Homochiral Preference Research Articles

Negatively-Charged Halide Adducts of Homochiral Serine Octamers

Negatively charged halide adducts of serine octamers, (Ser(8)+2Cl)(2-) and (Ser(8)+2Br)(2-), appear as magic number clusters in the negative ion electrospray mass spectra of solutions containing serine and the halide. Like the well-known protonated serine octamer, these negatively charged adducts are formed with homochiral preference and also undergo chiroselective substitution reactions with other amino acids. Tandem mass spectra of negatively charged halide adducts of serine octamers show that these ions also have a characteristic fragmentation signature. The fact that octamers of both polarities display analogous chemical properties suggests that these may be characteristics of the so-far-unknown neutral octamer. If serine played a key role in the origin of homochirality on the primitive earth, it was likely through both the neutral octamer and the ionic adducts. Unlike the octamers, the formation of halide-containing serine cluster ions of particular sizes is unfavorable under the conditions of the experiment. Signals corresponding to the ions (Ser(9)+2Br)(2-) and (Ser(15)+2Br)(2-) are particularly low in intensity, giving rise to gaps in the distribution of serine/bromide clusters in the negative ion electrospray mass spectra. These cluster sizes are likely to correspond to unstable "anti-magic number" clusters recently reported by Clemmer.

Amino acid cluster formation studied by electrospray ionization mass spectrometry

The association properties of natural and non-natural amino acids were studied in detail using electrospray ionization mass spectrometry. The results show a highly diverse cluster formation behavior of amino acids. There are differences regarding the degree of clustering (average cluster size), the presence or absence of one or several 'magic' clusters of special stability and the influence of chirality on cluster stability. Cluster formation does not show a good correlation with simple physico-chemical properties (such as solubility), indicating that it is a specific process and not only a simple aggregation during evaporation/ionization. A systematic study of cluster formation of serine derivatives reveals that all functional groups play a prominent role in the binding of the magic octamer. The results support the idea of the zwitterionic character of the octamer. Electrospray ionization of the side-chain acetylated serine shows the formation of a very stable tetramer with a strong preference for homochirality. The results suggest that Ser8 is made up of two tetramer subunits, held together by hydrogen bonds of the side-chain.

Spontaneous anti-resolution in heterochiral clusters of serine.

Chirality plays an important role in the formation and stability of noncovalent clusters which are made of chiral molecules. It is shown that clusters can exhibit a preference for both homochirality and heterochirality. Serine cluster formation is dominated by the formation of heterochiral aggregates, with the exception of the previously observed homochiral serine octamer. Thus, the majority of serine clusters lead to chiral anti-resolution, or the racemic mixing of enantiomers.

Tailor-made aggregates of α-hydroxy esters in supersonic jets

Hydrogen-bonded clusters of methyl glycolate and methyl α-hydroxyisobutyrate are investigated by ragout-jet FTIR-spectroscopy to elucidate the influence of α-C hydrogens on the unusual aggregation of α-hydroxy esters. Highly structured OH-stretching spectra, stagnation pressure studies, and argon coating experiments show that two α-C hydrogens enable the formation of small oligomers such as trimers and tetramers, whereas the lack of α-C hydrogens stops selective aggregation beyond dimers. Methyl lactate with a single chiral α-C hydrogen is suggested to be intermediate in its clustering behavior, with a homochiral preference for trimers and a strong heterochiral preference for tetramers.

Chiral Discrimination Effects in Langmuir Monolayers of 1-O-Hexadecyl Glycerol

Chiral discrimination effects in the monolayer of 1-O-hexadecyl glycerol (3-hexadecyl-oxy-propane-1,2-diol) amphiphile are theoretically investigated on the basis of a detailed coarse-grained molecular model. The theory uses experimentally obtained information for the average tilt and azimuthal orientation of a reference molecule and minimizes the energy of neighboring molecules using a theoretical framework developed previously. On the basis of the effective pair potential (EPP) of a neighboring pair of molecules, the mutual orientation at the minimum of EPP is obtained. The predicted curvature of the domains composed of enantiomeric amphiphiles based on the mutual orientation obtained from the EPP theory agrees with the experimentally observed handedness based on results from optical studies such as Brewster angle microscopy and fluorescence microscopy. The study revealed, for the first time, interesting crossover from heterochiral preference to homochiral preference of hexadecyl glycerol amphiphile. Th...

Unusually Stable Magic Number Clusters of Serine with a Surprising Preference for Homochirality

AbstractFor Abstract see ChemInform Abstract in Full Text.

Chiral Discrimination Effects in Langmuir Monolayers: Monolayers of Palmitoyl Aspartic Acid,N-Stearoyl Serine Methyl Ester, andN-Tetradecyl-γ,δ-dihydroxypentanoic Acid Amide

The chiral discrimination energies in domains composed of chiral amphiphilic monolayers are theoretically studied. First, calculations on simpler model systems are carried out which show that the depth of the pair potential well is dependent on the extent of coarse-graining the molecular structure. However, systematic coarse-graining of model systems shows that the consistent use of a set of parameters can correctly predict the chiral preference (homo- or heterochirality). The model calculation further suggests that, with a gradual loss of chirality of the model molecule, the chiral interaction is diminished. Calculations of the chiral discriminating pair potential of three chiral compounds, N-stearoyl serine methyl ester, N-palmitoyl aspartic acid, and N-tetradecyl-γ,δ-dihydroxypentanoic acid amide indicate homochiral preference. The preference is observed both in packing of a pair of molecules and the pair potential energy profile. The enantiomeric pairs are closely packed and have a lower minimum pair potential compared to the racemic pairs. The homochiral preference corroborates well with the domain features observed by BAM. The growth in both directions observed in racemic domains are suggestive of local chiral symmetry breaking, which is possible in the case of the homochirally preferred interaction. We also compare the results of the calculation of discrimination energy by the present approach and the results by other molecular models available in the literature. Whereas the basic conclusions of the present model and other models agree, the present model reveals a nontrivial distance and orientation dependence of the discrimination energy. The results are insensitive to the choice of parameters.

Nanocrystalline Aggregation of Serine Detected by Electrospray Ionization Mass Spectrometry: Origin of the Stable Homochiral Gas-Phase Serine Octamer

Electrospray ionization sampling of 0.01 M solutions of serine reveals an unusually abundant protonated serine octamer, which demonstrates a strong preference for homochirality. Ion mobility spectra yield an experimental cross section that is consistent with a single “cubic” structure. Density functional theory calculations support experimental observations which suggest that the structure of the serine octamer is closely related to and formed from a small subsection of the crystal structure of anhydrous serine. The formation, bonding, and experimental constraints placed on the serine octamer are discussed in light of the proposed nanocrystalline structure. The mass spectra of the analogous molecules threonine and homoserine exhibit several similar features with that of serine and the likely structures for the octameric species are compared with that of the serine octamer. Finally, implications for the spontaneous symmetry breaking of a racemic mixture and the origin of life are addressed.

Spontaneous chiral separation in noncovalent molecular clusters.

A new method is introduced to determine the extent to which spontaneous chiral separation occurs in small noncovalently bound clusters. Soft-sampling electrospray ionization was used to transfer noncovalent complexes from solution to the gas phase. Mixing D and L enantiomers with one of the pair isotopically labeled reveals the effect of chirality on cluster stability. The observed cluster distribution is compared to the predicted statistical distribution to determine any preference for homo- or heterochirality. Arginine, for example, forms a stable trimer with no preference for the chirality of the individual amino acids. Serine, however, forms a protonated octamer with a pronounced preference for homochirality. The implications of these results for the structures of the complexes are discussed along with the broader implications for the origins of homochirality in living systems (homochirogenesis).

Infrared Reflection−Absorption Spectroscopy of Racemic and Enantioenriched Methyl 17,18-Dihydroxyoctadecanoate at the Air−Water Interface

Surface pressure−area isotherms (Π−A) of racemic and enantioenriched methyl 17,18-dihydroxyoctadecanoate (rac- and (17R)-17,18-DHO) at the air−water interface are very similar, while these esters show significant differences in infrared reflection−absorption spectroscopy (IRRAS). Substantial shifts of the νas(CH2) from 2927 to 2917 cm-1 and of the ν(CO) bands from 1722 to 1742 cm-1 are observed upon monolayer compression from 0.8 to 0.2 nm2 per molecule. This shift occurs gradually for rac-17,18-DHO and sharply within a small compression interval for (17R)-17,18-DHO. This suggests first, a change from a weak heterochiral to a weak homochiral preference, and second a removal of the two hydroxyl groups from the air−water interface, while the ester function immerses into the uppermost water layer in the compressed monolayer. This arrangement appears to be stabilized by intermolecular hydrogen bonds.

The oxidation of [Co(edta)] 2− by [Co(phen) 3] 3+

[(Co(phen) 3][(Sb( R, R)-tartrate) 2] · 8H 2O, C 44H 44N 6O 20CoSb 2, crystallizes in the trigonal space group P3 221 (No. 154) with Z = 3, a = 18.861(4), c = 11.917(3) Å and R = 0.035 for 1800 reflections. The structure has been determined and the absolute configuration of (+) 589-[Co(phen) 3](ClO 4) 3 · 2H 2O formed from the compound by Cl 2 oxidation is confirmed as Λ. Reduction of [Co(phen) 3] 3+ by [Co(edta) 2− is outer-sphere with a second-order rate constant of 9.6 x 10 −3 M −1 s −1 at 25 °C and 0.10 M ionic strength. When Λ-[Co(phen) 3] 3+ is used as oxidant, chiral induction is observed and the [Co(edta)] − product shows a enantiomeric excess of 20% of the Λ isomer. This homochiral (ΛΛ) preference contrasts with the heterochiral (ΔΛ) preference found previously for reactions of [Co(edta)] 2− with [Fe(phen) 3] 3+ and [Os(phen) 3] 3+. Interpretation of the results is aided by 1H NMR relaxation studies of the ion pair {[Co(phen) 3] 3+, [Co(edta)] −}.

Stereoselective photosensitized decomposition of tris(oxalato)cobaltate(III) and tris(acetylacetonato)cobalt(III) by tris(2,2′-bipyridine)ruthenium(II)

The photosensitized decomposition of [Co(ox)3]3–(H2ox = oxalic acid) and [Co(acac)3](Hacac = acetylacetone) proceeded stereoselectively in the presence of optically active [Ru(bipy)3]2+(bipy = 2,2′-bipyridine). Homochiral (Δ-Δ or Λ-Λ) preference for [Co(ox)3]3– contrary to heterochiral (Δ-Λ) preference for [Co(acac)3] was found in aqueous solution according to the circular dichroism appearing upon decomposition of the racemic complex, and by the decomposition rate of each optical isomer. For the [Co(ox)3]3– complex the selectivity varied with the addition of an organic solvent such as methanol and acetonitrile following a decrease in the decomposition rate. The factors resulting in the different stereoselectivity between the similar tris-chelate complexes containing planar bidentate ligands have been investigated. Quenching rate constants for Δ-*[Ru(bipy)3]2+ obtained from emission lifetimes demonstrated the homochiral preference in the quenching process with both [Co(ox)3]3– and [Co(acac)3] in aqueous solution. Chromatographic experiments using a SP-sephadex C-25 column exchanged with Δ-[Ru(bipy)3]2+ demonstrated the higher stability of the Δ-Δ compared with the Δ-Λ pair. It is concluded that overall selectivity is controlled by the relative and competitive rates between the back electron transfer and decomposition processes.