The existence of two potential sites for protonation in the molecules pyridoxol and pyridoxal produces characteristic effects in their 13C nuclear magnetic resonance spectra. The analysis of these effects permits both the confirmation of assignments proposed by others and the establishment of the nature of the species existing in solution as a function of the pH. In particular it appears that, at pH values in the neighbolarhood of 7, both substances exist essentially as zwitterions, pyridoxal adopting the hemiacetal form.L'existence de deux sites de protonation dans les molCcules de pyridoxol et de pyridoxal se traduit au niveau de la rCsonance de chaque noyau 13C par des effects caractkristiques dont la rCsultante acquiert ainsi un caractbre spCcifique. L'analyse de ces effets permet de confirmer les attributions proposCes par ailleurs et d'Ctablir la nature des formes intervenant h chaque valeur du pH dans la structure de ces mol6cules. I1 apparait, en particulier, qu'aux pH voisins de 7, le pyridoxol et le pyridoxal existent essentiellement sous une forme zwitterionique, le pyridoxal Ctant par ailleurs sous une forme de type hCmi-acCtal.
A nnmher of phenol-and catechol-amines has been studied in aqueous solutions at acidic and alkaline pD by means of "C NMR. Resonance assignments are achieved for the f d y protonated forms and the influence of the side chain on the aromatic carbons is studied. The shielding effects associated with the various types of side chain tend to follow an additive relationship. The pD dependence of the ring and side chain carbon resonances in phenolamines is accounted for by considering the s u m of two separate and opposite effects related to ring and side chain deprotonations. Such a simple model fails to apply to the catechol derivatives. This difference is discussed in terms of the orientational dependence of the deprotonation shifts. It is shown that the protonated amino group is the less acidic group in phenol-and catechol-amines. * Author to whom correspondence should be addressed.reactions enzymatiques et, en particulier, une dbamination oxydative que nous nous proposons d'envisager dans la mesure ou il semble se prkciser que l'enzyme responsable (monoamine oxydase, MAO) possbde comme cofacteur le phosphate de pyridoxal. C'est ainsi que nous avons dCjB CtC amenks 2 caractkriser le comportement de ce cofacteur dans toute la gamme de pD (pD =indication du pH mbtre + 0,43) et B ktendre aujourd'hui cet examen B diverses amines (cf. Fig. 1).Aprks avoir proced6 aux attributions des diffkrents pics constituant les spectres de rCsonance, nous avons CtudiC l'influence exercee sur ces spectres par les variations de pD en prenant en consideration les microconstantes caractkrisant les Cquilibres entre les formes cationique (C), neutre (N), zwitterionique ( Z ) et anionique (A) (cf Fig. 2) sous lesquelles ces amines peuvent se presenter en nous attachant plus particulibrement B ttablir l'ordre de basicit6 des diffkrents sites (fonction phtnol, fonction amine) qui reste encore c o n t r o v e r~k .~~
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