The radioracemization of amino acids by ionizing radiation: geochemical and cosmochemical implications

Biomimetic crosslinking of collagen gels by energetic electrons: The role of L-lysine

Energetic electrons have recently evolved as a powerful tool for crosslinking bio-derived hydrogels without the need for adding potentially hazardous reagents. Application of this approach allows for synthesis of biomimetic collagen-derived networks of highly tunable properties and functionalization. Yet, the underlying reaction kinetics are still not sufficiently established at this point. While hydroxyl radicals are generated by energetic electron-induced hydrolysis of water and play a key role in introducing covalent bonds between network fibers, a detailed mechanistic understanding would significantly increase applicability. We present a comprehensive analysis of central aspects of the reactivity between the hydroxyl radical (^•OH) and collagen, elastin, glycine (Gly) and l-lysine (Lys). Pulse radiolysis (PR), solid state nuclear magnetic resonance (NMR), ultraviolet-visible absorption spectroscopy (UV/VIS) and electron spray ionization mass spectrometry (ESI-MS) shine light on distinct features of the crosslinking process. These highlight retained protein backbone integrity in collagen and elastin whilst Lys's ability to form several imine bonded Lys-Lys-species suggests striking similarities to crosslinking via lysyl oxidase catalysis in vivo. Thus, energetic electron based crosslinking opens the venue for customized hybrid gels of outstanding biomimicry and -compatibility. STATEMENT OF SIGNIFICANCE: Energetic electron beam treatment constitutes a highly attractive approach to establish chemical bonds between (bio) molecules. Although a convincing number of publications showed the versatility regarding crosslinking of bioderived hydrogels, insights into the underlying chemistry are still unestablished at this point. The present work unravels the mechanistics of energetic electron induced processes in collagen and elastin hydrogels as well as several abundant amino acids in aqueous solution. As key finding we demonstrate, that i) the connection between polymer chains is dominated by amino acid side chain interaction and ii) two single l-lysine molecules form an imine bond between the terminal amino group of one molecule and the delta carbon of the second molecule. We also consider the formation of H-bonds as a second crosslinking pathway. These findings open up for advanced, optionally spatially resolved biomaterials design.

Ronald C.D. Breslow (1931-2017): A career in review

Reviewed herein are key research accomplishments of Professor Ronald Charles D. Breslow (1931-2017) throughout his more than 60 year research career. These accomplishments span a wide range of topics, most notably physical organic chemistry, medicinal chemistry, and bioorganic chemistry. These topics are reviewed, as are topics of molecular electronics and origin of chirality, which combine to make up the bulk of this review. Also reviewed briefly are Breslow's contributions to the broader chemistry profession, including his work for the American Chemical Society and his work promoting gender equity. Throughout the article, efforts are made to put Breslow's accomplishments in the context of other work being done at the time, as well as to include subsequent iterations and elaborations of the research.

Stability toward High Energy Radiation of Non-Proteinogenic Amino Acids: Implications for the Origins of Life

A series of non-proteinogenic amino acids, most of them found quite commonly in the meteorites known as carbonaceous chondrites, were subjected to solid state radiolysis in vacuum to a total radiation dose of 3.2 MGy corresponding to 23% of the total dose expected to be taken by organic molecules buried in asteroids and meteorites since the beginning of the solar system 4.6 × 10⁹ years ago. The radiolyzed amino acids were studied by FT-IR spectroscopy, Differential Scanning Calorimetry (DSC) and by polarimety and Optical Rotatory Dispersion (ORD). It is shown that an important fraction of each amino acid is able to “survive” the massive dose of radiation, while the enantiomeric excess is partially preserved. Based on the results obtained, it is concluded that it is unsurprising to find amino acids even in enantiomeric excess in carbonaceous chondrites.

Isovaline, a rare amino acid, has anticonvulsant properties in two in vitro hippocampal seizure models by increasing interneuronal activity

PURPOSE

We investigated whether RS-isovaline, a unique amino acid found in carbonaceous meteorites and presumed extraterrestrial, has anticonvulsant properties in rat hippocampal slices in vitro.

METHODS

Extracellular recordings were obtained in the rat hippocampal CA1 pyramidal layer in two in vitro seizure models: perfusion of low (0.25 mm) Mg(2+) and high (5 mm) K(+) (LM/HK), or 100 μm 4-aminopyridine (4-AP). To investigate the underlying mechanisms of isovaline action, whole-cell recordings were obtained from CA1 pyramidal neurons and stratum oriens interneurons during 4-AP blockade of K(+) channels.

KEY FINDINGS

Perfusion of LM/HK produced seizure-like events (SLEs) or stimulus-evoked primary afterdischarges (PADs) with amplitudes of 0.9 ± 0.1 mV lasting 80 ± 14 s. Application of isovaline (250 μm) for 20-30 min abolished SLEs and PADs or attenuated seizure amplitude and duration by 57.0 ± 9.0% and 57.0 ± 12.0%, respectively. Similar effects were seen with isovaline in the 4-AP seizure model. Isovaline alone increased interneuronal spontaneous spiking from 0.9 ± 0.3 to 3.2 ± 0.9 Hz, increased input resistance by 21.6 ± 8.1%, and depolarized the resting membrane potential by 8.0 ± 1.5 mV; no changes in the firing or electrical properties of pyramidal neurons were observed. Coapplication of 4-AP and isovaline increased interneuronal spontaneous spiking from 1.0 ± 0.6 to 2.6 ± 0.8 Hz, whereas pyramidal neuronal spiking activity decreased from 0.6 ± 0.4 to 0.2 ± 0.1 Hz.

SIGNIFICANCE

Isovaline exhibited anticonvulsant properties in two hippocampal seizure models. This may lead to the development of a new class of anticonvulsants based on an unusual mechanism of action of this presumed extraterrestrial amino acid.

Enrichment of the amino acid L-isovaline by aqueous alteration on CI and CM meteorite parent bodies

The distribution and enantiomeric composition of the 5-carbon (C(5)) amino acids found in CI-, CM-, and CR-type carbonaceous meteorites were investigated by using liquid chromatography fluorescence detection/TOF-MS coupled with o-phthaldialdehyde/N-acetyl-L-cysteine derivatization. A large L-enantiomeric excess (ee) of the alpha-methyl amino acid isovaline was found in the CM meteorite Murchison (L(ee) = 18.5 +/- 2.6%) and the CI meteorite Orgueil (L(ee) = 15.2 +/- 4.0%). The measured value for Murchison is the largest enantiomeric excess in any meteorite reported to date, and the Orgueil measurement of an isovaline excess has not been reported previously for this or any CI meteorite. The L-isovaline enrichments in these two carbonaceous meteorites cannot be the result of interference from other C(5) amino acid isomers present in the samples, analytical biases, or terrestrial amino acid contamination. We observed no L-isovaline enrichment for the most primitive unaltered Antarctic CR meteorites EET 92042 and QUE 99177. These results are inconsistent with UV circularly polarized light as the primary mechanism for L-isovaline enrichment and indicate that amplification of a small initial isovaline asymmetry in Murchison and Orgueil occurred during an extended aqueous alteration phase on the meteorite parent bodies. The large asymmetry in isovaline and other alpha-dialkyl amino acids found in altered CI and CM meteorites suggests that amino acids delivered by asteroids, comets, and their fragments would have biased the Earth's prebiotic organic inventory with left-handed molecules before the origin of life.

Experimental evidence for beta-decay as a source of chirality by enantiomer analysis

Earlier experiments testing the Vester-Ulbricht beta-decay hypothesis for the origin of molecular chirality are reviewed, followed by descriptions of our own experiments involving attempted asymmetric radiolysis of DL-amino acids using quantitative gas chromotography as a probe for optical activity. Our radiation sources included 90Sr-90Y, 14C and 32P Bremsstrahlen, longitudinally polarized electrons from a linear accelerator and longitudinally polarized protons from a cyclotron. With the possible exception of the linear accelerator irradiations, these experiments failed to produce g.c.-detectable enantiomeric excesses, even at 50-70% gross radiolysis. We thus find no unambiguous support for the Vester-Ulbricht hypothesis in any of the attempted asymmetric radiolyses performed to date. Radioracemization, a possible reason for these failures, is discussed.

The radiolysis and radioracemization of amino acids on clays

L-Leucine and its hydrochloride salt have been deposited on the clay minerals kaolin and bentonite, and the amino acid/clay preparations have been irradiated in a 3000 Ci60Co gamma-ray source for radiation dosages that achieved 2-89% radiolysis of the leucine. The undecomposed leucine was thereupon recovered and both percent radiolysis and percent radioracemization were determined. Similar studies were made using solid L-leucine and its hydrochloride, and L-leucine in 0.1 M aqueous solution. It has been found that radiolysis and radio-racemization in these and the previously studied leucine systems follow pseudo-first-order rate laws, and the corresponding specific rate constants are evaluated and compared. Leucine and its hydrochloride salt proved to be the most stable to both radiolysis and radioracemization, followed by leucine and its HCl salt on kaolin, followed by leucine and its HCl salt on bentonite, with leucine (and its HCl and Na salts) in aqueous solution being least stable to both radiolysis and (except for the HCl salt) radioracemization. Implications of these observations as regards the Vester-Ulbricht mechanism for the origin of optical activity are discussed.

Parity violation and the evolution of biomolecular homochirality

Parity violation at the level of terrestrial biopolymers, as seen in proteins, DNAs, and RNAs, and parity violation at the level of nuclear processes, as evident in longitudinally polarized beta-particles and parity-violating energy differences (PVEDs), are discussed and their fundamental importances are emphasized. Attempts to find a causal connection between the unique homochirality of biopolymers and parity violation at the nuclear level, and speculations that the former is a consequence of the latter, are reviewed. Consideration of all lines of evidence leads to the conclusion that there is no substantiation for such a causal connection, and that the two levels of parity violation are entirely independent of each other.

Chemistry, nutrition, and microbiology of D-amino acids

Exposure of food proteins to certain processing conditions induces two major chemical changes: racemization of all L-amino acids to D-isomers and concurrent formation of cross-linked amino acids such as lysinoalanine. Racemization of L-amino acids residues to their D-isomers in food and other proteins is pH-, time-, and temperature-dependent. Although racemization rates of the 18 different L-amino acid residues in a protein vary, the relative rates in different proteins are similar. The diet contains both processing-induced and naturally formed D-amino acids. The latter include those found in microorganisms, plants, and marine invertebrates. Racemization impairs digestibility and nutritional quality. The nutritional utilization of different D-amino acids varies widely in animals and humans. In addition, some D-amino acids may be both beneficial and deleterious. Thus, although D-phenylalanine in an all-amino-acid diet is utilized as a nutritional source of L-phenylalanine, high concentrations of D-tyrosine in such diets inhibit the growth of mice. Both D-serine and lysinoalanine induce histological changes in the rat kidney. The wide variation in the utilization of D-amino acids is illustrated by the fact that whereas D-methionine is largely utilized as a nutritional source of the L-isomer, D-lysine is totally devoid of any nutritional value. Similarly, although L-cysteine has a sparing effect on L-methionine when fed to mice, D-cysteine does not. Because D-amino acids are consumed by animals and humans as part of their normal diets, a need exists to develop a better understanding of their roles in nutrition, food safety, microbiology, physiology, and medicine. To contribute to this effort, this multidiscipline-oriented overview surveys our present knowledge of the chemistry, nutrition, safety, microbiology, and pharmacology of D-amino acids. Also covered are the origin and distribution of D-amino acids in the food chain and in body fluids and tissues and recommendations for future research in each of these areas. Understanding of the integrated, beneficial effects of D-amino acids against cancer, schizophrenia, and infection, and overlapping aspects of the formation, occurrence, and biological functions of D-amino should lead to better foods and improved human health.

Chirality and life

The crucial role of homochirality and chiral homogeneity in the self-replication of contemporary biopolymers is emphasized, and the experimentally demonstrated advantages of these chirality attributes in simpler polymeric systems are summarized. The implausibility of life without chirality and hence of a biogenic scenario for the origin of chiral molecules is stressed, and chance and determinate abiotic mechanisms for the origin of chirality are reviewed briefly in the context of their potential viability on the primitive Earth. It is concluded that all such mechanisms would be nonviable, and that the turbulent prebiotic environment would require an ongoing extraterrestrial source for the accumulation of chiral molecules on the primitive Earth. A scenario is described wherein the circularly polarized ultraviolet synchrotron radiation from the neutron star remnants of supernovae engenders asymmetric photolysis of the racemic constituents in the organic mantles on interstellar dust grains, whereupon these chiral constituents are transported repetitively to the primitive Earth by direct accretion of the interstellar dust or through impacts of comets and asteroids.

Characteristics and formation of amino acids and hydroxy acids of the Murchison meteorite

Eight characteristics of the unique suite of amino acids and hydroxy acids found in the Murchison meteorite can be recognized on the basis of detailed molecular and isotopic analyses. The marked structural correspondence between the alpha-amino acids and alpha-hydroxy acids and the high deuterium/hydrogen ratio argue persuasively for their formation by aqueous phase Strecker reactions in the meteorite parent body from presolar, i.e., interstellar, aldehydes, ketones, ammonia, and hydrogen cyanide. The characteristics of the meteoritic suite of amino acids and hydroxy acids are briefly enumerated and discussed with regard to their consonance with this interstellar-parent body formation hypothesis. The hypothesis has interesting implications for the organic composition of both the primitive parent body and the presolar nebula.

Beta-decay, bremsstrahlen, and the origin of molecular chirality

A brief review is presented of the Vester-Ulbricht beta-decay Bremsstrahlen hypothesis for the origin of optical activity, and of subsequent experiments designed to test it. Certain of our experiments along these lines, begun in 1974 and involving the irradiation of racemic and optically active amino acids in a 61.7 KCi 90Sr-90Y Bremsstrahlen source, have now been completed and are described. After 10.89 years of irradiation with a total Bremsstrahlen dose of 2.5 X 10(9) rads, crystalline DL-leucine, norleucine, and norvaline suffered 47.2, 33.6, and 27.4% radiolysis, respectively, but showed no evidence whatsoever of asymmetric degradation. D- and L-Leucine underwent about 48% radiolysis and showed 2.4-2.9% radioracemization. Other samples in solution were too severely degraded to analyze. Probable intrinsic reasons for the failure of the Vester-Ulbricht mechanism to afford asymmetric radiolysis in the present and related experiments involving beta-decay Bremsstrahlen are enumerated.

Identification of some OH radical-induced products of lysozyme

OH radical reactions with lysozyme in gamma-irradiated N2O saturated aqueous solutions caused formation of allo-threonine, alpha-amino-n-butyric acid, o- and m- tyrosines, and 2- and 3-hydroxytyrosines. These identified radiolytic products were characterized by capillary gas chromatography-mass spectrometry as their trimethylsilyl derivatives after HCl-hydrolysis of irradiated lysozyme. Their initial G-values were also determined using gas chromatography. The possible use of these radiolytic products as monitors of radiation-induced damage to proteins and the sites of attack are also discussed.

The radiolysis and radioracemization of amino acids on silica surfaces

L-Leucine, deposited on both 1-quartz powder and on a commercial amorphous silica preparation (Syloid 63), has been subjected to irradiation in a 60Co gamma-ray source, and the ensuing radiolysis and radioracemization have been determined gas chromatographically. The radiolysis and radioracemization observed for leucine on 1-quartz were rather similar to those noted for a crystalline L-leucine control. L-Leucine on Syloid 63, however, was vastly more susceptible to radiolysis as compared to the L-leucine control, and radioracemization was also markedly enhanced - each increasing with larger radiation dosage. L-Isovaline showed a similar, but diminished, enhancement of radiolysis sensitivity when adsorbed on the Syloid surface, but underwent no radioracemization whatsoever. The divergent results of the control and quartz-leucine irradiations versus the Syloid-leucine and Syloid-isovaline irradiations are interpreted qualitatively in terms of the surface area parameters of the two silica adsorbents and the amino acid adsorbates.

Experiments on the abiotic amplification of optical activity

Our earlier experiments are briefly reviewed, involving the abiotic generation of optical activity by exposure of DL-amino acids to various "chiral" physical forces. The enantiomeric enrichments so obtained were low, however, and additional experiments were undertaken with the objective of abiotically enhancing such small enantiomeric excesses. D not equal to L Mixtures of leucine N-carboxy anhydride gave enantiomerically enriched polymers on partial polymerization, while valine NCA mixtures behaved oppositely. Leucine polymers were also found to hydrolyze stereoselectively, providing for additional enantiomeric enhancement. A repetitive sequence of partial polymerization-hydrolysis steps is suggested as a possible mechanism for the abiotic genesis of optically enriched polypeptides on the primitive Earth.