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- Periodic Temperature Variation as an Energy Source for Prebiotic Chemistry
- Christian Mayer, University of Duisburg-Essen, Germany
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- After the formation of basic biomolecules, some sort of evolution process started...
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- ... maybe something like the fascinating RNA World, or the intriguing GARD theory, or a sophisticated metabolic network...
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- power?
- ... but what could have provided theDriving Forceor theEnergy Sourcefor these processes?
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- What aboutperiodic temperature changeA century ago, they invented a clock that is permanently powered by daily variation of a single degree...
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- Hm? But how can a periodic variation of the temperature provide energy?
- Well, we actually deal with aHeat Engine!or, more precise,a generalizedCarnot Engine!
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- day
- night
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- This is how the clock works: rising temperature makes a gas expand, moving a pinwheel with the spring by one notch. On cooling down, a small lever disengages, the motion of the pinwheel is locked. The spring remains loaded and drives the clock.
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- Great! Let's try to simulate that on an RNA world model reaction, based on the Eyring theory and known thermodynamic data.Two RNA strands B and C are competing for a single strand A, with the duplex AC being slightly preferred versus AB. Then let's apply a periodic temperature variation and see what happens...
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- It takes up heat during the day and loses heat during the night. The difference between both heat quantities is then turned into usable energy, following the First and Second Law of Thermodynamics.
- In fact, in each cycle we see the duplexes AB and AC rapidly adopting the high temperature equilibrium, but lagging behind the low temperature equilibrium.
- Yes, and on the right we find small spontaneous bursts of heat in each cycle. They represent 2.1 kJ/mol of frozen chemical energy.
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- Just before these points of spontaneous release, any catalytic activity will facilitate uphill reactions, let it be RNA growth, steps in a metabolic network or micellar reproduction...
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- This principle is also valid for prebiotic chemical reactions. At rising temperatures, the equilibrium will shift following thermodynamics. On cooling down, the inverted process will be partially blocked by slowing kinetics. This corresponds to the disengaging lever...
- Well, for that let's go back to good old Carnot theory. Commonly, the cycle is represented either as a PV or as a TS diagram. PV doesn't make sense here, but TS does. This is the diagram for our RNA strand competition cycles. The enclosed area is the amount of usable work, the actual thermodynamic driving force provided by temperature cycling!
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- For details and more calculations, see here
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- But how much chemical energy will we get out of this heat engine cycle for general reactions?
- Great, So temperature cycling is a very strong and versatile power source for any prebiotic evolution process!But aren't there alternatives?
- Yes, there are, like wet-dry cycling or phase separation. But all of these alternatives are quite destructive. For example, they disintegrate vesicles or micelles...
- Temperature cycling is much softer on sensitive structures. Still, it remains a powerful driving force for chemical reactions.
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- All in all, periodic temperature variation is a universal power source for prebiotic reactions, even in presence of very sensitive structures
- It can occur on any planetary body, on moons and even on asteroids. With a given amplitude, it is especially efficient at lower temperatures
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- This given, the general amount of free enthalpy can be easily calculated using the van't Hoff equation. With some rearrangement, we can connect the chemical driving force on the left with the reaction enthalpy and the Carnot efficiency term on the right.
- So somewhere out there...
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- ... the clock of chemical evolution may be ticking!