Partial transfer of enantioselective chiralities from α-methylated amino acids, known to be of meteoritic origin, into normal amino acids

Prebiotic access to enantioenriched amino acids via peptide-mediated transamination reactions

The kinetic resolution of racemic amino acids mediated by dipeptides and pyridoxal provides a prebiotically plausible route to enantioenriched proteinogenic amino acids. The enzymatic transamination cycles that are key to modern biochemical formation of enantiopure amino acids may have evolved from this half of the reversible reaction couple. Kinetic resolution of racemic precursors emerges as a general route to enantioenrichment under prebiotic conditions.

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.

Synthesis of Unprotected 2-Arylglycines by Transamination of Arylglyoxylic Acids with 2-(2-Chlorophenyl)glycine

The transamination of α-keto acids with 2-phenylglycine is an effective methodology for directly synthesizing unprotected α-amino acids. However, the synthesis of 2-arylglycines by transamination is problematic because the corresponding products, 2-arylglycines, transaminate the starting arylglyoxylic acids. Herein, we demonstrate the use of commercially available l-2-(2-chlorophenyl)glycine as the nitrogen source in the transamination of arylglyoxylic acids, producing the corresponding 2-arylglycines without interference from the undesired self-transamination process.

Social Media, Peer Review, and Responsible Conduct of Research (RCR) in Chemistry: Trends, Pitfalls, and Promises

Over the last two decades, various themes inherent in the responsible conduct of research (RCR) in chemistry have been brought to light through prominent cases of research misconduct. This article will describe a few of these cases especially through the lens of social media such as blogs and Twitter. A case will be made that these wholly novel modalities of online discussion are now complementing, and in some cases even circumventing some of the limitations of traditional peer review in chemistry. We present in detail our evaluation of three recent cases of RCR along with several other social media illustrations. These cases have been selected to be representative and showcase several of the most prominent issues at the intersection of traditional and social-media based peer review. In each case, basic details are presented along with a brief discussion of the underlying issues-readers interested in deeper analysis of each subject are referred to a collection of relevant articles and websites. This perspective focuses on the most important RCR issues that have arisen in the past decade, a time which we believe coincides with the serious participation of the scientific community in general, and the chemistry community in particular, in social media-based, citizen-enabled peer-review. A discussion of important trends in RCR in the age of social media, outstanding developments in this area, and questions of enduring interest for the near future concludes the article.

Preparation of novel pyridine-based poly(ether amide)s via three different routes and investigation of their properties

A new diamine was prepared via two successive reactions: 6-chloronicotinoyl chloride was reacted with 4,4′-oxydianiline and the obtained dichloro product was reacted with 5-amino-1-naphthol. A novel series of poly(ether amide)s containing flexible ether groups, pyridine polar units, and bulky naphthyl units was prepared through polycondensation reactions of the diamine with four diacids via three different routes. Polyamides were prepared by phosphorylation reaction, indirect diacid chloride reaction, and using ionic liquid medium. The effect of reaction conditions on the physical and thermal properties of the polymers such as solution viscosity, solubility, crystallinity, thermal stability, and thermal behavior were investigated.

Enantiodifferent proton exchange in alanine and asparagine in the presence of H(2)(17)O

Using Time Domain (1)H Nuclear Magnetic Resonance with H (2) (17) O (H (2) (17) O-TD-(1)HNMR), we found [H (2) (17) O]- and pH-controlled chiral differences in proton exchange properties in alanine (Ala) and asparagine (Asn). To minimize and equalize chemical impurities, Asn enantiomers were purified by crystallization from racemic solution. At <0.1 M H (2) (17) O, a shift in isoelectric pH (pI) occurred, approximately 1.14 kJ mol(-1) L: -D: -Asn DeltaDeltaG (o)' in the 5.91-6.42 pH range. One potential source for this asymmetry is the enantio-different magnetic moments (L: mu upward arrow not equal D: mu downward arrow) produced by neutral ring currents in the chiral center, leading to enantio-different nuclear spin organization and charge distribution in the amino group. At >or=pI, dissimilar interactions may occur in the hydration of the amino group with H (2) (17) O (NH(2)/H (2) (17) O not equal NH(2)/H (2) (16) O; NH(3) (+)/H (2) (17) O not equal NH(2)/H (2) (17) O; L: -*C-NH(2)/H (2) (17) O not equal D: -*C-NH(2)/H (2) (17) O). As L: mu upward arrow not equal D: mu downward arrow, the L: -*C-amino and the D: -*C-amino groups are diastereo spin-isomers. The nuclear spin of (17)O may be parallel or antiparallel with the ortho-(1)H(1)H pair; hence two ortho-H (2) (17) O molecules exist, also diastereo spin-isomers. As the pK of H (2) (17) O is different from H (2) (16) O, dissimilarities between L: -*C- and D: -*C-amino groups are converted into proton exchange differences. During H (2) (17) O-TD-(1)HNMR, the H (2) (17) O molecule is a "probe" of the state of the amino group. Regarding prebiotic evolution: prebiotic chirality may not require stochastic symmetry breaking or preexisting chiral conditions; chemical chiral effects due to L: mu upward arrow not equal D: mu downward arrow are small and need chiral amplification to generate an enantiomeric excess significant for prebiotic evolution; and prebiotic symmetry breaking was homochiral because the effect of L: mu upward arrow and D: mu downward arrow on the amino group should be similar in all alpha amino acids.

An experimental investigation of the evolution of chirality in a potential dynamic peptide system: N-terminal epimerization and degradation into diketopiperazine

The APED model (activation-polymerization-epimerization-depolymerization) is a unique example of a chemical system that allows symmetry breaking through a dynamic process involving indirect network autocatalysis. In its simplest version, the autocatalytic behavior of this model partly relies on the reproduction of local chiral centers in dipeptides through an epimerization process, with a thermodynamic preference for homochiral chains. We studied the reactivity of di- and tripeptides, containing a N-terminal phenylglycine (Phg) residue, as model compounds for the experimental determination of the kinetic and thermodynamic parameters related to the N-terminal epimerization process. Although the N-terminal residue is prone to spontaneous epimerization, catalysis was required for the epimerization to reach the equilibrium state in reasonable time. Unexpectedly, the observed equilibrium diastereoisomeric excesses have shown a general tendency for more stable heterochiral peptides, especially strong in the case of dipeptides. In parallel to this process, a stereoselective peptide cleavage through diketopiperazine formation was observed. Contrary to the N-terminal epimerization of peptides, the diketopiperazine formation did not need any catalyst, and heterochiral peptides were shown to be dynamically unstabilized, as they were cleaved faster than homochiral peptides. The validity of the extrapolation of these results to other residues and longer peptide chains is discussed, and some directions for future developments of the theoretical model are given.

Imitating prebiotic homochirality on Earth

We show how the amino acids needed on prebiotic earth in their homochiral L form can be produced by a reaction of L-alpha-methyl amino acids-that have been identified in the Murchison meteorite-with alpha-keto acids under credible prebiotic conditions. When they are simply heated together they perform a process of decarboxylative transamination but with almost no chiral transfer, and that in the wrong direction, producing D-amino acids from the L-alpha-methyl amino acids. With copper ion a square planar complex with two of the reaction intermediates is formed, and now there is the desired L to L transformation, producing small enantioexcesses of the normal L-amino acids. We also show how these can be amplified, not by making more of the L form but by increasing its concentration in water solution. The process can start with a miniscule excess and in one step generate water solutions with L/D ratios in the over 90% region. Kinetic processes can exceed the results from equilibria. We have also examined such amplifications with ribonucleosides, and have shown that initial modest excesses of the D-nucleosides can be amplified to afford water solutions with D to L ratios in the high 90's. We have shown that the homochiral compound has two effects on the solubility of the racemate. On one hand it decreases the solubility of the racemate by its role in the solubility product, as a theoretical equation predicts. On the other hand, it increases the solubility of the racemate by changing the nature of the solvent, acting as a cosolvent with the water. This explains why the amplification, while large, is not as large as the simple theoretical equation predicts. Thus when credible examples are produced where small enantioexcesses of D-ribose are created under credible prebiotic conditions, the prerequisites for the RNA world will have been exemplified.

On-line sample stacking of peptides in capillary electrophoresis for the study of prebiotic reactions between alpha,alpha-dialkylated amino acids and amino acid N-carboxyanhydrides

The reaction between alpha,alpha-dialkylated amino acids and amino acid N-carboxyanhydrides is slow leading to low concentrations of products (peptides). The detection by capillary electrophoresis of the analytes contained in such samples is therefore a challenging issue. In this work, on-line sample pre-concentration methods based on field-amplified sample stacking have been implemented and compared. Because of the high ionic strength present in the sample matrix, samples were diluted with an organic solvent prior to analysis to decrease the sample conductivity. Different modes of sample injection (field amplified sample injection (FASI), hydrodynamic normal sample stacking (NSS) or large volume sample stacking (LVSS)) were compared. Pre-concentration factors of 20 for FASI, about 30-40 for NSS and 60 for LVSS were obtained for the analysis of (l,l) dipeptide of valine in a large excess of isovaline and 0.2M of ionic strength. For LVSS application and resolution optimisation, a new non-covalent coating based on the partial modification of the capillary surface was used to tune the electroosmotic flow magnitude and to pump the sample matrix out of the capillary. This on-line sample pre-concentration step allowed confirming that oligopeptides including alpha,alpha-dialkylated amino acids are formed during the reaction between alpha,alpha-dialkylated amino acids and N-carboxyanhydride amino acids.

Amplification of enantiomeric concentrations under credible prebiotic conditions

Solutions with as little as 1% enantiomeric excess (ee) of D- or L-phenylalanine are amplified to 90% ee (a 95/5 ratio) by two successive evaporations to precipitate the racemate. Such a process on the prebiotic earth could lead to a mechanism by which meteoritic chiral alpha-alkyl amino acids could form solutions with high ee values that were needed for the beginning of biology.