Chloride ion binding to bacteriorhodopsin at low pH: an infrared spectroscopic study.

Bacteriorhodopsin (bR) and halorhodopsin (hR) are light-induced ion pumps in the cell membrane of Halobacterium salinarium. Under normal conditions bR is an outward proton transporter, whereas hR is an inward Cl- transporter. There is strong evidence that at very low pH and in the presence of Cl-, bR transports Cl- ions into the cell, similarly to hR.

Structural factors controlling ligand binding to myoglobin: a kinetic hole-burning study.

Using temperature-derivative spectroscopy in the temperature range below 100 K, we have studied the dependence of the Soret band on the recombination barrier in sperm whale carbonmonoxy myoglobin (MbCO) after photodissociation at 12 K. The spectra were separated into contributions from the photodissociated species, Mb*CO, and CO-bound myoglobin. The line shapes of the Soret bands of both photolyzed and liganded myoglobin were analyzed with a model that takes into account the homogeneous bandwidth, coupling of the electronic transition to vibrational modes, and static conformational heterogeneity.

Ca(2+)- and Al(3+)-induced conformational transitions of amyloid fragment H-Ile-Ile-Gly-Leu-Met-NH2.

The effect of Ca2+ and Al3+ binding on the conformation of the neurotoxic amyloid fragment H-Ile-Ile-Gly-Leu-Met-NH2 [betaA(31-35)NH2] was studied in trifluoroethanol solutions and in the presence of liposomes. Comparative circular dichroism and Fourier-transform infrared spectroscopic studies revealed that the peptide forms a specific 1:1 complex with Ca2+ which coordinates the polar amide carbonyl groups of the peptide backbone. The results suggest the importance of a folded structure in the complexation of Ca2+.

pH-induced structural changes in bacteriorhodopsin studied by Fourier transform infrared spectroscopy.

Previous C13-NMR studies showed that two of the four internal aspartic acid residues (Asp-96 and Asp-115) of bacteriorhodopsin (bR) are protonated up to pH = 10, but no accurate pKa of these residues has been determined. In this work, infrared spectroscopy with the attenuated total reflection technique was used to characterize pH-dependent structural changes of ground-state, dark-adapted wild-type bacteriorhodopsin and its mutant (D96N) with aspartic acid-96 replaced by asparagine. Data indicated deprotonation of Asp-96 at high pH (pKa = 11.

Alternative translocation of protons and halide ions by bacteriorhodopsin.

Bacteriorhodopsin (bR568) in purple membrane near pH 2 shifts its absorption maximum from 568 to 605 nm forming the blue protein bRacid605, which no longer transports protons and which shows no transient deprotonation of the Schiff base upon illumination. Continued acid titration with HCl or HBr but not H2SO4 restores the purple chromophore to yield bRHCl564 or bRHBr568. These acid purple forms also regain transmembrane charge transport, but no transient Schiff base deprotonation is observed.

Orientation of the chromophore plane in purple membrane.

Excitation of bacteriorhodopsin (BR) in its beta absorption band drives a photocycle identical in the millisecond range, to that excited in the alpha band of BR. The relative contribution of the two transition dipoles distinguished in the beta band to the initiation of the photocycle was established by photoselection experiments. Having this information the orientation of the chromophoric plane was specified by electric dichroism measurements.