. 69, 239 (1991). In this study, the protonation of simple Schiff bases by organic acids in various solvents is investigated by Raman spectroscopy and the spectral response of the C=Nf stretching mode is correlated with the macroscopic properties of the medium surrounding the imine. Upon protonation, the unperturbed C=N stretch increases in frequency due to the combination of the coupling with the C=N+H bend and a partial rehybridization of the C=N bond. Depending on the proton location relative to that of its counterion, one mechanism may prevail over the other. Intensity measurements show that protonation is mostly controlled by the hydrogen bonding properties of the solvent and by the polarity of the medium. However, whatever the solvent, full proton transfer cannot be achieved with organic acids with pK, similar to the COOH of amino acids found in visual or bacterial pigments, meaning that the protonation mechanism may depend upon particular properties of the protein.Key words: imines, protonation, Raman spectroscopy, rhodopsin, Schiff base.

The percentages of protonation were determined for a conjugated Schiff base trans, trans-2,4-heptadienylidene tertbutylamine in the presence of 3-chloropropionic acid (CPR) and dichloroacetic acid (DCA) in three solvents of different polarities. In dioxane, a solvent of low polarity, protonation is only important for the strong acid DCA (50-60%). By using solvents of higher polarities, protonation is seen to increase and is almost complete for DCA in ethanol. When water molecules are added to these systems, hydrolysis of the Schiff base, measured inside the time span of the experiments (10 min), occurs readily in dioxane, is very slow in a chloroform-dioxane mixture (9: l), and is totally absent in a mixture of ethanol-dioxane (1 :9). Results indicate that water does not mediate protonation in all three sets of solvent combinations. The results are also discussed in terms of the possible role that water molecules could have in the visual and bacterial pigments. Les pourcentages de protonation ont CtC determines pour le cas d'une base de Schiff conjuguke, la trans,trans-2,4-heptadiknylidkne tert-butylamine, en presence de l'acide 3-chloropropionique (CPR) et de l'acide dichloroacCtique (DCA) dans trois solvants de differentes polarites. Dans le dioxane, un solvant de faible polaritk, la protonation n'est importante que pour l'acide fort DCA (50-60%). En utilisant des solvants de polarit6 plus klevke, la protonation augmente et est B peu pres complkte pour le DCA dans I'ethanol. Quand de l'eau est ajoutCe a ces systemes, l'hydrolyse de la base de Schiff, mesuree 21 l'intkrieur du temps d'enregistrement des spectres (10 min), est rapide dans le dioxane, est beaucoup plus lente dans un melange de chloroforme-dioxane (9: 1) et est totalement absente dans un melange d'kthanoldioxane (1 :9). Les resultats indiquent que l'eau ne favorise aucunement la protonation et cela vaut pour les trois ensem-I