Sugar-Induced Blue Membrane: Release of Divalent Cations during Phase Transition of Purple Membranes Observed in Sugar-Derived Glasses

Abstract

The formation of blue membrane from purple membranes (PM) has been observed in glassy films made from PM and various sugars. The phase transition of PM at about 70 degrees C causes the complexation of divalent cations to be weakened. The vicinal diol structures in sugars are capable to complex divalent cations and delocalize them throughout the matrix as long as its glass transition temperature is lower than the phase transition temperature of PM. The loss of divalent cations from bacteriorhodopsin (BR), the only protein in PM, causes the formation of blue membrane (BM), which is accompanied by a loss of beta-sheet structure observable in the infrared spectrum. Glassy sugars are particular useful to observe this transition, as sugar entrapment does not restrict conformational changes of BR but rather retards them. The material obtained was named sugar-induced blue membrane (SIBM). The formation of SIBM is inhibited by the addition of divalent cations. Furthermore, SIBM is reverted immediately to PM by addition of water. A characteristic time dependence of the thermal reversion of SIBM to PM proves that the phase transition of PM triggers the release and uptake of divalent cations and the corresponding color change.