Hypercondensation of an amino acid: synthesis and characterization of a black glycine polymer

Chemical, Thermal, and Radiation Resistance of an Iron Porphyrin: A Model Study of Biosignature Stability

Metal complexes of porphyrins and porphyrin-type compounds are ubiquitous in all three domains of life, with hemes and chlorophylls being the best-known examples. Their diagenetic transformation products are found as geoporphyrins, in which the characteristic porphyrin core structure is retained and which can be up to 1.1 billion years old. Because of this, and their relative ease of detection, metalloporphyrins appear attractive as chemical biosignatures in the search for extraterrestrial life. In this study, we investigated the stability of solid chlorido(2,3,7,8,12,13,17,18-octaethylporphyrinato)iron(III) [FeCl(oep)], which served as a model for heme-like molecules and iron geoporphyrins. [FeCl(oep)] was exposed to a variety of astrobiologically relevant extreme conditions, namely: aqueous acids and bases, oxidants, heat, and radiation. Key results are: (1) the [Fe(oep)]⁺ core is stable over the pH range 0.0-13.5 even at 80°C; (2) the oxidizing power follows the order ClO^- > H₂O₂ > ClO₃^- > HNO₃ > ClO₄^-; (3) in an inert atmosphere, the iron porphyrin is thermally stable to near 250°C; (4) at high temperatures, carbon dioxide gas is not inert but acts as an oxidant, forming carbon monoxide; (5) a decomposition layer is formed on ultraviolet irradiation and protects the [FeCl(oep)] underneath; (6) an NaCl/NaHCO₃ salt mixture has a protective effect against X-rays; and (7) no such effect is observed when [FeCl(oep)] is exposed to iron ion particle radiation. The relevance to potential iron porphyrin biosignatures on Mars, Europa, and Enceladus is discussed.

One Step up the Ladder of Prebiotic Complexity: Formation of Nonrandom Linear Polypeptides from Binary Systems of Amino Acids on Silica

Evidence for the formation of linear oligopeptides with nonrandom sequences from mixtures of amino acids coadsorbed on silica and submitted to a simple thermal activation is presented. The amino acid couples (glutamic acid+leucine) and (aspartic acid+valine) were deposited on a fumed silica and submitted to a single heating step at moderate temperature. The evolution of the systems was characterized by X-ray diffraction, infrared spectroscopy, thermosgravimetric analysis, HPLC, and electrospray ionization mass spectrometry (ESI-MS). Evidence for the formation of amide bonds was found in all systems studied. While the products of single amino acids activation on silica could be considered as evolutionary dead ends, (glutamic acid+leucine) and, at to some extent, (aspartic acid+valine) gave rise to the high yield formation of linear peptides up to the hexamers. Oligopeptides of such length have not been observed before in surface polymerization scenarios (unless the amino acids had been deposited by chemical vapor deposition, which is not realistic in a prebiotic environment). Furthermore, not all possible amino acid sequences were present in the activation products, which is indicative of polymerization selectivity. These results are promising for origins of life studies because they suggest the emergence of nonrandom biopolymers in a simple prebiotic scenario.

The Influence of Mineral Matrices on the Thermal Behavior of Glycine

On the Hadean-Early Archean Earth, the first islands must have provided hot and dry environments for abiotically formed organic molecules. The heat sources, mainly volcanism and meteorite impacts, were also available on Mars during the Noachian period. In recent work simulating this scenario, we have shown that neat glycine forms a black, sparingly water-soluble polymer ("thermomelanoid") when dry-heated at 200 °C under pure nitrogen. The present study explores whether relevant minerals and mineral mixtures can change this thermal behavior. Most experiments were conducted at 200 or 250 °C for 2 or 7 days. The mineral matrices used were phyllosilicates (Ca-montmorillonites SAz-1 and STx-1, Na-montmorillonite SAz-1-Na, nontronite NAu-1, kaolinite KGa-1), salts (NaCl, NaCl-KCl, CaCl2, artificial sea salt, gypsum, magnesite), picritic basalt, and three Martian regolith simulants (P-MRS, S-MRS, JSC Mars-1A). The main analytical method employed was high-performance liquid chromatography (HPLC). Glycine intercalated in SAz-1 and SAz-1-Na was well protected against thermomelanoid formation and sublimation at 200 °C: after 2 days, 95 and 79 %, respectively, had either survived unaltered or been transformed into the cyclic dipeptide (DKP) and linear peptides up to Gly6. The glycine survival rate followed the order SAz-1 > SAz-1-Na > STx-1 ≈ NAu-1 > KGa-1. Very good protection was also provided by artificial sea salt (84 % unaltered glycine after 200 °C for 7 days). P-MRS promoted the condensation up to Gly6, consistent with its high phyllosilicate content. The remaining matrices were less effective in preserving glycine as such or as peptides.

Inhabited or Uninhabited? Pitfalls in the Interpretation of Possible Chemical Signatures of Extraterrestrial Life

The "Rare Earth" hypothesis-put forward by Ward and Brownlee in their 2000 book of the same title-states that prokaryote-type organisms may be common in the universe but animals and higher plants are exceedingly rare. If this idea is correct, the search for extraterrestrial life is essentially the search for microorganisms. Various indicators may be used to detect extant or extinct microbial life beyond Earth. Among them are chemical biosignatures, such as biomolecules and stable isotope ratios. The present minireview focuses on the major problems associated with the identification of chemical biosignatures. Two main types of misinterpretation are distinguished, namely false positive and false negative results. The former can be caused by terrestrial biogenic contaminants or by abiotic products. Terrestrial contamination is a common problem in space missions that search for biosignatures on other planets and moons. Abiotic organics can lead to false positive results if erroneously interpreted as biomolecules, but also to false negatives, for example when an abiotic source obscures a less productive biological one. In principle, all types of putative chemical biosignatures are prone to misinterpretation. Some, however, are more reliable ("stronger") than others. These include: (i) homochiral polymers of defined length and sequence, comparable to proteins and polynucleotides; (ii) enantiopure compounds; (iii) the existence of only a subset of molecules when abiotic syntheses would produce a continuous range of molecules; the proteinogenic amino acids constitute such a subset. These considerations are particularly important for life detection missions to solar system bodies such as Mars, Europa, and Enceladus.

Formation of Diastereoisomeric Piperazine-2,5-dione from DL-Alanine in the Presence of Olivine and Water

DL-Alanine (Ala) was heated with/without powdered olivine and water at 120 °C for 8 days to investigate the formation of the diastereoisomers of piperazine-2,5-dione (diketopiperazine, DKP). When only DL-Ala was heated with a small amount of water, 3.0 % of DL-Ala changed to cis- and trans-DKP after 8 days. DKPs were not detected after heating when no water was added. The presence of a small amount of water is important factor controlling peptide production rates under thermal conditions. When DL-Ala was heated with olivine powder for 8 days, the yields of cis- and trans-DKP were 6.8 and 4.9 %, respectively. The high yield of cis-DKP compared with trans-DKP was attributed to greater thermal stability of cis-DKP. After heating for 8 days, the diastereoisomeric excess of cis-DKP without olivine was 7.3 %, whereas a much higher value of 16.3 % was obtained in the presence of olivine. Taken together, these results show that olivine is not only an efficient catalyst for the formation of DKPs but that it also play a significant role in determining the diastereoisomer selectivity of these cyclic dipeptides.

Controlled radical polymerization of an acrylamide containing L-alanine moiety via ATRP

Homopolymerization of an optically active acrylamide having an amino acid moiety in the side chain, N-acryloyl-L-alanine (AAla) was carried out via atom transfer radical polymerization (ATRP) at room temperature using 2-hydroxyethyl-2'-methyl-2'-bromopropionate (HMB) or sodium-4-(bromomethyl)benzoate (SBB) as initiator in pure water, methanol/water mixture and pure methanol solvents. The polymerization reaction resulted in the optically active biocompatible amino acid-based homopolymer in good yield with narrow molecular weight distribution. The number average molecular weight increased with conversion and polydispersity was low. The structure and molecular weight of synthesized polymer were characterized by (1)H NMR, FT-IR spectroscopic techniques and size-exclusion chromatography.