Heterogeneous media, such as micro-structured aqueous environments, could offer an alternative approach to the synthesis of biopolymers with novel functions. Structured media are here defined as specialized, self-assembled structures that are formed, e.g, by amphiphiles, such as liposomes, emulsion compartments and lipid-bilayer lattices. Another kind of media is represented by self-assembled phases in the reaction medium, e.g., in water-ice matrices that are formed by two co-existing aqueous phases (a solid phase and a concentrated liquid phase) when an aqueous solution is cooled below its freezing point, but above the eutectic point. These media have the capacity to assemble chemical molecules or complex catalytic assemblies into unique configurations that are unstable or unavailable in bulk aqueous phases. Reactions can then proceed which do not readily occur in homogeneous solutions. To gauge the potential of this idea, we have investigated the non-enzymatic polymerization of RNA from monomers in the presence of various catalysts. Metal-ion catalyzed condensation of activated monomers into RNA polymers proceed in the water-ice eutectic phase very efficiently1,2. In template-directed RNA polymerization, the initial elongation rates clearly depended on the complementarity of the monomers with the templating nucleobases3. However, metal-ion catalyzed reactions deliver RNA analogs with heterogeneous linkages. Moreover, the usefulness of this medium in the form of quasi-compartmentalization extends beyond metal-ion catalysis reactions, as we have recently demonstrated the catalytic power of a dipeptide, SerHis, for the regioselective formation of phosphodiester bonds. These results in conjonction with the synthesis of nucleobases at -78˚C, the demonstration of ribozyme activity (RNA ligase ribozyme4 and polymerase activity5) suggest that the cold conditions and the resulting eutectic phase formation could have allowed an RNA-supported world on our Earth and possibly on other icy planets. References 1Kanavarioti, A, Monnard, P-A and Deamer, DW (2001) Astrobiology, 1, 271. 2Monnard, P-A, Kanavarioti, A and Deamer, DW (2003) J. Am. Chem. Soc., 125, 13734. 3Monnard, P-A and Szostak, JW (2008) J. Inorg. Biochem., 112, 1104. 4Vlassov, A, Johnston, BH, Landweber, LF, and Kazakov, SA (2004) Nucl. Acids. Res., 32, 2966. 5Attwater, J, Wochner, A, Pinheiro, VB, Coulson, A and Holliger, P (2010) Nat. Commun., 1, 76.