We have coupled a matrix assisted laser desorption source to an electric
deflection set-up and to a mass spectrometer to measure the electric
dipole of gas phase molecules. The electric dipole is a direct probe of
the electronic properties of the molecule in its ground state. It can
also be used as a tool to determine the geometric structure of an
isolated molecule or molecular complexe. This technique has been used to
study the conformation of amino-acids and polypeptides. We will in
particular discuss the secondary structures of gas phase polyalanines
and the influence of a charge and of the temperature on these structures