Boundary structures and the non-polar organic components of the murchison carbonaceous chondrite

Abstract

Amphiphilic compounds were presumably present in the prebiotic environment, and at some point must have been involved in the first cellular life forms. An interesting question concerns the plausible identity of the amphiphiles. Fatty acids are among the simplest amphiphiles, and can self-assemble into vesicular membranes (1,2). However, such membranes are relatively labile, and it is likely that more complex amphiphilic mixtures were required for stable membranes as shown in prebiotic simulations (3,4,5). Although simulations serve a useful purpose, a second approach is to investigate the physical properties of organic mixtures known to have been synthesized abiotically by natural processes. One such mixture is the organic content of carbonaceous chondrites produced in the early solar system (6). The results to be reported here were obtained with samples of the Murchison meteorite, which earlier studies have shown to contain complex organic polymers together with a minor fraction of carboxylic acids and traces of alkanes, alcohols and amino acids (see ref. 7 for review). Samples of the chondrite were extracted with organic solvents and the extracted material was investigated by surface chemistry, infrared and fluorescence spectrophotometry, and microscopic methods (8). Parallel extractions of the Allende chondrite and silica powder were carried out. The Allende chondrite is relatively poor in organic content, and, together with the silica, served as a control against contaminants introduced during extraction. Chloroform-methanol extraction of powdered meteoritic material was carried out at alkaline pH ranges, and non-polar organic components were defined as those substances soluble in the chloroform phase. The non-polar extracts typically contained a few tenths mg per gram meteorite, and were faint yellow in color, with fluorescence peaks at 395 (excitation) and 475 (emission). Infrared spectrophotometry showed absorption bands characteristic of -C-H, C=O and -O-H stretching frequencies, which is consistent with earlier reports of phenolic and carboxylic acid derivatives of hydrocarbons. Titrations of non-polar material dispersed in water indicated pK values of 6.5 and 9.3. The extracted material was surface active, and spread at the airwater interface to produce films with surface areas of 0.45 m 2