Abstract
The visible absorption of bacteriorhodopsin (bR) is highly sensitive to pH, the maximum shifting from 568 nm (pH 7) to similar to 600 nm (pH 2) and back to 565 nm (pH 0) as the pH is decreased further with HCl. Blue membrane ( lambda sub(max) > 600 nm) is also formed by deionization of neutral purple membrane suspensions. Low-temperature, magic angle spinning super(13)N NMR was used to investigate the transitions to the blue and acid purple states. The super(15)N NMR studies involved ( epsilon - super(15)N)lysine bR, allowing a detailed investigation of effects at the Schiff base nitrogen. The super(15)N resonance shifts similar to 16 ppm upfield in the neutral purple to blue transition and returns to its original value in the blue to acid purple transition. Thus, the super(15)N shift correlates directly with the color changes, suggesting an important contribution of the Schiff base counterion to the "opsin shift."