Structure and reactivity in Langmuir films of amphiphilic alkyl and thio-alkyl esters of α-amino acids at the air/water interface

The structure and reactivity of alkyl esters of several alpha-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.