life Review Monosaccharides and Their Derivatives in Carbonaceous Meteorites: A Scenario for Their Synthesis and Onset of Enantiomeric Excesses George Cooper 1,*, Andro C. Rios 1,2,* and Michel Nuevo 1,3 ID 1 NASA-Ames Research Center, Moffett Field, CA 94035, USA; [email protected] 2 Blue Marble Space, 1001 4th Ave, Ste 3201, Seattle, WA 98154, USA 3 Bay Area Environmental Research Institute, NASA Research Park, Moffett Field, CA 94035, USA * Correspondence: [email protected] (G.C.); [email protected] (A.C.R.) Received: 5 June 2018; Accepted: 22 August 2018; Published: 27 August 2018 Abstract: Carbonaceous meteorites provide the best glimpse into the solar system’s earliest physical and chemical processes. These ancient objects, ~4.56 billion years old, contain evidence of phenomena ranging from solar system formation to the synthesis of organic compounds by aqueous and (likely) low-temperature photolytic reactions. Collectively, chemical reactions resulted in an insoluble kerogen-like carbon phase and a complex mixture of discrete soluble compounds including amino acids, nucleobases, and monosaccharide (or “sugar”) derivatives. This review presents the documented search for sugars and their derivatives in carbonaceous meteorites. We examine early papers, published in the early 1960s, and note the analytical methods used for meteorite analysis as well as conclusions on the results. We then present the recent finding of sugar derivatives including sugar alcohols and several sugar acids: The latter compounds were found to possess unusual “D” enantiomeric (mirror-image) excesses. After discussions on the possible roles of interstellar grain chemistry and meteorite parent body aqueous activity in the synthesis of sugar derivatives, we present a scenario that suggests that most of Earth’s extraterrestrial sugar alcohols (e.g., glycerol) were synthesized by interstellar irradiation and/or cold grain chemistry and that the early solar disk was the location of the initial enantiomeric excesses in meteoritic sugar derivatives. Keywords: carbonaceous meteorites; interstellar photolysis; enantiomeric excess; monosaccharide; sugars; sugar acid; aldoses; aldonic acid; sugar alcohol; glycerol 1. Introduction A subset of meteorites classified as “carbonaceous” preserves a valuable glimpse of early solar system chemical and physical processes. This is particularly true of the CI, CM, and CR classes of carbonaceous meteorites, which are viewed as the most unaltered (pristine) since their formation [1]. The majority of carbonaceous meteorites are assigned to the “carbonaceous chondrite” class of meteorites, and we will therefore refer to them as CCs. Their constituent pre-solar grains [2] are an intriguing illustration of their antiquity, as they include the oldest known condensates in the solar system, ~4.56-billion-year-old calcium-aluminum inclusions in the Allende and other CCs [3]. Presolar grains also contain evidence of even earlier events such as the origin of the solar system. Isotopes (60Fe, etc.) show that supernovae likely had a role in triggering solar system formation [3–6]. SiC grains, direct condensates from supernova, are found in several CCs such as the Murchison meteorite [5,7]. The analysis of Cr isotopes in several oxide grains from the Orgueil CC also implicates supernovae in solar system formation [3,8]. In addition to supernovae, the presence of multiple other types of stars at (or before) the birth of the solar system [2–7] indicates that sufficient ambient radiation was available for the chemical synthesis Life 2018, 8, 36; doi:10.3390/life8030036 www.mdpi.com/journal/life Life 2018, 8, x 2 of 29 In addition to supernovae, the presence of multiple other types of stars at (or before) the birth of Lifethe2018 solar, 8, 36system [2–7] indicates that sufficient ambient radiation was available for the chemical2 of 29 synthesis of organic compounds. For example, analysis of CCs yielded strong evidence of the role of gas-phase photolysis in shaping the properties of certain meteoritic organic sulfur compounds, of organic compounds. For example, analysis of CCs yielded strong evidence of the role of gas-phase sulfonates acids (R-SO3−) (see Section 3.3.3). Characteristics of the organic phase of CCs provide photolysisevidence that in shaping organic thesynthetic properties processes of certain were meteoritic taking place organic throughout sulfur compounds, solar system sulfonates formation acids and − (R-SOevolution.3 ) (see This Section is significant 3.3.3). Characteristics for origin of oflife the research organic phaseas CCs of provide CCs provide the only evidence window that organicto this syntheticprebiotic organic processes chemistry were taking and can place be throughoutstudied in gr solareat detail. system The formation most studied and evolution.CC, to date, This is the is significantMurchison for meteorite. origin of The life researchfall of this as meteorite CCs provide (in theMurchison, only window Australia, to this 1969 prebiotic) was organicdirectly chemistryobserved and canit was be studied collected in greatvery detail. soon Theafter. most This studied minimized CC, to date,contamination is the Murchison at a time meteorite. when Thethe falllunar of thisprogram meteorite of NASA (in Murchison, had developed Australia, 1969)protocols was directlyfor the observed sampling and and it was collection collected veryof very soon clean after. Thisextraterrestrial minimized samples. contamination In the at last a time few when decades, the lunar examination program of NASAseveral had CCs developed has revealed protocols that the for theorganic sampling carbon and consists collection of an of insoluble very clean “macromole extraterrestrialcular” samples. carbon In phase, the last ~50–99% few decades, of the total examination organic ofcarbon several [9–12], CCs hasas revealedwell as multitude that the organic of discreet carbon and consists relatively of an insolublelabile soluble “macromolecular” organic compounds carbon phase,[9,10,13,14] ~50–99% that of demonstrated the total organic an carbonactive [9organic–12], as chemistry well as multitude network of discreetin the pre-biological and relatively labilesolar solublesystem. organicIdentified compounds meteoritic [9, 10organic,13,14] thatcompound demonstrateds are numerous an active organic[9] and chemistry many are network biologically in the pre-biologicalrelevant. They solar include system. amino Identified acids [9,13,14], meteoritic polyaromatic organic compounds hydrocarbons are numerous nucleobases [9] and [15,16], many and are biologicallyearly reports relevant. of monosaccharides They include amino (Section acids 2.1) [9 ,and13,14 monosaccharide], polyaromatic hydrocarbons derivatives [17]. nucleobases Hereafter, [15, 16we], andwill earlyrefer to reports monosaccharides of monosaccharides as simply (Section “sugars”, 2.1 )or and refer monosaccharide collectively to monosaccharides derivatives [17]. Hereafter,and their wederivatives will refer as to “polyols” monosaccharides (Figure 1). as In simply this article “sugars”, we focus or referon aldehyde collectively sugars to monosaccharidesor “aldoses” and their and theiracid and derivatives alcohol asderivatives. “polyols” (FigureCollectively,1). In thisthese article compounds we focus are on essentia aldehydel components sugars or “aldoses” of DNA, andRNA, their polysaccharides, acid and alcohol and derivatives. metabolic processes, Collectively, and these are compoundsthought to have are essential been necessary components for the of DNA,formation RNA, of polysaccharides, Earth’s first life and forms. metabolic Glucose processes, and mannose and are thoughtare familiar to have examples been necessary of polyols for thein formationmodern biological of Earth’s processes, first life forms. but Glucoseglyceric andacid mannose (and other are familiarsugar acids) examples also of has polyols a critical in modern and biologicalubiquitous processes, role in biology but glyceric [18]. In acid this (and article, other we sugar will acids)intermittently also has ause critical the term and ubiquitous“aldonic acid” role into biologyrefer to [the18]. acid In this form article, of a we sugar will in intermittently which all carbons use the are term bonded “aldonic to acid”an OH to refergroup. to theFor acidexample, form ofglyceric a sugar acid in whichis the allaldonic carbons acid are form bonded of glycer to analdehyde, OH group. while For example,2-methylglyceric glyceric acidacid isis thea “deoxy” aldonic acidsugar form acid of (Figure glyceraldehyde, 1). while 2-methylglyceric acid is a “deoxy” sugar acid (Figure1). Figure 1.1. Example structures of polyol classes: (1) glyceraldehyde; ( 2) glyceric acid (an aldonic acid); ((33)) glycerol;glycerol; ((44)) 2-methylglyceric2-methylglyceric acid.acid. A note on pristine meteorites: although meteorites generally categorized as “pristine” do show A note on pristine meteorites: although meteorites generally categorized as “pristine” do show relatively less aqueous and thermal alteration, recent observations have reemphasized the fact that relatively less aqueous and thermal alteration, recent observations have reemphasized the fact that that some do contain evidence of (very localized) aqueous activity. For example, from a study of that some do contain evidence of (very localized) aqueous activity. For example, from a study of components of CR2‒3 and CO3.0 carbonaceous meteorites [19], the authors concluded that components of CR2-3 and CO3.0 carbonaceous