Development of homochiral peptides in the chemical evolutionary process: Separation of homochiral and heterochiral oligopeptides

Abstract

Living organisms have one-handed structures of L-amino acids in proteins and D-sugars in nucleic acids. Although the origins of each one-handed structure (or homochirality) have been discussed for many years, these discussions have been restricted to monomeric compounds, such as amino acids and monosaccharides, or their stereospecific condensation reactions. Oligomers of these compounds have to be considered in the accumulation processes of homochirality because of the differences in physical properties of the diastereomers. High-performance liquid chromatography (HPLC) and the calculation of the partition coefficient values showed that the peptides having heterochiral sequences like L-Ala-D-Ala or D-Ala-L-Ala were more hydrophobic than the peptides having homochiral ones (L-Ala-L-Ala and D-Ala-D-Ala). Similar results were given from the calculation of most linear dipeptides and all cyclic ones composed of Gly, Ala, Val, or Asp. In addition, longer homo-oligopeptides composed of Ala, Val, or Asp also gave similar results. This general tendency would be useful for the separation of diastereomeric oligopeptides in water. The results also suggest that the separation of the homochiral peptides from the heterochiral ones by their solubility in water could have progressed in a primitive hydrosphere.

Hydrophobicity of Peptides Containing D‐Amino Acids

“…Results and Discussion. -Experimental log P values, log P exp , of amino acids and oligopeptides have been correlated to their log k' values reported in the literature [23]. The present study corrects the correlation by using the better established log P exp values [38] [39] of amino acids (Tables 1 and 2).…”

“…1 shows that there is a linear correlation between log P exp and log k' (log P exp ¼ 2.844 log k' -2.843; square of correlation coefficient: R 2 ¼ 0.9912) in the cases of aliphatic and acidic amino acids ( Table 1), but not in the cases of aromatic and basic amino acids. Both aromatic and basic amino acids deviate from the linear correlation [23]. On the line in Fig.…”

“…It has been clarified [23] that most of the latter oligopeptides are usually more hydrophobic than the former. Separation of diastereoisomeric peptides by reversed-phase high-performance liquid chromatography (RP-HPLC) has provided experimental results demonstrating that many heterochiral peptide diastereoisomers are more retarded than their homochiral peptide diastereoisomers on RP-HPLC.…”

Epimerization of Cyclic Alanyl-Alanine in Basic Solutions

“…Cyclic dipeptides have been suggested as easily formed peptides in the chemical evolutionary process 22 and important compounds in the development of homochirality 23 , because of theireasy formation from amino acids and the difference between diastereomers in hydrophobicity.The topic of this research is the simple chiral cyclic dipeptides (cyclo-(alanyl-alanine): Homo-L-CP (1) and Hetero-CP (2)) shown in Figure 1.In particular, the epimerization and ring-opening reactions are investigated in basic aqueous solutions.The epimerization of Homo-L-CP (1) and (3) will proceed at first from the hydrogen abstraction step to give solvated anion intermediates (4) and (5) 15 . These transform to their enantiomers.…”

“…(2),and Cyclo-(D-Ala-L-Ala) (3) were purchased from Sigma Chemicals Inc. Authentic linear peptides, L-Ala-L-Ala and D-Ala-L-Ala were prepared by the manner in the literature 22 .…”