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National audience; The widespread emergence of bacterial resistance has led to an urgent need to develop new strategies to regain the efficacy of antibacterials. One of the emerging concept is to target the bacterial membrane bilayer. Aminoglycosides are among the most potent antimicrobials to treat severe infections. In the search for new antibiotics, we have synthesized derivatives of the small aminoglycoside, neamine in the aim to obtain amphiphilic antibiotics able to disturb bacterial membrane bilayer. One to four hydroxyl functions of neamine were capped with phenyl, naphthyl, pyridyl, or quinolyl rings. The 3',4'-, 3',6- and the 3',4',6-2-naphthylmethylene (2NM) derivatives were active against both sensitive and resistant S. aureus strains. The trisubstituted derivative, also showed marked antibacterial activity against Gram (-) bacteria, including resistant strains (1). Regarding its mechanism of action, it showed only a weak and aspecific binding to a model bacterial 16S rRNA as well as a lower ability to decrease 3H leucine incorporation into proteins in P.aeruginosa, suggesting it acts through a mechanism probably involving membrane destabilization. To understand the molecular mechanism involved, we determined the ability of 3’,4’,6-tri-2NM neamine to interact with the bacterial membranes of P. aeruginosa or models mimicking these membranes. Using Atomic Force Microscopy (AFM), we observed a decrease of P. aeruginosa cell thickness. In models of bacterial lipid membranes, we showed a lipid membrane permeabilization in agreement with the deep insertion of 3’,4’,6-tri-2NM neamine within lipid bilayer as predicted by modeling. This new amphiphilic aminoglycoside bound to lipopolysaccharides and induced P. aeruginosa membrane depolarization. All these effects were compared to those obtained with neamine, the disubstituted neamine derivative (3’,6-di-2NM neamine), conventional aminoglycosides (neomycin B and gentamicin) as well as to compounds acting on lipid bilayers like colistin and chlorhexidine. All together, the data showed that 3’,4’,6-tri-2NM neamine derivatives target the membrane of P. aeruginosa (2). This should offer promising prospects in the search for new antibacterials against resistant drug or biocide strains.

The production of deep water in the North Atlantic is one of the puzzle in understanding oceanic influence in climate changes. In this work, Nd and Pb isotopes were measured on the fine fraction of Labrador Sea sediments. Our aim is to record the relative contribution of fine particle supplies carried by the North Atlantic deep components into Labrador Sea. Based on characterization of potential geographical sources of particles, three main sources contribute to sediment mixture at core location: an old Precambrian crustal material from Canada, Greenland and Scandinavia (NAS), a Paleozoic or younger crustal material from East Greenland, Europa, and Scandinavia (YC) and a volcanic source from Iceland, Faeroe and Reykjanes Ridge (MAR). For the last 12 kyr, clay isotope signatures indicate two mixtures of sediment sources. The first mixture is composed of proximal material from Labrador Sea margins and distal deep current-driven crustal source. From 6.5 kyr onward, the mixture is characterized by the crustal and volcanic components. Since the significant decrease in proximal deglacial supplies, the evolution of the relative contributions of sediment sources suggests major changes in relative contributions of the deep water masses carried by the Western Boundary Undercurrent (WBUC) over the past 8.4 kyr. The progressive intensification of WBUC was associated mainly with the transport of North East Atlantic Deep Water mass until 6.5 kyr and with Denmark Strait Overflow Water thereafter. The establishment of the modern circulation at 3 kyr suggests a reduced influence of the Denmark Strait Overflow Water, synchronous with the full appearance of the Labrador Sea Water mass. Our isotopic dataset emphasizes several changes in the relative contribution of the two major components of North Atlantic Deep Water throughout the Holocene. However, if the clay-size fraction gives informations about the inception and presence of a deep current, it does tell nothing about the strength of this current. Moreover, clays are not likely to be deposited in case of high-strength deep current, resulting in an uncomplete and/or biased reconstruction of deep current evolution. To overcome this problem, it is necessary to look also at coarser fractions. Pb and Nd isotope compositions were thus analysed by MC-ICP-MS on different grain-size fractions (clay-size < 2 `µm, cohesive silt 2-10 µm, fine silt 10-30 µm and coarse silt 30-60 µm) on Late Glacial and Holocene sediments. Four grain-size fractions were investigated for a set of 12 samples from core MD99-2227:. Our results show a clear variation of Pb concentration and isotopic signatures according to grain-size (Nd data still in progress). The observed shifts are interpreted in terms of changes in deep current strength. This approach allows to monitor deep current changes through time, whatever the strength of paleocurrent.

Nd and Pb isotopes were measured on the fine fraction of one sediment core drilled off southern Greenland. This work aims to reconstruct the evolution of deep circulation patterns in the North Atlantic during the Holocene on the basis of sediment supply variations. For the last 12 kyr, three sources have contributed to the sediment mixture: the North American Shield, the Pan-African and Variscan crusts, and the Mid-Atlantic Ridge. Clay isotope signatures indicate two mixtures of sediment sources. The first mixture (12.2-6.5 ka) is composed of material derived from the North American shield and from a "young'' crustal source. From 6.5 ka onward the mixture is characterized by a young crustal component and by a volcanic component characteristic of the Mid-Atlantic Ridge. Since the significant decrease in proximal deglacial supplies, the evolution of the relative contributions of the sediment sources suggests major changes in the relative contributions of the deep water masses carried by the Western Boundary Undercurrent over the past 8.4 kyr. The progressive intensification of the Western Boundary Undercurrent was initially associated mainly with the transport of the Northeast Atlantic Deep Water mass until 6.5 ka and with the Denmark Strait Overflow Water thereafter. The establishment of the modern circulation at 3 ka suggests a reduced influence of the Denmark Strait Overflow Water, synchronous with the full appearance of the Labrador Seawater mass. Our isotopic data set emphasizes several changes in the relative contribution of the two major components of North Atlantic Deep Water throughout the Holocene.

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The orientation of the different helical regions of the mitochondrial precursor protein apocytochrome c has been studied using circular dichroism on isolated fragments of this protein associated with oriented films composed of various phospholipids [de Jongh, H. H. J., Goormaghtigh, E., & Killian, J. A. (1994) Biochemistry (preceding article in this issue)]. Both the N and C terminus adopt helical structures in a membrane environment. The middle region can also be helical, but only in the presence of the N-terminal domain of the protein. In the presence of the unsaturated lipids dioleoylphosphatidylcholine and dioleoylphosphatidylglycerol, all three helices are found to have a preferred orientation perpendicular to the membrane normal, whereas in the presence of the saturated lipids dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol, the terminal helices are preferentially oriented parallel to the membrane normal. In films composed of dioleoylphosphatidylserine, it is found that the N-terminal helix is oriented preferentially perpendicular, whereas the C-terminal helix is aligned more parallel to the membrane normal. The differences in preferred orientation between the terminal helices are demonstrated by molecular modeling of the helices at a water-lipid interface. The results are discussed in light of the translocation of apocytochrome c over the outer mitochondrial membrane, an important step in the import process of this protein in mitochondria.