The Serine octamer cluster in

physical chemistry Physical chemistry is the study of macroscopic and microscopic phenomena in chemical systems in terms of the principles, practices, and concepts of physics such as motion, energy, force, time, thermodynamics, quantum chemistry, statistical ...

is an unusually stable cluster consisting of eight

serine Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α- amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − for ...

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...

s (Ser) implicated in the origin of

homochirality Homochirality is a uniformity of chirality, or handedness. Objects are chiral when they cannot be superposed on their mirror images. For example, the left and right hands of a human are approximately mirror images of each other but are not their ow ...

. This cluster was first discovered in

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a '' mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is u ...

experiments.

Electrospray ionization Electrospray ionization (ESI) is a technique used in mass spectrometry to produce ions using an electrospray in which a high voltage is applied to a liquid to create an aerosol. It is especially useful in producing ions from macromolecules becau ...

of an

aerosol An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be natural or anthropogenic. Examples of natural aerosols are fog or mist, dust, forest exudates, and geyser steam. Examples of ant ...

of serine in methanol results in a mass spectrum with a prominent ion peak of 841 corresponding to the Ser₈+H⁺

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

. The smaller and larger clusters are virtually absent in the spectrum and therefore the number 8 is called a magic number. The same octamer ions are also produced by rapid evaporation of a serine solution on a hot (200-250 °C) metal surface or by

sublimation Sublimation or sublimate may refer to: * ''Sublimation'' (album), by Canvas Solaris, 2004 * Sublimation (phase transition), directly from the solid to the gas phase * Sublimation (psychology), a mature type of defense mechanism * Sublimate of mer ...

of solid serine. After production, detection again is by mass-spectroscopic means. For the discussion of homochirality, these laboratory production methods are designed to mimic prebiotic conditions. The cluster is not only unusually stable but also unusual because the clusters have a strong homochiral preference. A

racemic In chemistry, a racemic mixture, or racemate (), is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule or salt. Racemic mixtures are rare in nature, but many compounds are produced industrially as racemates. ...

serine solution produces a minimum amount of cluster and with solutions of both

enantiomer In chemistry, an enantiomer ( /ɪˈnænti.əmər, ɛ-, -oʊ-/ ''ih-NAN-tee-ə-mər''; from Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''(méros)'' 'part') – also called optical isomer, antipode, or optical ant ...

s a maximum amount is formed of both homochiral D-Ser₈ and L-Ser₈. In another experiment cluster formation of a racemic mixture with

deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two Stable isotope ratio, stable isotopes of hydrogen (the other being Hydrogen atom, protium, or hydrogen-1). The atomic nucleus, nucleus of a deuterium ato ...

enriched L-serine results in a product distribution with hardly any 50/50 D/L clusters but a preference for either D or L enantioenriched clusters. A model for

chiral amplification In chemistry, an enantiomer ( /ɪˈnænti.əmər, ɛ-, -oʊ-/ ''ih-NAN-tee-ə-mər''; from Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''(méros)'' 'part') – also called optical isomer, antipode, or optical ant ...

is proposed whereby enantioenriched clusters are formed from a non-racemic mixture already enriched by L-serine as a result of a

mirror-symmetry breaking Homochirality is a uniformity of chirality, or handedness. Objects are chiral when they cannot be superposed on their mirror images. For example, the left and right hands of a human are approximately mirror images of each other but are not their ow ...

process. Cluster formation is followed by isolation and on subsequent dissociation of the cluster a serene solution forms with a higher concentration of L-serine than in the original mixture. A cycle can be maintained in which each turn results in an incremental enrichment in L-serine. Many such cycles eventually result in

enantiopure In chemistry, an enantiomer ( /ɪˈnænti.əmər, ɛ-, -oʊ-/ ''ih-NAN-tee-ə-mər''; from Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''(méros)'' 'part') – also called optical isomer, antipode, or optical anti ...

L-serine. This model has been experimentally verified.

Chiral transmission Homochirality is a uniformity of chirality, or handedness. Objects are chiral when they cannot be superposed on their mirror images. For example, the left and right hands of a human are approximately mirror images of each other but are not their ow ...

is assumed to take place through so-called substitution reactions of serine clusters. In these reactions, a serine monomer in a cluster can be replaced by another small biologically relevant molecule. For instance Ser₈ reacts with

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, usi ...

(Glc) to the Ser₆ + Glc₃ + Na⁺ cluster. Moreover, the cluster of synthetic L-glucose with Ser₈ is less abundant than that with the biological D-glucose.

References

{{reflist Physical chemistry Stereochemistry

See also

References