PCCP Seminar: David Osborn - Sandia Nat'l Laboratory

Sugar Formation without Life:  Pyruvic Acid and the Story of an Activated Aldehyde

Sugars, a class of carbohydrates, are one of four types of macromolecules required to enable life as we know it.  Almost all sugars are synthesized by plants, but the structure of plants requires carbohydrates, leading to the question of how sugars formed in a prebiotic environment, i.e., sugar formation without life.  Although the formose reaction (“form-”aldehyde to sugars,
“-ose”) has long been known to enable sugar formation without life, it requires aqueous solution, a strong base, and a specific counter ion.  Furthermore, this reaction has a long and confounding induction period that led Ronald Breslow to speculate on the involvement of an ill-defined “activated aldehyde” in its mechanism.  Meteorites older than the Earth contain sugars, providing evidence that sugar formation is likely possible without liquid water or warm temperatures.  In this talk, I’ll discuss the photodissociation of pyruvic acid (CH₃-C(O)-C(O)OH), an important keto-acid that is consumed in Earth’s troposphere primarily by photodissociation rather than OH-initiated oxidation.  Using the universal technique of Multiplexed Photoionization Mass Spectrometry, I will present direct evidence that 351-nm photodissociation of pyruvic acid produces CO₂ + methylhydroxycarbene (CH₃-C-OH), a high energy isomer of acetaldehyde.  From studies with d₁-pyruvic acid we argue that decarboxylation occurs at least partially by tunneling.  This photolytic source of a hydroxy-carbene opens the door to the gas-phase study of carbene reactivity.  We then show that the reaction of methylhydroxycarbene with acetaldehyde is indeed fast and forms only the ketose sugar acetoin (CH₃-C(O)-CH(CH₃)OH), and not the aldose that is also a credible possible product. Contrary to the mechanistic assumption that singlet carbenes react by insertion, this reaction proceeds via a hydrogen-bonded pre-reactive complex that submerges the reaction barrier below the energy of reactants, with the result that this reaction is rapid even at ultralow temperatures, and could be a source of sugars in cold, dry, prebiotic environments.