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International audience; The photoionization of N2 plays a key role in initiating the formation of complex organic molecules in the nitrogen-rich atmosphere of Saturn’s largest moon, Titan. To date, only a handful of laboratory studies have explored the reactivity of N2+ ions with hydrocarbons─limited to methane, acetylene, and ethylene─at the low temperatures relevant to Titan. Here, the rate coefficients, product identity, and branching ratios of the ion–molecule reactions of N2+ with C3H4 isomers, namely, propyne CH3CCH and allene CH2CCH2, were measured between 24 and 72 K in uniform supersonic flows. The rate coefficients are collisional and their temperature dependence is in remarkable agreement with capture models. The outcomes of both reactions are similar: they proceed primarily via dissociative charge transfer, leading to the formation of C3H3+ (main product, >70%) and C3H2+ (between 9 and 17%), whereas a second, nondissociative charge-transfer mechanism leading to C3H4+ becomes slightly more prominent as the temperature decreases (from 3 to 12%). C3H3+ is plausibly formed predominantly as the smallest aromatic cation, cyclopropenyl c-C3H3+, by following the lowest-energy pathway for the decomposition of allene and propyne cations. The measured rate coefficients and branching ratios were included in a photochemical model of Titan’s atmosphere. The results point toward a secondary role of N2+ + C3H4 reactive pathways in the production of c-C3H3+.

Background Thoroughly understanding the temporal associations between cognitive and functional dimensions along the dementia process is fundamental to define preventive measures likely to delay the disease’s onset. This work aimed to finely describe the trajectories of cognitive and functional declines, and assess their dynamic bidirectional relationships among subjects at different stages of the dementia process. Methods We leveraged extensive repeated data of cognition and functional dependency from the French prospective COGICARE study, designed to better characterize the natural history of cognitive and functional declines around dementia diagnosis. Cognition was measured by the Mini-Mental State Examination, the Isaacs Set Test for verbal fluency, the Benton Visual Retention Test for visuo-spatial memory, and Trail Making Test Part B for executive functioning. Functional dependency was measured by basic and instrumental activities of daily living. The study included 102 cognitively normal, 123 mildly cognitively impaired, and 72 dementia cases with a median of 5 repeated visits over up to 57 months. We used a dynamic causal model which addresses the two essential issues in temporal associations assessment: focusing on intra-individual change and accounting for time. Results Better cognitive abilities were associated with lower subsequent decline of the functional level among the three clinical stages with an intensification over time but no reciprocity of the association whatever the clinical status. Conclusion This work confirms that the progressive functional dependency could be induced by cognitive impairment. Subjects identified as early as possible with clinically significant cognitive impairments could benefit from preventive measures before the deterioration of activities of daily living and the appearance of dementia clinical signs. Supplementary Information The online version contains supplementary material available at 10.1186/s13195-021-00887-4.

Understanding the mechanisms governing the formation of complex organic molecules in the interstellar medium and cold planetary atmospheres relies in particular on the exploration of ionic processes under extreme conditions. The CRESU-SIS experimental setup, developed to study ion-neutral reactions in the gas phase at low temperatures, is able to meet this challenge. This unique instrument, which combines a selective ion source (SIS) with the CRESU reactor which generates a uniform supersonic flow at very low temperature, makes it possible to measure the rate coefficients of the reactions between ions and neutral molecules and to determine the branching ratios between the different exit channels. The reactivity of N2+, Ar+, and H2CN+ ions with hydrocarbons (e.g CH4, C2H4 ...) has been studied experimentally and analyzed using quantum calculations. The reaction between N2+ and the linear isomers of C3H4 (allene and propyne) has been included into a photochemical model for the atmosphere of Titan., La compréhension des mécanismes régissant la formation des molécules organiques complexes dans le milieu interstellaire et les atmosphères planétaires froides repose notamment sur l’exploration de processus ioniques en conditions extrêmes. Le dispositif expérimental CRESU-SIS, développé pour étudier les réactions ion-neutres en phase gazeuse à basse températures, est en mesure de répondre à ce défi. Cet instrument unique, qui combine une source d’ions sélective (SIS) au réacteur CRESU qui génère un écoulement supersonique uniforme de très basse température, permet de mesurer les coefficients de vitesse de réactions entre des ions et des molécules neutres et de déterminer les rapports de branchement entre les différentes voies de sortie. La réactivité des ions N2+, Ar+, et CH2CN+ avec des hydrocarbures (e.g CH4, C2H4…) a été étudiée expérimentalement et analysée à l’aide de calculs quantiques. La réaction entre N2+ et les isomères linéaires de C3H4 (allene et propyne) a été incluse dans un modèle photochimique pour l’atmosphère de Titan.