COOPERATIVITY OF THE DEHYDRATION BLUE-SHIFT OF BACTERIORHODOPSIN

Compositional heterogeneity reflects partial dehydration in three-dimensional crystals of bacteriorhodopsin

Absorption, fluorescence and excitation spectra of three-dimensional bacteriorhodopsin crystals harvested from a lipidic cubic phase are presented. The combination of the spectroscopic experiments performed at room temperature, controlled pH and full external hydration reveals the presence of three distinct protein species. Besides the well-known form observed in purple membrane, we find two other species with a relative contribution of up to 30%. As the spectra are similar to those of dehydrated or deionized membranes containing bacteriorhodopsin, we suggest that amino acid residues, located in the vicinity of the retinal chromophore, have changed their protonation state. We propose partial dehydration during crystallization and/or room temperature conditions as the main source of this heterogeneity. This assignment is supported by an experiment showing interconversion of the species upon intentional dehydration and by crystallographic data, which have indicated an in-plane unit cell in 3D crystals comparable to that of dehydrated bacteriorhodopsin membranes. Full hydration of the proteins after the water-withdrawing crystallization process is hampered. We suggest that this hindered water diffusion originates mainly from a closure of hydrophobic crystal surfaces by lipid bilayers. The present spectroscopic work complements the crystallographic data, due to its ability to determine quantitatively compositional heterogeneity resulting from proteins in different protonation states.

Water and carboxyl group environments in the dehydration blueshift of bacteriorhodopsin

The proton channels of the bacteriorhodopsin (BR) proton pump contain bound water molecules. The channels connect the purple membrane surfaces with the protonated retinal Schiff base at the membrane center. Films of purple membrane equilibrated at low relative humidity display a shift of the 570 nm retinal absorbance maximum to 528 nm, with most of the change occurring below 15% relative humidity. Purple membrane films were dehydrated to defined humidities between about 50 and 4.5% and examined by Fourier transform infrared difference spectroscopy. In spectra of dehydrated-minus-hydrated purple membrane, troughs are observed at 3645 and 3550 cm-1, and peaks are observed at 3665 and 3500 cm-1. We attribute these changes to water dissociation from the proton uptake channel and the resulting changes in hydrogen bonding of water that remains bound. Also, in the carboxylic acid spectral region, a trough was observed at 1742 cm-1 and a peak at 1737 cm-1. The magnitude of the trough to peak difference between 1737 and 1742 cm-1 correlates linearly with the extent of the 528 nm pigment. This suggests that a carboxylic acid group or groups is undergoing a change in environment as a result of dehydration, and that this change is linked to the appearance of the 528 nm pigment. Dehydration difference spectra with BR mutants D96N and D115N show that the 1737-1742 cm-1 change is due to Asp 96 and Asp 115. A possible mechanism is suggested that links dissociation of water in the proton uptake channel to the environmental change at the Schiff base site.

Photochemistry in dried polymer films incorporating the deionized blue membrane form of bacteriorhodopsin

The preparation and photochemical properties of dried deionized blue membrane (dIbR600; lambdamax approximately 600 nm, epsilon approximately 54, 760 cm-1 M-1, f approximately 1.1) in polyvinyl alcohol films are studied. Reversible photoconversion from dIbR600 to the pink membrane (dIbR485; lambdamax approximately 485 nm) is shown to occur in these films under conditions of strong 647-nm laser irradiation. The pink membrane analog, dIbR485, has a molar extinction coefficient of approximately 39,000 cm-1 M-1 (f approximately 1.2). The ratio of pink --> blue and blue --> pink quantum efficiencies is 33 +/- 5. We observe an additional blue-shifted species (dIbR455, lambdamax approximately 455 nm) with a very low oscillator strength (f approximately 0.6, epsilon approximately 26,000 cm-1 M-1). This species is the product of fast thermal decay of dIbR485. Molecular modeling indicates that charge/charge and charge/dipole interactions introduced by the protonation of ASP85 are responsible for lowering the excited-state all-trans --> 9-cis barrier to approximately 6 kcal mol-1 while increasing the corresponding all-trans --> 13-cis barrier to approximately 4 kcal mol-1. Photochemical formation of both 9-cis and 13-cis photoproducts are now competitive, as is observed experimentally. We suggest that dIbR455 may be a 9-cis, 10-s-distorted species that partially divides the chromophore into two localized conjugated segments with a concomitant blue shift and decreased oscillator strength of the lambdamax absorption band.

Retention of bacteriorhodopsin activity in dried sol-gel glass

The D96N mutant form of bacteriorhodopsin (bR) purple membrane fragments isolated from Halobacterium salinarium has been entrapped in sol-gel glass and dried. The photo-activity of the bR was monitored during drying over a period of 60 days. The entrapped bR was initially dried at 4 degrees C followed by room temperature storage. Samples showing constant activity after storage at 4 degrees C lost further activity at room temperature, eventually reaching a level where the activity remained constant. Upon further drying at 100 degrees in a vacuum oven the glass lost an additional 11.4 +/- 1.3% weight and all remaining activity.