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Ultraviolet circular dichroism spectroscopy in the interval from 190 to 240 nm and infrared spectroscopy in the region of the amide I band (1,600 cm-1 to 1,700 cm-1) has been used to estimate the alpha-helix content and the beta-sheet content of bacteriorhodopsin. Circular dichroism spectroscopy strongly suggests that the alpha-helix content is sufficient for only five helices, if each helix is composed of 20 or more residues. It also suggests that there is substantial beta-sheet conformation in bacteriorhodopsin. The presence of beta-sheet secondary structure is further suggested by the presence of a 1,639 cm-1 shoulder on the amide I band in the infrared spectrum. Although a structural model consisting of seven alpha-helical rods has been generally accepted up to this point, the spectroscopic data are more consistent with a model consisting of five alpha-helices and four strands of beta-sheet. We note that the primary amino acid sequence can be assigned to segments of alpha-helix and beta-sheet in a way that does not require burying more than two charged groups in the hydrophobic membrane interior, contrary to the situation for any seven-helix model.

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The direction of orientation of the protein bacteriorhodopsin within the purple membrane of Halobacterium halobium has been determined by selected-area electron diffraction of membranes preferentially oriented by adsorption to polylysine. Purple membrane is known to adsorb preferentially to poly sine by its cytoplasmic surface at neutral pH and by its extracellular surface at Iow pH. To maintain the adsorbed membranes in a well-ordered state in the electron microscope, an improved technique of preparing frozen specimens was developed. Large areas of frozen-hydrated specimens, devoid of bulk water, were obtainable after the specimen was passed through a Ca stearate film at an air-water interface. High-resolution microscopy was used to relate the orientation observed in the electron diffraction patterns to the orientation of the projected structure that is obtained from images. We have found that the three-dimensional structure determined by Henderson Purple membrane is a specialized region of the plasma membrane of Halobacterium halobium, a halophilic, photosynthetic microorganism. The lipid content of the purple membrane is similar to, but not identical with, that of the rest of the plasma membrane (1, 2). However, the purple membrane contains only a single protein, bacteriorhodopsin (3), which is arranged in the plane of the membrane in a highly ordered, crystalline array (4). The purple color of bacteriorhodopsin is due to the presence of retinal linked to a lysine residue by a Schiff base, as in the visual pigment rhodopsin. Bacteriorhodopsin differs from rhodopsin in that it is not bleached upon absorption of light. The bacteriorhodopsin molecule instead undergoes a cyclic photoreaction, during which hydrogen ions are translocated from the inside of the cell to the outside medium (5). The special function of bacteriorhodopsin, and of the purple membrane, is therefore to act as a photon-driven proton pump, establishing a concentration gradient of hydrogen ions across the plasma membrane. The chemical potential associated with this pH gradient is then used by the cell for the synthesis of ATP. A recent review summarizes the role of bacteriorhodopsin in providing this photosynthetic capability to H. halobium (6).The molecular structure of bacteriorhodopsin has been determined at about 7-A resolution by high-resolution electron microscopy of unstained, hydrated membranes (7, 8). The protein was found to contain seven a-helices packed parallel to one another and spanning the full thickness of the membrane. The electron microscope structure analysis confirmed many inferences that were made in earlier x-ray diffraction studies of the purple membrane (9, 10). However, the orientation of the three-dimensional model of bacteriorhodopsin, relative to the cytoplasmic and extracellular surfaces of the membrane, was not determined in the initial electron microscope structural study. It is important to know the orientation of the structural model of bacteriorhodopsin relative to the inside and the outside of the c...

This paper reports the irritant effects associated with formaldehyde exposures in mobile homes. Week-long, integrated formaldehyde concentrations were measured using passive monitors in summer and winter while the mobile home residents continued their normal activities. Information on acute health problems, chronic respiratory/allergic illnesses, smoking behavior, demographic variables, and time spent at home was obtained on over 1000 individuals during the sampling period. Measured formaldehyde concentrations varied from under the limit of detection (0.01 ppm) to 0.46 ppm. Formaldehyde exposure was estimated for each individual by multiplying the concentration measured in his or her home by the time he or she spent at home. Irritant effects were found to be associated with formaldehyde exposure after controlling for age, sex, smoking status, and chronic illnesses using a logistic procedure. Some of the interaction terms found to be significant indicated that there were synergistic effects between formaldehyde exposure and chronic health problems.