Guanine Assembly at Ice–Water Boundaries under Early Earth–Like Na+/K+ Ionic Conditions

Freeze–thaw cycles in icy environments have been proposed as potential drivers of prebiotic molecular organization, yet the behaviour of individual nucleobases under such fluctuating conditions remains insufficiently understood. Here, we show that guanine undergoes progressive hypochromicity during repeated freeze–thaw cycling under early Earth–like Na⁺/K⁺ ionic conditions, consistent with increased stacking‑related and hydrogen‑bonding interactions. The hypochromicity observed after five days partially recovers at 5 °C and is further restored by brief heating to 40 °C, consistent with a partially reversible interaction. In contrast, cytosine exhibits no detectable spectral changes and remains predominantly monomeric under identical conditions. Quantum chemical calculations reveal that guanine dimers and K⁺‑stabilized quartets possess substantially greater association energies than cytosine assemblies, providing a physicochemical basis for the observed differences. These findings suggest that freeze–thaw–driven guanine organization at ice–water boundaries may represent a plausible route toward prebiotic molecular structuring in cold, icy environments.