at 1.0 mg/mL. The ice nucleation temperature decreased as the hydrophobic l-alanine composition and the molecular weight of PAK increased. The thermal hysteresis (TH) of PAK was about 0.05 °C at 1.0 mg/mL and increased to 0.2 °C as the molecular weight increased from 4 to 15 kDa. The ice crystal size of ice cream showed a similar trend to IRI activity. To understand the underlying chemistry of the IRI effect of PAK, circular dichroism spectroscopy, a dynamic ice shaping study, and Fourier-transform infrared spectroscopy were carried out. These studies suggest that the α-helicity, hydrophobic/hydrophilic balance, molecular weight of PAK, and extent of interactions between polymers and ice crystals are related to the IRI activity, which extends to ice nucleation inhibition, TH of ice formation, ice shaping, and even the ice crystal size of ice cream.