Compatible solutes accumulate in the cytoplasm of halophilic microorganisms, enabling their survival in a high-salinity environment. Ectoine is such a compatible solute. It is a zwitterionic molecule that strongly interacts with surrounding water molecules and changes the dynamics of the local hydration shell. Ectoine interacts with biomolecules such as lipids, proteins, and DNA. The molecular interaction between ectoine and biomolecules, in particular the interaction between ectoine and DNA, is far from being understood. In this paper, we describe molecular aspects of the interaction between ectoine and doublestranded DNA (dsDNA). Two 20 base pairs-long dsDNA fragments were immobilized on a gold surface via a thiol-tether. The interaction between the dsDNA monolayers with diluted and concentrated ectoine solutions was examined by means of X-ray photoelectron and polarization modulation infrared reflection absorption spectroscopies (PM IRRAS). Experimental results indicate that the ability of ectoine to bind water reduces the strength of hydrogen bonds formed to the ribose-phosphate backbone in the dsDNA. In diluted (0.1 M) ectoine solution, DNA interacts predominantly with water molecules. The sugar−phosphate backbone is involved in the formation of strong hydrogen bonds to water, which, over time, leads to a reorientation of the planes of nucleic acid bases. This reorientation destabilizes the strength of hydrogen bonds between the bases and leads to a partial dehybridization of the dsDNA. In concentrated ectoine solution (2.5 M), almost all water molecules interact with ectoine. Under this condition, ectoine is able to interact directly with DNA. Density functional theory (DFT) calculations demonstrate that the direct interaction involves the nitrogen atoms in ectoine and phosphate groups in the DNA molecule. The results of the quantum-chemical calculations show that rearrangements in the ribose-phosphate backbone, caused by a direct interaction with ectoine, facilitates contacts between the O atom in the phosphate group and H atoms in a nucleic acid base. In the PM IRRA spectra, an increase in the number of IR absorption modes in the base pair frequency region proves that the hydrogen bonds between bases become weaker. Thus, a sequence of reorientations caused by interaction with ectoine leads to a breakdown of hydrogen bonds between bases in the double helix.

Topography is a critical feature driving formation and dynamics of protein and lipid domains within biological membranes. The yeast plasma membrane (PM) has provided a powerful model system to study lateral domain formation, including characteristic BAR domain-induced PM furrows. Currently, it is not clear how the components involved in the establishment of these furrows cooperate to precisely regulate local PM topography. Here we report opposing functions for the Sur7 and Nce102 families of tetraspanner proteins in modulating membrane curvature and domain topography. Using STED nanoscopy and freeze-fracture EM we found that Sur7 tetraspanners form multimeric strands at the upper edges of PM furrows, which counteract the forces exerted by BAR domain proteins and prevent membrane tubulation. In contrast, Nce102 tetraspanners are located basal to the Sur7 proteins and promote BAR domain-induced curvature. The segregation of the two tetraspanner-based nanodomains is further supported by differential distribution of ergosterol to the upper edge of furrows and PIP2 lipids at the furrow base. These findings suggest a general role of tetraspanner proteins in sculpting local membrane domains.

Lack of long-time stability of dsDNA-based supramolecular assemblies is an important issue that hinders their applications. In this work, 20 base pairs long dsDNA fragments [(dCdG) 20 -65 %] composed of 65 % dCdG and 35 % dAdT nucleotides were tethered via a thiol to the surface of a gold electrode. The selfassembled (dCdG) 20 -65 % monolayer was immersed in solutions containing ectoine, a compatible solute. Electrochemical results showed that these monolayers were stable for one month. In situ IR spectroscopy indicated that ectoine interacts weakly with the phosphate-ribose backbone, dehydrating the phosphate groups and stabilizing the A-DNA conformation. This structural reorganization led to a reorientation of nucleic acid base pairs and a local disruption of the double-helix structure. However, the conformation and orientation of the dsDNA fragment was stable in the À 0.4 < E < 0.3 potential range. As a direct interaction between ectoine and dsDNA, the enzymatic reaction of exonuclease VII hydrolyzing the ester phosphate bond in ssDNA, was blocked. We show that the addition of a compatible solute to the electrolyte solution stabilized the dsDNA structure despite structural rearrangements.

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