Protonation of a conjugated Schiff base by weak to strong acids: a ¹³C and ¹⁵N nuclear magnetic resonance study

Abstract

DANIEL COSSETTE and DANIEL VOCELLE. Can. J. Chem. 65, 661 (1987).The interactions (mostly protonation) between several carboxylic and mineral acids and trans,trans-2,4-heptadienylidene tert-butylamine (HBA) were determined by I3c and I5N nuclear magnetic resonance spectroscopies. Results indicate that protonation is only partial with weak acids in both polar and nonpolar solvents. No interaction between polar groups located on the acids and the polyenic chain of HBA seems to occur. The chemical shifts of all carbon atoms and also of the Schiff base nitrogen are linearly correlated with the acidity strength of the carboxylic acids.DANIEL COSSETTE et DANIEL VOCELLE. Can. J. Chem. 65, 661 (1987).Les interactions (surtout la protonation) entre plusieurs acides carboxyliques et minCraux avec la trans,trans-heptadiknylidkne-2,4 tert-butylamine (HBA) ont Ct C dCterminCes par la rCsonance magnCtique nuclCaire du I3C et du I5N. Les rCsultats indiquent que la protonation est partielle pour les acides faibles aussi bien pour les solvants polaires que non-polaires. Aucune interactionentre les groupes polaires des acides et la chaine polyCnique de HBA n'a pu &tre dCcelCe. Les dkplacements chimiques de tous les atomes de carbone et aussi de l'azote de la base de Schiff sont IinCairement correles avec la force de l'aciditk des acides carboxyliques.

Spectroscopic studies on the protonation of Schiff bases

“…Halides of SB are questionable models for rhodopsin chromophores. All these results were confirmed by the NMR measurements of Vocelle and his co-workers [46,47].…”

NMR Constraints on the Location of the Retinal Chromophore in Rhodopsin and Bathorhodopsin

“…The model in Figure 2d incorporates a water molecule hydrogen bonded to the Schiff base to account for the high C=N stretching frequency (~1660 cm-1) and large ND isotope shift (~30 cm-1) which indicate that the SB is strongly hydrogen-bonded (Mathies et al, 1977;Narva & Callender, 1980;Bagley et al, 1985;Bassov et al, 1987), possibly due to water in the binding site (Rafferty & Shichi, 1981;Cossette & Vocelle, 1987;Tallent et al, 1992). New evidence for water bound near the SB in rhodopsin comes from the recent observation of rapid hydrogen/deuterium exchange of the Schiff base proton (Deng et al, 1994).…”