Confirmation-dependent organic phosphor reveals amino acid nanoaggregates in ice with insight 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 prompt us to examine the frozen samples with cryo-transmission electron microscopy (cryo-TEM), which directly reveals uniform nanoaggregates. These nanoaggregates create distinct local microenvironments that influence the 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 may represent a prerequisite condition for polymerization, offering new insights into how primitive icy environments could have influenced peptide-based prebiotic chemistry.