Intracellular water dynamics in Haloarcula marismortui, an extremely halophilic organism originally isolated from the Dead Sea, was studied by neutron scattering. The water in centrifuged cell pellets was examined by means of two spectrometers, IN6 and IN16, sensitive to motions with time scales of 10 ps and 1 ns, respectively. From IN6 data, a translational diffusion constant of 1.3 ؋ 10 ؊5 cm 2 s ؊1 was determined at 285 K. This value is close to that found previously for other cells and close to that for bulk water, as well as that of the water in the 3.5 M NaCl solution bathing the cells. A very slow water component was discovered from the IN16 data. At 285 K the waterprotons of this component displays a residence time of 411 ps (compared with a few ps in bulk water). At 300 K, the residence time dropped to 243 ps and was associated with a translational diffusion of 9.3 ؋ 10 ؊8 cm 2 s ؊1 , or 250 times lower than that of bulk water. This slow water accounts for Ϸ76% of cell water in H. marismortui. No such water was found in Escherichia coli measured on BSS, a neutron spectrometer with properties similar to those of IN16. It is hypothesized that the slow mobility of a large part of H. marismortui cell water indicates a specific water structure responsible for the large amounts of K ؉ bound within these extremophile cells.Haloarcula marismortui ͉ water structure and dynamics ͉ extreme haplophile T he study of the specific properties of water in biological systems continues to yield fascinating surprises. Haloarcula marismortui, an archaeal extreme halophile isolated from the Dead Sea, attracted our attention some years ago because of its high selectivity for, despite a high membrane permeability (1). In distinction to other organisms, K ϩ was retained within the cell, even in the absence of metabolism, with a half-time of exchange with the outside medium of Ͼ24 h (2). At the time, the only systems known with a very high binding selectivity were antibiotics such as nonactin and valinomycin. The structure responsible for their specificity is a ''cage'' of six to eight carbonyl oxygen atoms arranged in space according to a definite pattern. If the same principle were to be responsible for K ϩ binding in H. marismortui, 24-32 moles oxygen per liter of cell water would be required, enormous amounts of oxygen atoms that cannot be supplied exclusively by the organic components of the cell. It is therefore suggested that the oxygen atoms required might be furnished by the ordering of water molecules forming a tertiary system of water, KCl, and cell proteins.Evidence in favor of more than a single phase of water in cell pellets of H. marismortui came initially from H-NMR measurements (3). The visible intensity of water in such pellets accounted for all of the total water present (101 Ϯ 7%). When the pellets were slowly cooled to below Ϫ19°C (the temperature at which the culture medium froze), 40% of the water signal remained visible. From analysis of the dynamic properties of the intracellular water, it was concluded ...
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