Amino Acid Nanoaggregates in Water Ice Revealed by Conformation-Dependent Organic Phosphor: Implication for Prebiotic Chemistry

Freezing-induced enrichment of organic solutes within ice has long been hypothesized to play a pivotal role in prebiotic chemistry and the origins of life, yet direct evidence for the in-ice aggregation of dilute, water-soluble organics has remained elusive. Here, we employ a conformation-sensitive organic phosphor, 2-phenylbenzothiazole iodide (SNI), to uncover the formation of amino-acid nanoaggregates in water ice. Unusual, amino-acid-specific phosphorescence signatures from SNI-guided investigations prompted us to examine the frozen samples with cryo-transmission electron microscopy (cryo-TEM), which directly revealed uniform nanoaggregates. These nanoaggregates create distinct local microenvironments that influence photophysical properties of SNI, inducing distinct ground-state conformations that lead to conformation-dependent phosphorescence. Complementary theoretical calculations, molecular dynamics simulations, and temperature-variable Raman spectra suggest that amino acids, such as alanine, undergo a temperature-dependent proton-transfer process from their ammonium to carboxylate groups, enhancing their hydrophobicity and triggering aggregation upon freezing. This enrichment of amino acids in ice sets the stage for subsequent polymerization reactions, shedding new light on how primitive icy environments could have facilitated the emergence of peptide-based prebiotic chemistry.