Equilibrium and non-equilibrium furanose selection in the ribose isomerisation network

The exclusive presence of beta -D-ribofuranose in nucleic acids is still a conundrum in prebiotic chemistry, given that pyranose species are substantially more stable at equilibrium. However, a precise characterisation of the relative furanose/pyranose fraction at temperatures higher than about 50 degrees C is still lacking. Here, we employ a combination of NMR measurements and statistical mechanics modelling to predict a population inversion between furanose and pyranose at equilibrium at high temperatures. More importantly, we show that a steady temperature gradient may steer an open isomerisation network into a non-equilibrium steady state where furanose is boosted beyond the limits set by equilibrium thermodynamics. Moreover, we demonstrate that nonequilibrium selection of furanose is maximum at optimal dissipation, as gauged by the temperature gradient and energy barriers for isomerisation. The predicted optimum is compatible with temperature drops found in hydrothermal vents associated with extremely fresh lava flows on the seafloor. Furanose species have a key role in the chemistry of life despite their instability over pyranose ones. The authors, through NMR characterization of the anomeric ratios at equilibrium and a non-equilibrium theoretical treatment, show that a steady temperature gradient, at temperatures relevant to the early Earth, favors furanose over pyranose isomers.