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Dielectric and Calorimetric Studies of Hydrated Purple Membrane

Peter Berntsen ^*, Rikard Bergman ^*, Helén Jansson ^*, Martin Weik  and Jan Swenson ^*

^* Department of Applied Physics, Chalmers University of Technology and Göteborg University, Göteborg, Sweden; and Institut de Biologie Structurale, Grenoble, France

Correspondence: Address reprint requests to Peter Berntsen, Dept. of Applied Physics, Göteborg University, SE-41296 Göteborg, Sweden. E-mail: peterber{at}fy.chalmers.se; or to Jan Swenson, f5xjs{at}fy.chalmers.se.

Purple membranes (PM) from halobacteria were hydrated to 0.4 and 0.2 g H₂O/g of PM and studied by dielectric spectroscopy and differential scanning calorimetry between 120 and 300 K. The dielectric process, attributed to a local (ß) relaxation of the confined supercooled water, shows an Arrhenius temperature behavior at low temperatures. In the case of the most hydrated PM a small deviation from the Arrhenius behavior occurs at 190–200 K together with a pronounced endothermic process and an increased activation energy. The observed crossover is accompanied by a reduction of the interlayer spacing due to the partial loss of the intermembrane water. All these effects at 200 K are consistent with a scenario where the local relaxation process merges with a nonobservable -relaxation of the interlayer water, giving rise to a more liquid-like behavior of the interfacial water. For the less hydrated sample the effects are less pronounced and shift to a slightly higher temperature.

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