Characterization of indochrome C: a novel blue pigment with cryoprotective potential from Arthrobacter antioxidans QL17T isolated from Mount Everest.

A water-soluble blue pigment, indochrome C, was isolated from Arthrobacter antioxidans QL17T and structurally characterized as a glycosylated diazaindophenol (C20H23N3O12) bearing a C-linked pyranose moiety. Its biosynthesis was strongly cold-enhanced, with yields at 5 °C and 15 °C approximately threefold higher than those at 25 °C. Pigment-deficient and overproducing mutants generated by heavy-ion beam mutagenesis confirmed its role in freeze-thaw protection. Indochrome C significantly improved bacterial survival during freezing, reduced intracellular oxidative stress, and exhibited strong ABTS and DPPH radical scavenging activity. Splat cooling assays demonstrated concentration-dependent ice recrystallization inhibition, comparable to that of trehalose on a mass basis. Differential scanning calorimetry revealed moderate freezing point depression, reduced melting enthalpy, and a lower fraction of freezable water, while molecular dynamics simulation supported its interaction with the ice-surface through hydrogen bonding. Indochrome C remained stable over pH 4-10 and at temperatures up to 60 °C, and no adverse effects were observed under the tested conditions. These findings establish indochrome C as a promising natural blue pigment with cryoprotective potential in food and biotechnology.