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SALT AND pH‐DEPENDENT CHANGES OF THE PURPLE MEMBRANE ABSORPTION SPECTRUM
Author(s) -
Kimura Yoshiaki,
Ikegami Akira,
Stoeckenius Walther
Publication year - 1984
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1984.tb05353.x
Subject(s) - chemistry , chromophore , absorption (acoustics) , absorption spectroscopy , salt (chemistry) , membrane , titration , analytical chemistry (journal) , conformational change , crystallography , inorganic chemistry , chromatography , stereochemistry , photochemistry , biochemistry , physics , quantum mechanics , acoustics
—Purple membrane suspensions change their color to blue and the absorption maximum shifts to 608 nm when the membrane is deionized on a cation exchange column or when it is washed first with < 2N NaCl followed by deionized water. The deionized chromophore is essentially identical with the chromophore produced by lowering the pH of the native membrane to < 4.0 (p K < 3.0). However, the deionized membrane does not aggregate and can be obtained in the pure state. The original purple color of the membrane is restored by addition of around 1 m M Na + , K + or 10 μ M Mg 2+ , Ca 2+ , Sr 2+ , Mn 2+ , Pb 2+ or La 2+ when the protein concentration is 5μ M . The required salt concentrations decrease with decreasing pH. Direct measurement of bound Ca 2+ by atomic absorption spectroscopy yields a ratio of Ca 2+ to protein of <2 and a binding constant of 1.4 × 10 6 . Titration of the spectral change with salts at different pH values shows a linear relation between the pH and the logarithm of the salt concentration, with a 1:1 ratio for Na + and 1:2 ratio for Ca 2+ . These relations are well predicted by Gouy‐Chapman theory; however, the accompanying release of protons, changes of the CD spectrum, the complex kinetics of the spectral change during reconstitution with salt and preliminary X‐ray diffraction results all suggest that conformational changes may be occurring in the protein.