Les paires d’ions, atomiques ou moléculaires, sont naturellement omniprésentes, dans l’eau de mer, les aérosols et jusqu’au sein des organismes vivants.

Following UV absorption, many biomolecular systems are endowed with mechanisms of excited-states deactivation that ensure their photochemical stability. These (ultra)fast processes indeed provide an efficient way to dissipate the electronic energy into vibration, avoiding then structural damages which could affect their biological function.

Les propriétés des molécules flexibles sont fonction de leur forme et leurs applications intéressent de multiples domaines (médicaments, nouveaux matériaux…). Un enjeu majeur dans ce domaine est de pouvoir contrôler leur repliement pour in fine contrôler leurs propriétés.

Many complex molecular systems absorb light in the UV spectral range, including those of paramount biological importance, like DNA bases or proteins. The excited states created by UV absorption are endowed with mechanisms of deactivation which are of major importance for the photochemical stability of these species. The majority of these processes are ultrafast and provide a rapid and efficient way to dissipate the electronic energy into vibration, thus avoiding photochemical reactions.

The description of the interactions controlling the shape of a protein is crucial in understanding the cellular mechanisms, but is still difficult to achieve on biological systems because of their complexity. In this context, the use of model molecules makes accessible to experiments many biological problems lying at the heart of current societal issues. Gas-phase IR/UV spectroscopy of small peptides is an outstanding example.

By comparison with experimental data obtained specifically for this purpose by scientists of IRAMIS / SPAM, it is possible to select the best ab initio method able to give the parameters of "force field" models to faithfully reproduce the structure of assemblies of atoms, as complex as proteins (macromolecule composed of amino acids) or peptides (small polymers of amino acids, n <50).

One of the great mysteries of life is its lack of symmetry at the molecular scale. Many biological molecules, like amino acids, exist a priori in two asymmetrical forms, "right” and “left”. Such molecules are referred to as “chiral” . Like the two hands, these forms cannot be superimposed but are mirror images of each other (Fig.). However, in living organisms, proteins are exclusively built up from “left” amino acids.

SPAM / Lab. Francis Perrin (URA CEA-CNRS 2453), CEA Saclay, 91191 Gif-sur-Yvette Cedex, France Les protéines, molécules indispensables à la vie des organismes vivants, sont des polymères synthétisés à partir de l’ADN des cellules. Leur fonction dépend de leur constitution mais aussi de leur structure.