The thermodynamic cis r 'trans equilibrium in stilbene and its derivatives can be established at room temperature only by the use of catalysts such as atomic iodine. The latter has now been used to determine the above equilibrium in a number of cases. The fraction of cis-isomer at equilibrium is ca. 0,002 in stilbene and the para-substituted derivatives investigated, but increases to 0.07 in ap-difluorostilbene and 2.4.6-trimethylstilbene, and to 0.20 in a-methylstilbene. These values were used to calculate the respective differences in free energy. Catalysed isomerization, in particular if the catalyst is a product of irradiation, may, under certain conditions, occur concurrently with direct or sensitized photoisomerization, and distort the results of the latter reactions. It is therefore important to distinguish between true photoisomerization, involving the excited states of the molecules undergoing isomerization, and photocatalysed isomerization, involving only the ground states of these molecules.
It has been previously found that lipids were required to maintain intact the tetrameric structure of the receptor for immunoglobulin E (IgE) (Fc epsilon R) in detergent solutions [Rivnay, B., Rossi, G., Henkart, M., & Metzger, H. (1984) J. Biol. Chem. 259, 1212-1217, and references cited therein]. Failure of commercially obtained lipids to provide sufficient protection, however, underscored the necessity for development of additional analytical approaches. In order to identify the phospholipid distribution in the intimate natural environment of this receptor, both the plasma membrane vesicles and the ligand-receptor complex (IgE-Fc epsilon R) have been isolated by affinity chromatography. The phospholipids of both preparations were compared. After extensive washing with detergent lipid micelles, IgE-Fc epsilon R retained 0.1-1% of the total phospholipids in the purified plasma membrane. The receptor-bound lipids were shown to contain phosphatidylcholine and sphingomyelin; the content of the latter lipid was enriched 2-5-fold compared with that in the plasma membranes. This pattern was observed with several detergents employed for purification and under a variety of experimental conditions. In light of the general distribution of choline phospholipids in the outer leaflet of plasma membranes, this enrichment may not be a characteristic of this particular receptor exclusively. These observations should be particularly helpful in studies on aggregation-induced functions of the isolated Fc epsilon receptor. In general, the methods employed enable isolation of purified and lipid-protected integral proteins and also provide an appropriate reference source of intact membrane vesicles. These qualities render this approach useful in similar studies of other membrane proteins.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.