Effect of selected anions and solvents on the electronic absorption, nuclear magnetic resonance, and infrared spectra of the N-retinylidenebutylammonium cation

Blatz, Paul E.; Mohler, Jonathan H.

doi:10.1021/bi00682a005

Cited by 63 publications

(33 citation statements)

References 13 publications

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“…Hence, the frequency distribution of VC=N+ seems rather a function of the nature of the counterion. This is consistent with the limited effect of the anion on the VC=N+ vibration position in MeOH, where the hydrogen bond network around the chromophore is known to play a buffer electrostatic role (or a levelling effect) (23). The negative charge probably defines the proton position and, in this kind of complex, the proton can be found close to the nitrogen atom (i.e., forming a front H-bond), or close to its counterion (involving a back H-bond), or again hesitating between both moieties (3,15).…”

Section: Resultssupporting

confidence: 84%

“…4 cm-'. Since methanol or other alcohols are known to form strong hydrogen bonds with C=N (20,23,24), the results might be explained accordingly: that is, hydrogen bonding could better localize the 7~ electrons on the C=N group and contributes to increase its vibrational frequency. This might also explain the increase in intensity of the band although this enhancement is probably only related to the variation of the dielectric constant.…”

Section: Resultsmentioning

confidence: 99%

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Study of the protonation of simple Schiff bases in solvents of various polarity by means of Raman spectroscopy

Alex¹,

Turcotte²,

Fournier³

et al. 1991

Can. J. Chem.

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. 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.

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Section: Resultssupporting

confidence: 84%

Section: Resultsmentioning

confidence: 99%

Study of the protonation of simple Schiff bases in solvents of various polarity by means of Raman spectroscopy

Alex¹,

Turcotte²,

Fournier³

et al. 1991

Can. J. Chem.

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“…Assignments were based on first-order coupling constant information and comparison to previous work (14,17,18). In iminium salts 1-3 the aldiminium proton appears as a doublet of doublets.…”

Section: Resultsmentioning

confidence: 99%

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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“…At present the opsin shift in bR is accounted for in terms of a combination offactors: (a) different interactions between the positively charged Schiff base nitrogen and the counterion in the pigment relative to retinal protonated Schiff base in methanol (11)(12)(13)(14)(15)(16); (b) a planar s-trans ring-chain conformation (14,(17)(18)(19)(20); (c) an ion-pair (rather than a single opsin charge) in the vicinity of the ,B-ionone ring (14,17,18). The smaller (2400-2500 cm-') opsin shifts observed in the case of bR480(III) and bR527(IV) are comparable to those reported for §Difference between the pKa of bR570 and that of the corresponding pigment.…”

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confidence: 99%

Controlling the pKa of the bacteriorhodopsin Schiff base by use of artificial retinal analogues.

Sheves¹,

Albeck²,

Friedman³

et al. 1986

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

116

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Artificial bacteriorhodopsin pigments based on synthetic retinal analogues carrying an electron-withdrawing CF3 substituent group were prepared. The effects of CF3 on the spectra, photocycles, and Schiff base pKa values of the pigments were analyzed. A reduction of 5 units in the pKa of the Schiff base is observed when the CF3 substituent is located at the C-13 polyene position, in the vicinity of the protonated Schiff base nitrogen. The results lead (D) to the unambiguous characterization of the (direct) titration of the Schiff base in bacteriorhodopsin and (ii) to the conclusion that the deprotonation rate of the Schiff base during the photocycle (i.e., the generation of the M412 intermediate) is determined by a structural change in the protein.Bacteriorhodopsin (bR), the photosynthetic protein pigment in the purple membrane of Halobacterium halobium, operates as a light-driven proton pump leading to ATP synthesis (see refs. 1-3 for reviews). The chromophore of the lightadapted form of bacteriorhodopsin, absorbing at 570 nm (bR570), is all-trans-retinal bound to the e-amino group of a lysine residue via a protonated Schiff base. Accumulated evidence (1) suggests that the protonated polyene structure is not only responsible for the visible absorption of the pigment but may also participate directly in the proton pump. Consequently, a specific pumping mechanism involving the translocation of the Schiff base proton was proposed (4), based on the assumption that, during the photocycle, the pKa of the Schiff base is markedly reduced (1,4).It is thus evident that determination of the pKa value of the retinyl chromophore is of primary importance for understanding the function of bR on a molecular level. By using flow and stationary methods, Druckmann et al. (5) PKa values that are markedly reduced relative to retinal. This approach has been used to confirm that the observed pKa value of 13.3 ± 0.3 is directly associated with the Schiff base of bR570. Another important aspect of this work is the use of the artificial control ofthe acid-base properties of the Schiffbase to study the possible effects of the corresponding pKa changes on the photocycle of bR. This is especially relevant to the phase in the cycle associated with the formation of the M412 intermediate (Xma, 412 nm) in which the Schiff base undergoes deprotonation. If proton transfer from the Schiff base is rate-determining in the overall deprotonation process, then the rate of M412 formation is expected to be affected by changes in the value of pKa. MATERIALS AND METHODSSynthesis of Artificial Retinal Analogues. Chromophore 2 (Fig. 1), prepared previously by Gartner et al. (6) and by Asato et al. (7), was synthesized by a modified route (Scheme 1). Condensation of aldehyde 5 with 1,1,1-trifluoroacetone in ethanol at 25°C in the presence of 2 equivalents of potassium hydroxide led, after separation of isomers, to ketone 6. Emmons-Horner condensation with the sodium salt of triethyl phosphonoacetate at 25°C followed by reduction with diisobutylalu...

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Effect of selected anions and solvents on the electronic absorption, nuclear magnetic resonance, and infrared spectra of the N-retinylidenebutylammonium cation

Cited by 63 publications

References 13 publications

Study of the protonation of simple Schiff bases in solvents of various polarity by means of Raman spectroscopy

Study of the protonation of simple Schiff bases in solvents of various polarity by means of Raman spectroscopy

Structure and wavelength modification in retinylidene iminium salts

Controlling the pKa of the bacteriorhodopsin Schiff base by use of artificial retinal analogues.

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