Chiral Transmission between Amino Acids: Chirally Selective Amino Acid Substitution in the Serine Octamer as a Possible Step in Homochirogenesis

Abstract

Pasteur×s discovery [1] that tartaric acid can occur in two enantomeric forms raised still unanswered questions [2±6] concerning the origin of chirality in biological systems (homochirality). Homochirogenesis, the set of events leading to the almost exclusive preference for one enantiomeric form over the other in the suite of biological compounds that make up living organisms, can be considered to have involved three steps: 1) symmetry breaking, 2) chiral enrichment, and 3) chiral transmission.The results of CD spectroscopy accord well with the amide proton/deuterium exchange NMR measurements performed in [D 4 ]methanol, where 4 lost its amide signal intensity in 15 minutes, while the amide protons of 5 survived for more than 1 hour. The lower exchange rate for heptamer 5, together with the IR and CD results, supports the view that the folding occurs in a cooperative manner as the chain length increases.Herein we have shown that the 1R,2S-ACPC pentamer and heptamer adopt a self-stabilizing six-strand secondary structure in the solution phase. From earlier results, we know that the model system composed of (1R,2R)-trans-ACPC monomers adopts the highly stable 12-helix conformation [16] (Figure 6). The change in the relative configuration of the ACPC residues in these models provides an efficient control over the Figure 6. Comparison of the secondary structures induced by the different relative configurations of the ACPC residues.

Chiral enrichment of serine via formation, dissociation, and soft-landing of octameric cluster ions

“…The novel experimental facts shown here support the hypothesis [2,5,8] that serine, through the formation of its homochiral octamers, may have played a key role in homochirogenesis. This role, as argued elsewhere [2,5,8], involved the transmission of chirality to other molecules.…”

“…This role, as argued elsewhere [2,5,8], involved the transmission of chirality to other molecules. The observed increase in enantiomeric excess in cycles of formation and dissociation of serine octamers allows a prebiotic scenario where slight differences in enantiomeric composition might have given rise to octamers of both L-and D-chirality in greatly different proportions.…”

“…These studies support the hypothesis [2,5] that serine octamers might have been involved in prebiotic mechanisms that lead to homochirogenesis. It has been demonstrated that protonated serine octamers undergo chiroselective reactions with compounds of fundamental importance to biochemistry, including other amino acids [5], glyceraldehyde [8], and glucose [8]. The chiral selection in these reactions occurs with preference for the enantiomers present in life (L-amino acids, Dsaccharides).…”

Formation and stability of charged amino acid clusters and the role of chirality

“…Examples of magic numbers are 13, 55, 147 for closed shells of atoms held together by van der Waals forces and 2, 8, 20, 40 for the number of valence electrons in metal clusters. Much of the work related to biomolecular clusters has focused on clusters of the serine amino acid [8,[12][13][14][15][16][17][18][19]. For these clusters, eight is found to be a magic number and this octamer further shows a preference for homochirality [8,12].…”

A review of recent advances in mass spectrometric methods for gas-phase chiral analysis of pharmaceutical and biological compounds

“…Chirality is an intrinsic property of the biomolecular building blocks of life, being incorporated into the essential components that are necessary for life such as amino acids, sugars, proteins, nucleic acids, and polysaccharides [3,4]. The use of enantiomerically pure compounds as drugs has continuously attracted much interest in pharmaceutical industry.…”