CONFORMATIONAL changes in coherent arrays of amphiphiles, such as phospholipids, are believed to be important in the function of biological membranes1. Danielli and Davson2 suggested that the bimolecular lipid leaflet comprised the basic structural unit in many natural membranes, but only recently has this fundamental structure become available for direct experimental investigation3. These so-called black lipid films have d.c. resistances of 106–109 Ω cm2 (refs. 4 and 5), but it is known that with certain preparations the resistance characteristics are markedly affected by the composition of the aqueous environment5–7. This communication outlines some preliminary observations on the effects of pH, in the presence of sodium and calcium ions, on the d.c. resistance of egg lecithin–cholesterol films.

CONSIDERABLE interest has recently been centred on the difficult separation of m-xylene and p-xylene by gas chromatography both from the point of view of the analysis of mixtures containing these isomers and the assessment of the performance of high efficiency columns. Martin and James1 predicted some years ago that it should be possible to achieve a separation using benzyldiphenyl as a stationary phase with a high-efficiency column and this was recently demonstrated by Zlatkis2. Similar partial separations have been described3–5 with other stationary phases in long packed columns and by using long capillary columns6. Langer7 has described the use of tetrahalophthalic esters as stationary phases which cause the meta isomer to be eluted before the para—the reverse of the usual order. (In an earlier paper8, the relative retention volumes of p-xylene and m-xylene were incorrectly quoted in the reverse order for both n-hexatriacontane and benzyldiphenyl.)