Crystal structure of N-methyl-N-phenylretinal iminium perchlorate; a structural model for the bacteriorhodopsin chromophore

Santarsiero, Bernard D.; James, Michael N.G.; Mahendran, M.; Childs, Ronald F.

doi:10.1021/ja00181a067

Cited by 52 publications

(42 citation statements)

References 2 publications

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“…1 and 2, the two salts have remarkably similar structures and packing arrangements within the crystal lattice. As was found in our earlier work with an N-phenyl substituted retinylidene iminium salt (22), the cations in 2 and 3 are bowed in the plane defined by atoms N, C14, C15, and C21 and bent normal to this plane, Figs. 1 and 2.…”

Section: Comparison Of the Structures Of The Two Saltssupporting

confidence: 66%

Structure and wavelength modification in retinylidene iminium salts

Elia¹,

Childs²,

Britten³

et al. 1996

Can. J. Chem.

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Abstract:The spectroscopic and structural properties of the perchlorate and triflate salts of N-n-butyl-retinylidene imine, 2 and 3, have been examined in solution and solid phases. In solution these salts were found to exhibit very similar UV and NMR spectroscopic properties. However, in the solid state marked differences in their absorption spectra (2, A, , = 504 nm; 3, A, , , = 445 nm) and I3c NMR spectra were found. The structures of the two salts were determined by X-ray crystallography. The cations in each of the salts were shown to have very similar conformations, detailed structures, and packing in their crystal lattices. The differences in the spectroscopic properties of the salts in the solid state could not be accounted for on the basis of any structural differences in the cations themselves. In terms of cation-anion interactions, a strong hydrogen bonding interaction was found in each case between the N-H proton and an oxygen atom of the counterion. However, there were significant differences between the two salts in terms of the N-H..O internuclear distances (2, N-H...O = 2.939(7) A and in 3,2.85(1) A). The results are strongly suggestive that the wavelength and positive charge delocalization in retinylidene iminium salts are controlled by variation of the distance between the anion and the proton bonded to the Schiff base nitrogen atom. The work reported here represents the first examples of secondary retinylidene iminium salts containing an N-alkyl substituent to be successfully analyzed by X-ray crystallography. The relationship of these observations in the solid state to the spectroscopic properties of the natural visual pigments is explored.Key words: retinylidene iminium salts, iminium salts, cation-anion interactions. Resum6: On a Ctudie les propriCtCs spectroscopiques et structurales des perchlorate et triflate de la N-n-butyl-ritinylidene imine, 2 et 3, tant en solution qu'en phase solide. En solution, on a trouvC que ces sels prCsentent des propriCtCs spectroscopiques UV et RMN qui sont tres semblables. Toutefois, ii I'ttat solide, on a observe des difftrences marqutes dans leurs spectres d'absorption (2, A, , , = 504 nm; 3, A, , , = 445 nm) et dans leurs spectres RMN du I3c. On a determint les structures des deux sels par diffraction des rayons X. Dans chacun de ces sels, les cations ont des conformations, des structures dCtaillCes et des empilements trks semblables. On ne peut pas expliquer les differences observCes dans les propri6tCs spectroscopiques des sels ii l'etat solide sur la base de differences de structures dans les cations eux-mtmes. En termes d'interactions cation-anion, on a observe une forte interaction de liaison hydrogene entre le proton N-H et l'atome d'oxygkne du contre-ion. Toutefois, il existe des differences importantes entre les deux sels en termes de distances internuclCaires (2, N-H...O = 2,939(7) A et dans 3,2,85(1) A. Les rksultats suggerent fortement que la longueur d'onde et la dClocalisation de la charge positive dans les sels rktinylidkne iminium est contr6...

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Section: Comparison Of the Structures Of The Two Saltssupporting

confidence: 66%

Structure and wavelength modification in retinylidene iminium salts

Elia¹,

Childs²,

Britten³

et al. 1996

Can. J. Chem.

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“…Crystal structures of conjugated iminium salts have also shown previously for 1-hydroxyallyl cations and conjushown the all-E conformation to be the present in the solid gated iminium salts through low-temperature I3c NMR state (16,17).…”

Section: Resultsmentioning

confidence: 52%

Thermal cyclizations of protonated poly-unsaturated aldehydes

Elia¹,

Childs²,

Shaw³

1992

Can. J. Chem.

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. Protonated 2,4-hexadienal (IH) and 2,4,6-octatrienal (2H), prepared by protonation of the analogous aldehydes in FS03H, isomerized to give cyclized products 3H and 4H at 30°C and -20°C respectively. The rate constants for the cyclization of 1H were measured in both FS03H and CF3S03H. It was found that the rate constant for isomerization decreased when CF3S03H was used as the reaction medium. It is suggested that the thermal cyclizations of 1H and 2H involve diprotonated species with protonation occurring on oxygen. 1H and 2H underwent photoisomerization at -78°C in FS03H to yield 5H and 7H respectively. It was found that 5H underwent a cyclization reaction at -40°C to give 6H, which subsequently rearranged to give 3H at -10°C. However, 7H was found to isomerize to 2H at -50°C. The mechanisms of the thermal isomerization are discussed. GEORGE R. ELIA, RONALD F. CHILDS et GARY S. SHAW. Can J . Chem 70, 2065Chem 70, (1992. L'hexa-2,4-diknan (IH) et l'octa-2,4,6-triknal (2H protonts, prCpares par protonation des aldehydes correspondants par le FS03H, s'isomCrisent pour donner les produits cycliques 3H et 4H, a 30°C et -20°C respectivement. On a mesurC les constantes de vitesse pour la cyclisation du produit l H , tant dans le FS03H que dans le CF3S03H. On a trouvC que la constante de vitesse pour 1'isomCrisation diminue lorsqu'on utilise le CF3S03H comme milieu rkactionnel. I1 est suggCrC que les cyclisations thermiques des composes 1H et 2H impliquent des espkces diprotonCes dans lesquelles la protonation se fait sur l'oxygkne. Dans le FS03H, 2 -78"C, les composCs 1H et 2H subissent des photo-isomkrisations qui conduisent respectivement aux composCs 5H et 7H. On a trouvC que le composC 5H subit une reaction de cyclisation i -40°C qui conduit au produit 6H qui se transpose subsCquemment pour donner le compose 3H a -10°C. Toutefois, on a trouvC que, a -50"C, le composC 7H s'isomkrise en composC 2H. On discute des mecanismes des isomCrisations thkrmiques.[Traduit par la redaction]The chemistry of polyenyl carbocations has been investigated extensively (1 -3). The characteristic reaction of this important class of carbenium ions is ring closure of a pentadienyl fragment to form a substituted cyclopentenyl cation. This cyclization reaction has been shown to proceed via a stereoselective 4-n electron electrocyclic ring closure (4, 5). These reactions are generally facile, occurring, for example, at very low temperatures when the ions are generated as long-lived species in strong acid media.There have been fewer studies reported on the chemistry on the hydroxy-or alkoxy-substituted polyenyl carbenium ions, despite the ease by which these ions can be prepared by the protonation of unsaturated carbonyl compounds and their derivatives (6). One important example is the cyclization of protonated cross-conjugated dienones to form protonated cyclopentenones, a reaction that forms the key step in what has become known as the Nazarov reaction (7). This cationic cyclization reaction parallels the cyclization reactions observed ...

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“…1 that the halide salts ofthe all-trans-retinal pSB of aniline are in the 6-s-trans conformation. These compounds therefore have the same 6-s conformation in the solid state as the perchlorate salt of the pSB of all-trans-retinal with N-methylaniline (24). A striking feature of Fig.…”

Section: Methodsmentioning

confidence: 78%

“…Conjugation of the aniline and polyene 1T systems can also be ruled out for three reasons. First, x-ray crystallography of the closely analogous 6-s-trans pSB of retinal with N-methylaniline has shown that the dihedral angle between the phenyl ring and the polyene chain is 46.2 ± 0.5°and the length of the bond between the phenyl ring and the nitrogen is 1.428 + 0.005 A (24). Second, from the point of view of steric repulsion between the phenyl ring and the pSB counterion, the planes of the polyene chain and the phenyl ring should be skewed.…”

Section: Methodsmentioning

confidence: 99%

Synergy in the spectral tuning of retinal pigments: complete accounting of the opsin shift in bacteriorhodopsin.

Griffin

Herzfeld

1994

Proc. Natl. Acad. Sci. U.S.A.

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UV-visible and solid-state NMR studies of a series of6-s-trans protonated Schiffbases of retinal with aniline show that the bathochromic shift induced by weakening the imine counterion is sificandy greater in the 6-s-trans conformation than in the 6-s-cis conformation. Based on the observed magnitude of this coupling between the electronic effects of6-s isomerization and imine counterion strength in the mbdel compounds, the large opsin shift and unusual chemical shifts in light-adapted bacteriorhodopsin can be fully explained. These phenomena therefore do not require a negative point charge or polarizability effects in the chromophore binding pocket. The results are consistent with an effective center-to-center distance between the Schiff base and its counterion of about 4 A in light-adapted bacteriorhodopsin.Retinal pigments are found widely in nature, in organisms as evolutionarily distant as archae, algae, and mammals. In most cases, these integral membrane proteins are responsible for the first step of signal transduction. However, at least in archae, some retinal pigments function as energy transducers, by executing light-driven transport of ions. Certainly, for the visual pigments, and perhaps also for the ion pumps, it is important that the protein responds to light in the appropriate part of the spectrum. Since the chromophore in each known case is a protonated Schiff base (pSB) of retinal with a lysine residue, it is generally appreciated that the tuning of the chromophore must be due to the characteristics of the binding pocket in the protein. The shift of the wavelength of maximal visible absorbance (Amx), relative to the value of440 nm for the chloride of retinalbutylimine in methanol (1) (iv) The remaining identified mechanism for the opsin shift involves isomerization around the 6-s bond connecting the polyene chain to the ionone ring. Due to the methyl groups on the ionone ring, retinal in solution prefers the skewed 6-s-cis conformation. Isomerization to the planar 6-s-trans conformation results in a bathochromic shift due to extension ofthe polyene ir system to include the double bond in the ring. The magnitudes ofthe opsin shifts associated with these four mechanisms have been investigated using a variety of retinal analogs with appended charges (5, 6), disrupted ir systems (7, 8), and locked conformations (9), as well as variations of the pSB counterion (10-14) and solvents of varying polarizability (15)(16)(17).Where more than one opsin shift mechanism is at work, it has generally been assumed that the effects on the energy of excitation (or equivalently on .max = A 1 ) are additive.However, this has led to apparent inconsistencies. Here, we investigate the possibility of synergistic effects. We focus on the combination of the 6-s isomerization and the strength of the pSB counterion because (i) MATERIALS AND METHODSPreparation of pSBs. The pSBs were never exposed to white light. All syntheses and NMR experiments were conducted under red light or in darkness. To synthesize the pSBs,...

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Crystal structure of N-methyl-N-phenylretinal iminium perchlorate; a structural model for the bacteriorhodopsin chromophore

Cited by 52 publications

References 2 publications

Structure and wavelength modification in retinylidene iminium salts

Structure and wavelength modification in retinylidene iminium salts

Thermal cyclizations of protonated poly-unsaturated aldehydes

Synergy in the spectral tuning of retinal pigments: complete accounting of the opsin shift in bacteriorhodopsin.

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