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J. Phys. Chem. 1994, 98, 41164779
Vibrational Analysis of a Retinal Protonated Schiff Base Analog H. Deng,? L. Huang,t M. Groesbeek,* J. Lugtenburg,* and R. H. Callender'*t Department of Physics, City College of City University of New York, New York, New York 10031, and Department of Chemistry, University of Leiden, 2300 RA Leiden, The Netherlands Received: November 15, 1993; I n Final Form: March 11, 1994'
A vibrational analysis of a retinal protonated Schiff base analog has been conducted by Raman measurements on this Schiff base and its isotopically labeled derivatives and by calculations using a b initio methods. The calculated Raman spectra of the model and its isotopically labeled derivatives are in good agreement with the experimental data. Based on comparisons between the calculated Raman spectra of C=N syn and C=N anti Schiff bases, a number of criteria to determine the Schiff base C=N configuration from Raman spectroscopy are developed. Two of these criteria have been previously proposed on the basis of empirical or semiempirical calculations. The agreement between experiment and calculations suggests that a b initio methods are reliable enough to interpret the vibrational spectra of retinal protonated Schiff bases in various configurations and environments.
Introduction
SCHEME 1
Retinal isomerization reactions are functionally important in the proton pump cycle of bacteriorhodopsin (bR) and in the bleaching sequence of rhodopsin. Different models of retinal Schiff base isomerization reactions in rhodopsin and bR after absorbing a photon have been For example, there are two suggestions on the configurational changes of the retinal Schiff base in bR upon absorbing a photon: one is from all-trans in bR to 13-cis in K, the first photoproduct, and other is from all-trans to 13,14-dicis (cf. Fodor et a1.2 and Zhou et a1.s). Interestingly, each of them appears to be consistent with certain experimental and theoretical studies. This is due to the fact that much of the experimental observation on the photoproduct of bR at the atomic scale is from vibrational spectroscopy, such as resonance Raman and FTIR, and theoretical analyses of the data have not achieved an agreed conclusion despite extensive vibrational analyses of the retinal Schiff base of a large amount of data from I3Cand deuterium-labeled retinal Schiff bases, either free in solution or bound to pigments, which have been used to determine the conformation and double-bond configuration of the retinal protonated Schiff base (RPSB) and its interactions with its protein environment.2-3."16 Most analyses involve the use of the Wilson FG method which relies heavily on empirical parametrization for each conformation or configuration of RPSB. Although this approach can provide the most accurate normalmode assignments to the vibrational spectrum of a Schiff base with a fixed configuration, it has rather limited value in predicting an unknown structure and/or external perturbation from a given vibrational spectrum. Partly to overcome this limitation, semiempirical quantum chemical methods, such as MIND013 and MNDO, have become more widely ~ s e d . ~ + These ~ J ~ methods are capable of predicting values of force constants of a molecule with unknown structures and the changes of these force constants upon external perturbations. While the calculated normal modes, which can be evaluated by their frequencies and shifts upon isotopic labeling against experimental data, are fairly good in some cases, they are rather poor in others, as has been shown before and will also be shown below. In this paper, we probe the possibility of interpreting thz vibrational spectra quantitatively by a b initio methods. This is done by performing a b initio and semiempirical calculations on a protonated Schiff base (PSB) model compound (Scheme 1; City College of City University of New York. Leiden. Abstract published in Advonce ACS Abstrocts, April 15, 1994.
8 University of
0022-3654/94/2098-4776%04.50/0
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shown in the C=N anti configuration) with either C=N anti or C=N syn configuration, and the accuracy of the theories is evaluated against Raman data of the model. This model is small enough so that calculations are feasible and yet retains many of the molecular features which, as a first step, allow the prediction of spectral changes with regard to the various configurations of RPSB. Besides, the configuration about the C=N bond of RPSB in pigments bears on a number of important pigment properties, including pigment color regulation and the dynamics and efficient conversion of light to chemical energy that occurs in the primary photophysical step.2,7J1J2J4J7This is so because the charge distribution associated with the protonated Schiff base moiety and its environment is key to these issues.18-20 On the basis of our results, three experimental criteria are found which may be used as "fingerprints" in the vibrational spectra of RPSBs for the C=N bond configuration. These criteria are compared with those obtained in previous s t ~ d i e s . ~ J I - ~ ~ J ~ Results and Discussion
Figure 1 shows the Raman spectra of the PSB model and several isotopically labeled derivatives in aqueous solution. Most of the major bands observed in Figure 1A can be readily assigned on the basis of their isotopic shifts upon I3C, l5N, or deuterium labeling. Far example, the strong band at 1684 cm-l is assigned to C=N stretch based on its 21-cm-1 shift when the Schiff base nitrogen is labeled by ISN (Figure 1C). The 1631-cm-1 band, which is sensitive to the l3C labeling (Figure lE), but not to l5N labeling (Figure 1C), is assigned to the C=C stretch mode. When the PSB nitrogen is deuterated, the C=N stretch mode at 1684 cm-I shifts down to 1657 cm-1 due to its decoupling from the N D rock motion (Figure 1B). The 1497-cm-1 band in Figure 1A is assigned to the N H rock based on its disappearance upon Schiff 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 18, 1994 4111
Letters E. 14,15-13C
a '0
C. 'SN
g
H. 15D,ND
D. j5ND
1000
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1600
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Figure 1. Raman spectra of PSB in H10 (A)and its ND (B),15N (C),
15ND(D),14,15-"C(E), 14,15-13CND(F),15D(G),and 15D,ND(H) derivatives. The spectra were measured with a 514.5-nm laser line from an argon ion laser at a power level of 200-300 mW on an optical multichannel analyzer system, which has been described in detail elsewhere.21 Solvent Raman bands was subtracted. Resolution of the spectrometer is 8 cm-1. A spectral calibration is done for each measurement using the known Raman lines of toluene,and absoluteband positions are accurate to within *3 cm-'. base deuteration. The 1378-cm-1 band is very sensitive to the deuterium labeling on C1s and thus can be assigned to the combination of C14H/C15H rocks. In 15D-labeled Schiff base spectrum (Figure lG), the 979-cm-1 band is assigned to the ClsD rock and the band at 1350 cm-1 is assigned to the Cl4H rock. Finally, the band at 1232 cm-l can be assigned to the C14-C15 stretch based on its 13-cm-1 shift when both Cl4 and Cl5 are labeled with 13C (Figure 1E). It is interesting to note that, upon Schiff base deuteration, this band apparently shifts up 15 cm-I to 1247 cm-1 (Figure lB), which also contains significant Cl4C1sstretch as shown by its 10-cm-1 shift in thespectrum of doubly 13C labeled, deuterated Schiff base (Figure 1F). Previous N M R studies on RPSB have shown that the chemical shift of the C15H proton is sensitive to C=N configuration: its chemical shift is >8.5 ppm when C=N is syn and
