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Structure and Reactivity in Langmuir Films of Amphiphilic Alkyl and Thio-alkyl Esters of α-Amino Acids at the Air/Water Interface
journal contribution
posted on 2004-06-03, 00:00 authored by Ran Eliash, Isabelle Weissbuch, Markus J. Weygand, Kristian Kjaer, Leslie Leiserowitz, Meir LahavThe structure and reactivity of alkyl esters of several α-amino acids self-assembled at the air/water interface
have been investigated as part of our studies on mechanisms that are possibly relevant for the generation of
homochiral prebiotic peptides. Grazing incidence X-ray diffraction (GIXD) studies of monolayers of racemic
and enantiopure alkyl esters and thio-esters of alanine on the water surface demonstrated that these racemates
self-assemble in the form of mixed solid solutions, because of disorder of the headgroups of the two enantiomers
(enantiomeric disorder) within the two-dimensional (2D) crystallites. Matrix-assisted laser-desorption ionization
time-of-flight Mass Spectrum (MALDI−TOF MS) analysis of the products collected from the air/water interface
indicated the formation of low-molecular-weight oligopeptides (primarily dimers) and, in the case of some of
the thioesters, small quantities of trimers and tetramers. Mass spectrometric studies on the diastereoisomeric
distribution of the oligopeptides, starting from deuterium enantio-labeled monomers, demonstrated binomial
statistics, such as that in reactions occurring in an isotropic environment. The alkyl esters of phenylalanine
and tyrosine did not form 2D crystallites at the air/water interface, and, upon polycondensation, they yielded
only dipeptides. The enantiomeric disorder within the 2D crystallites of the monomers of the alkyl esters and
thioesters of racemic serine was absent. Polycondensation of these esters, however, yielded only dipeptides
and tripeptides and they were not investigated further. In contrast to previous reports, the present studies
demonstrate that this reaction does not proceed beyond the dipeptide stage and, therefore, cannot be regarded
as a plausible system for the generation of prebiotic peptides.