Effects of solvents on a model of rhodopsin consisting of a conjugated imine and substituted acetic acids

M~LAN HODOSCEK and DUSAN HADZI. Can. J. Chem. 63, 1528 (1985). The proton potential functions of Schiff base -formic acid complexes were calculated in the ab-initio scheme with STO-3G and 4-31G basis sets without and with the inclusion of a perturbing charge. The comparison of the curves with the retinal Schiff base and allylimine obtained by using the short base encouraged further calculations with the longer base on the truncated system only. The curve obtained with the 4-31G set on the allylimine -formic acid complex is of a double minimum type with the deeper well still at the formyl oxygen. Inclusion of a perturbing negative charge near the hydrogen bond reverses the depth of the wells making the N-protonated form more stable.MILAN HODOS~EK et DUSAN HADZI. Can. J. Chem. 63, 1528 (1985). Faisant appel i des schemas ab-itzitio et utilisant des bases STO-3G et 4-31G avec ou sans I'inclusion d'une charge perturbante, on a calcul6 les fonctions de potentiel des protons de complexes entre I'acide formique et des bases de Schiff.Une comparaison des courbes relatives a la base de Schiff du retinal et de I'allylamine, obtenues a I'aide de la base la plus courte, nous a encourage i proceder des calculs plus approfondis que nous avons effectues uniquement sur le systeme tronque en utilisant un base de calcul elargie. La courbe que nous avons obtenue pour le complexe de l'acide formique et de l'allylimine, en utilisant une base de calcul 4-31G, comporte un double minimum et le puit le plus profond est encore fix6 a l'oxygene. L'inclusion, pres de la liaison hydrogene, d'une charge perturbantc negative renverse la profondeur des puits et rend ainsi la forme N-protonke plus stable.[Traduit par le journal] Introduction In the series of hydrogen bonds involved in the light transduction mechanisms of both rhodopsin and bacteriorhodopsin (1, 2), the bond between the retinal Schiff base nitrogen (N,,) and a proton offering group in the opsin deserves particular attention. In explaining the bathochochromic shift that occurs upon bonding of retinal to the apoprotein it was necessary to invoke both a positive charge on N16 of the retinal Schiff base and a negative charge in the vicinity of C14 of retinal (refs. 3, 4 and references therein). Although the electronic absorption of the retinylic chromophore in the ground state could be explained equally well by assuming covalent N' -H bonding or by a strong hydrogen bond between Ni6 and a proton donor belonging to the apoprotein (5), it was the interpretation of the resonance Raman spectra of rhodopsin that led to the prevalent opinion in favour of protonated N16 (6, 7). However, the I3C nmr spectra of rhodopsin appeared to oppose this view (8) and also the results of differential kinetic infrared spectroscopy were interpreted in terms of the initially unprotonated N16 (9). Sandorfy and co-workers have elaborated a scheme (10-15) reconciling both extreme views, i.e. the protonated and unprotonated Schiff base. The essence of this scheme is a strong hydrogen bond with a double minim...

Section: Resultsmentioning

confidence: 99%

Ab-initio calculations on the retinal Schiff base – formic acid and allylimine – formic acid hydrogen bonds

Hodošček¹,

Hadži²

1985

“…All-trans-retinylidene tert-butylamine (RtBA) was prepared according to a method already described (20). The use of the tert-butyl derivative instead of the more customary n-butylamine (RnBA) in the model of the visual pigment is based on the rationale that it is far more stable.…”

Section: Methodsmentioning

confidence: 99%

A ¹³C and ¹⁵N nuclear magnetic resonance study of the protonation of a retinal Schiff base by acids of different pK_as and in solvents of different polarities

Cossette¹,

Vocelle²

1987

Self Cite

DANIEL COSSETTE and DANIEL VOCELLE. Can. J. Chem. 65. 1576 (1987). . .I3c and 1 5~ nuclear magnetic resonance spectra of mixtures of all-trans-retinylidene tert-butylamine (RtBA) and substituted acetic acids in equimolar concentrations in both CDC13 and CD30D were studied in relation to two questions: the nature of the state of protonation of a retinal Schiff base in the presence of carboxylic and mineral acids in solvents of different polarities, and secondly, to determine the nature of the interactions (if any) between the polar groups located on the acids and the polyenic chain of the retinal derivative. Using carboxylic acids of different pK,s and three mineral acids, we present results that indicate that, in solvents of low polarity, weak acids can only partially protonate RtBA while, in a more polar milieu, the protonation percentages are higher. With polar groups such as NO2, CI, Br, I, and CN, no specific interaction could be found between the polyenic chain of the imine and these groups. Variations in the intensity of certain peaks of RtBA were noted and these could possibly be related to the presence of the polar groups on the acids. Comparisons of our results with those obtained on rhodopsin and bacteriorhodopsin indicate that the milieu surrounding the chromophore must be polar if protonation is complete, while in a less polar environment partial protonation can be expected.DANIEL COSSETTE et DANIEL VOCELLE. Can. J. Chem. 65, 1576Chem. 65, (1987. Les spectres de rCsonance magnitique nuclkaire de I3C et de 1 5~ de mClanges de tout-trans-ritinylidkne tert-butylamine (RtBA) et d'acides acitiques substituis, en concentrations Cquimolaires dans des solvants comme CDCI3 et CD30D, ont itC ttudiis relativement deux questions : la nature de l'itat de protonation d'une base de Schiff du rCtinal en prisence d'acides carboxyliques dans des solvants de polaritis diffkrentes, et deuxiemement, de diterminer la nature des interactions (si elles existent) entre les groupes polaires localisks sur les acides et la chaine polyinique. Par l'utilisation d'acides carboxyliques ayant des pK,s variis et Cgalement de trois acides minCraux, nous montrons des resultats qui indiquent que dans des solvants de faibles polaritCs, les acides faibles ne peuvent protoner RtBA que partiellement tandis qu'en des milieux plus polaires, la protonation peut &tre complkte. Avec des groupes comme NO2, Cl, Br, I et CN, aucune interaction spicifique n'a pu &tre trouvke entre la chaine polyCnique de I'imine et ces groupes. Des variations dans l'intensiti de certains pics de RtBA ont CtC aperques qui peuvent possiblement &tre reliCes i i la presence des groupes polaires des acides. Des cornparaisons de nos rksultats avec ceux obtenus a partir de rhodopsine et de bactCriorhodopsine indiquent que le milieu entourant le chromophore doit &tre polaire pour une protonation complkte tandis qu'en milieu rnoins polaire une protonation partielle est probable.

“…We chose substituted acetic acids (XCH2C02H) as protonating agents, chloroform and methanol as solvents and two different Schiff bases: trans, trans-2,4 heptadienylidene tert-butylamine (HtBA) and all-trans retinylidene tert-butylamine (RtBA). In a series of articles (141)(142)(143) we have shown that, in a non-polar solvent, weak carboxylic acids are not able to fully protonate a conjugated Schiff base. Actually, with acids in the 4.5 pK, range, hardly any protonation can be observed.…”

Section: The Saga O F the Resonance Raman Assault On Rh And Br Startementioning

confidence: 99%

The photochemical event in rhodopsins

Sándorfy¹,

Vocelle²

1986

. 64, 225 1 (1986).The photochemical step in the functioning of visual and bacterial rhodopsins entails cis-trans or trans-cis isomerization and changes in the state of protonation of the retinylidene Schiff base chromophore. In this review our present knowledge on these two events is discussed as well as the role of interactions between the chromophore and the surrounding protein, opsin. The relation between protonation and hydrogen bonding at the Schiff base nitrogen, the problems of stabilization of the proton bridge and charge separation are also discussed. A new proposal is made which implies Schiff base to counter-ion proton translocation with concomitant isomerization and reprotonation of the chromophore.