134
J. Phys. Chem. 1980, 84, 134-139
(23) F. P. Del Greco and F. Kaufman, Discuss. Faraday Soc., 33, 128 (1962). (24) A. A. Westenburg and N. de Haas, J. Chem. Phys., 58, 4066 (1973). (25) D. W. Trainor and C. W. von Rosenberg, Jr., J. Chern. Phys., 61, 1010 (1974). (26) See discussion in the text.
(27) R. Zellner, K. Erler, and D. Field, Syrnp. (Int.) Combust., [Proc.], 7&h, 939 (1977). (28) G. Black and G. Porter, Proc. R . Soc. London, Ser. A , 266, 185 (1962). (29) S. Gordon and W. A. Mulac, Int. J. Chem. Kinet., Symp. No. 1 , 289 (1975).
Photochemistry of Linear Polyenes Related to Vitamin A. 13-Demethylretinal and 14-Methylretinal Walter H. Waddell" and John L. West Department of Chemistry, Carnegle-Mellon University, Pittsburgh, Pennsylvania 152 13 (Received April 19, 1979) Publication costs assisted by the National Institutes of Health
The photochemistry of all-trans-l3-demethylretinal(2) and alE-trans-14-methylretinal(3) is examined in nonpolar and polar solvents. Upon extended irradiation into the first absorption band, a photoequilibrium mixture is established that contains a number of isomeric photoproducts. High pressure liquid chromatographic methods are employed to isolate and purify each reaction product, and their absorption spectral and photochemical properties are examined. Primary photoproducts and quantum yields ($PI) of trans cis or cis trans photoisomerization for all-trans-, $.-cis-,9-cis-, and ll-cis-13-demethylretinaland all-trans-, 9-cis-, 11-cis-,and 13-cis-14-methylretinalare determined in a nonpolar solvent. The variation in 4pI and the relative photoproduct ratios of the isomers of 13-demethylretinal and 16methylretinal differ from those of the corresponding isomers of retinal. The differences in quantum yields, primary products, and their relative ratios are analyzed in terms of the structural changes resulting from incorporation or removal of the alkyl substituent of the C-13-C-14 carbon-carbon double bond. +
-
Introduction photoisomerization of the chromophore in rhodopsin, we have examined the solution photochemistry of isomers of The visual pigment rhodopsin is composed of the 11two synthetic retinals, 13-demethylretinal (2) and 14cis-retinyl chromophore' bound covalently via a protonated Schiff base linkage2p3to the terminal amino group of a CH3 CH 3 lysineP6 in the protein opsin. Upon absorption of a photon of light by rhodopsin the chromophore is isomerized to the all-trans form, initiating a series of chemical which alter the conformation of the protein. all-transRetinal (1) and opsin are the final products of this 1,all- trans-retinal bleaching process, which occurs with an efficiency of approximately 67%.101'1 A number of stable artificial pigments have been prepared from bovine opsin and a variety of chromophores. They include three retinal isomers, 7-cis-,12g-cis-,' and 2, all- trans- 13-deme thylre tinal 9-~is,l3-cis-retinal,'~ and a number of retinal analogues, 9-cis-, 11-cis-,and 9-cis,13-cis-14-methylretinal;'4J5 94s13-demethyl-14-methylretinal;15 and certain cis isomers of 9-demethylretinal,16 13-demethylretinal,17 and 9,13-demethylretinal.18 The physical and photochemical properties of these artificial pigments have not been examined 3, all- trans- 14-meth y lre tinal in detail. The photochemical properties of the isomeric retinals methylretinal (3). In particular, 4pI of all-trans-, 7-cis-, have been the subject of numerous investigations. Quan9-cis-, and ll-cis-13-demethylretinal and all-trans-, 9-cis-, tum yields of trans cis or cis trans photoisomerization 11-cis-,and 13-cis-14-methylretinal are determined upon (dpI)have been measured in solution upon direct excitairradiation in a nonpolar solvent. High pressure liquid tion19-22and triplet ~ e n s i t i z a t i o n ,and ~ ~ the ~ ~ primary ' ~ ~ ~ ~ ~ ~ chromatographic methods are employed to determine the photoproducts have been determined by using high presprimary products and their relative ratios obtained upon sure liquid chromatographic (LC) t e c h n i q ~ e s . ~ The ~ J ~ - ~ ~ irradiation of each molecule. photochemistry of the all-trans isomer of several retinal Experimental Section analogues has also been examined;2ghowever, there have Materials. 3-Methylpentane (3MP), the solvent for been no photochemical studies of the cis isomers of these quantum yield and absorption spectral studies, was pursynthetic retinals. Since this information may be imporchased as 99.4% pure (Phillips Petroleum) and was distant in helping to understand the nature of the trans tilled from dri-Na (Baker Chemicals) prior to use. Ethanol cis photoisomerization of the chromophore in bacterio(Gold Shield, IMC Chemicals) was distilled from calcium rhodopsin30and retinochrome,31 as well as the cis trans
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0022-3654/80/2084-0134$01.00/00 1980 American Chemical Society
Photochemistry of Linear Polyenes
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The Journal of Physical Chemistry, Vol. 84, No. 2, 1980 135
chloride imrnlediately prior to use. A 7 all-trans-13-Demethylretinal(2) was prepared by man40 I ganese dioxide oxidation of the corresponding vitamin A analogue,a2which was synthesized according to the procedure of van den Tempe1 and H u i ~ m a n .all-trans-14~~ Methylretinal (3) was synthesized folilowing the procedure of Tanis, Brown, and N a k a n i ~ h i . all-trans-2 ~~ and allt r a n s 4 were isolated and purified by preparative liquid chromatographic methods. A number of cis isomers of 2 and 3 were prepared by irradiating the corresponding all-trans isomers iin ethanol until a photoequilibrium mixture was established. They were isolated by analytical high pressure LC methods. Chromatography. Preparative separations weire accomplished by using a Waters Prep LCJSystem 500 that was modified for UV detection by using a Waters Model 440 0 250 300 350 400 450 absorbance detector.32 Separation conditions of all-trans-2 \IAVELENGTH nm and all-trans-3 from their respective reaction mixtures Flgure 1. The room temperature absorption spectra of all-frans- (-), ~ $ 12 ~ X2 were similar to those previously r e p ~ r t e d : ~two 9-cis- (----), 9-cis- (-), and 11-cis13-demethylretinal in 3in. Waters silica gel columns, 5-870 anhydrous ether in methylpentane, corrected for surface reflections and sotvent absorption. hexane, 250 mL/min flow rate, and 365-nm UV detection. Analytical separations were performed by using a Waters account of this procedure has been presented.38 ALC/GPC 204 liquid chromatograph. The separation conditions arc similar to those previously reported,21~26~29~a2 The final three steps in the synthesis of 2 and 3 and all spectroscopy, photochemistry, and chromatography exnamely, one 1 2 X 0.25 in. Waters p-porasil column, 2-4% periments were carried out under red lighting. anhydrous ether in hexane, 2-4 mL/min, ca. 2000 psi, and 365-nm UV detection. All compounds were purified to Results greater than 99.5% isomer pure. At room temperature, all-trans-13-demethylretinal(2) Spectroscopy. Isomers of 2 were identified by using IH has an absorption maximum A)(, of 366 nm with a molar NMR spectra recorded on a H F 250 MHz NMR spec) M-l cm-' in a 3extinction coefficient ( E ~ of~ 43000 trometer operated in the correlation mode.% The all-trans methylpentane (3MP) solution. Like that of all-transand 13-cis isomers of 3 were identified by using lH NMR retinal (l),the first absorption band of all-trans-2 is broad spectra. 9 4 s - and 114.9-3were identified by using high and diffuse with only hints of fine structure (Figure 1). pressure LC retention and absorption spectral data.14J5 This first band maximizes at a slightly shorter wavelength Absorption spectra were recorded at room temperature in than does the corresponding band of all-trans-retinal; quartz cuvets with Perkin-Elmer Model 575 and Cary however, t max is measurably lower.39 Model 14 spectrophotometers. Extinction coefficients for Photoequilibrium Studies. After irradiation of allisomers of 13-demethylretinal were obtained by direct trans-13-demethylretinal,high pressure LC analysis reveals measurement of 3-methylpentane solutions. the formation of a number of photoproducts. In order to photochemistry. Photoequilibrium mixtures were obobtain the maximum number of photoproducts, an,d to tained by irradiating 3-methylpentane, ethanol, and aceselectively optimize the photochemical formation of each tonitrile solutions of all-trans-2 and all-trans-3 with product a detailed study of the photochemistry of allmonochromatic radiation until the high pressure LC peak trans-2 was made. The exact composition of the photoratios were constant, A variety of excitation sources were chemical mixture (the number and relative ratio of peaks) employed for this purpose. They include a Hanovia 450-W is dependent upon the irradiation time, excitation waveHg arc lamp with 436-nm filter solution,37 a Hanovia length, and nature of the solvent. Upon extended irra1000-W Hg-Xe light and Aminco-Bowman 1/4-m monodiation a photoequilibrium composition is established in chrom,ator, and the 150-W xenon source of an Amincoeach solvent a t a specified excitation wavelength (Table Bowman spectrofluorimeter. Quantum yields of' photoisomerizution were determined I). That a photoequilibrium mixture is obtained was established by irradiation of all-trans-%until the number by irradiation of 1-2 mL of 10-4-10-5M retinal analogue and ratio of the high pressure LC peaks were constant. In for several minutes a t room temperature by using the addition, a photoproduct (9-cis-13-demethylretinal) was 150-W xenon lamp and 1/4-m monochromator of an Amirradiated under the same reaction conditions until a inco-Bowman spectrofluorimeter, 350-nm excitation, 7-nm constant high pressure LC peak ratio was obtained. The bandpass. The lamp flux was calibrated by using potasnumber and ratio of the high pressure LC peaks and their sium ferrioxalate a ~ t i n o m e t r y . The ~ ~ extent of photorelative retentions for this experiment was essentially isomerization was determined from multiple planimeter identical with that obtained upon extended irradiation of tracings of the high pressure LC peaks which were corall-trans-2. The three major photoproducts were isolated rected for individual isomer response at the detection wavelength. Quantum yields were calculated by using the and purified by using high pressure LC techniques, and equation employed in our previous s t ~ d i e s : ~ ~ ~ ~characterized ~ ' ~ ~ ~ ~ by ~ ~use of lH NMR chemical shift data. Thus, 7-cis-, 9-cis-, and 11-cis-13-demethylretinal were NAVCA Figure 1 displays the absorption spectra of +PI = these isomers of 13-demethylretinal, and Table I1 is a where NAis Avogadro's number, Vis the volume irradiated summary of their absorption spectral properties. In adin liters, C is the molar concentration, A is the percentage dition, a small shoulder is detectable on the long retention conversions, t is the irradiation time in seconds, F is the side of 9-cis-2 and is thought to be 7-cis,9-cis-13-delamp flux in photons per seconds, and A is the percentage methylretinal, ,,X = 350 nm. Similarly, the shoulder of light absorbed at the excitation wavelength. A detailed present on the long retention side of 11-cis-2may indicate (-.-e)
136
The Journal of Physical Chemistry, Vol. 84, No. 2, 1980
Waddell and West
TABLE I: Photoequilibrium Composition of Isomers of 13-Demeth y Iretinala
TABLE 111: Photoequilibrium Composition of Isomers of 14-Methylretinala
% isomer distributionC
% isomer distributionC
A,,~ nm
alltrans
7cis
9cis
11cis
350 390 430
83 83 83
0 0 0
7 7 9
10 9 7
hexane
ethanold
350 390 430
51 38 37
3 3 3
21 22 30
23 35 27
ace tonitriled
350 390 430
44 38 35
6 6 8
24 25 30
25 30 26
solvent 3-methylpentane
hb allnm
trans
9cis
11cis
390 430 450
41 32 26
3 3 4
0 0
et hanold
390 430 450
40 27 23
acetonitriled
350 390 430
32 20 19
solvent
a Obtained upon independent irradiation of all-transand 9-cis-13-demethylretinal t o this composition; corrected for individual isomer response of the detector. 7-nm bandpass. i 0 . 5 ; the remainder of the mixture was not analyzed by NMR. Based upon high pressure LC retention data it is thought that 7-cis, 9-cis- and 7-cis,ll-cis-13demethylretinal are formed; however, these isomers were not characterized by NMR spectroscopy.
Amax,
5 5
5 6 7
19 31 36
32 32 32
4 4 3
7 6 8
37 49 49
22 22 23
1 2
4
0
t
i
Emax,
nm
M-l cm-'
all-trans 7-cisb 9-cis 11-cis 13-cis
368 359 363 363 366
47 500 44 100 39800 26 200 38600
13-derneth~lretinal~ all-trans 7-cis 9-cis 11-cis
366 356 362 365
43 25 30 27
14-methylretinald
373 365 338 358
46 000 35000 18000 35000
retinala
isomer
2
49 60 62
a Obtained by independent irradiation of all-trans- and 13-cis-14-methylretinal to this composition; corrected for individual isomer response of the detector. 7-nm bandA trace of 7-cis-14-methylretinal (appass. t0.5. proximately 1%) is thought formed.
TABLE 11: Absorption Spectral Properties of Isomeric Retinals and Related Polyenes compd
13- 9-cis, cis 13-cis
000 600 000 000 hAVELEVGTH i r n
all-trans 94s 11-cis 13-cis
Figure 2. The room temperature absorption spectra of all-trans- (-), 9-cis- (-.-), 1 I-cis- (-e--), and 1 3 4 s - (----) 14-methylretinal in 3methylpentane, corrected for surface reflections and solvent absorptions. ,
I
; t
the presence of 7-cis,ll-cis-2, or another dicis isomer; however, since NMR spectra were not obtained for these minor (e1% ) photoproducts their structure is uncertain. Irradiation of an ethanol solution of all-trans-14methylretinal (3) results in the formation of several cis isomers.14J5 Again, the photoequilibrium composition depends upon the nature of the solvent and the excitation wavelength (Table 111), hence we were able to optimize the yield of selected photoproducts. 9-cis-, 11-cis-,13-cis-, and 9-cis,l3-cis-14-methylretinal are formed as the major photoisomers upon extended irradiation of all-trans-14methylretinal. Their absorption spectra are shown in Figure 2 and the absorption spectral proper tie^'^,'^ are summarized in Table 11. Based upon high pressure LC retention data, 7-cis-14-methylretinal is thought to be a minor photopraduct. Quantum Yield Studies. Quantum yields of trans cis or cis trans photoisomerization of all-trans-, 9-cis-, 11-cis-, and 13-cis-14-methylretinal were measured in the nonpolar solvent 3-methylpentane at room temperature by using potassium ferrioxalate a~tinometry.~' Primary products of each photochemical reaction were determined by using high pressure liquid chromatographic (LC) techniques. Table IV is a summary. Generally, the
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I ' -
n-Hexane solution; data from ref 39. n-Heptane; 3-Methylpentane; average of several data from ref 40. trials obtained by direct measurement; reproducibility of 2-5%. n-Hexane: data from ref 15. a
+
t..
-
3 ." c,r,
8 i
-
L . -
0"
100
700
I R R A D 1 4 T I C \ T l h E r e ondk
Figure 3. A plot of the quantum yield of photoisomerizationvs. irradiation time for 9-cis-I 4-methylretinal. The shaded and unshaded circles represent data obtained on separate solutions. Fresh aliquots were used for each determination. The least-squares fit of the data (-) is presented.
percentage conversions of the photochemical reactions used for determination of the quantum yields of photoisomerization and primary photoproduct data reported in Table IV were maintained at less than 3% in order to avoid any secondary photochemical reactions. Figure 3 shows a plot of the data for 9-cis-14-methylretinal vs. irradiation time. Fresh aliquots of a solution were used for each determination. A least-square fit of the data points reveals that dpI = 0.51 at the zero time intercept, a value that is within the standard deviation (h0.07) of the experimental mean, = 0.47. A plot of the appearance of the primary
The Journal of Physical Chemistty, Vol. 84, No. 2, 1980
Photochemistry of Linear Polyenes
TABLE IV : Photoisomerization Quantum ~
Yields of Isomeric Retinals and Related Polyenesa isomer @PI products
compd
retinal
all-transd 9-cise 11-cis 1h i s e
-
I
re1 ratiosC
0.08 t 0.02 0.18 0.25 0.05 0.21
9-cis/l3-cis trans/g-cis,13-cis trans trans
1:4 2:l
+_
13-demethylretinal
all-transf 74s 9-cis 11-cis
0.022 t 0.003 0.39 i 0.09 0.62 i. 0.12 0.60 * 0.12
9-cis/ 11-cis transidicisg trans/dicish trans
1:l 1:l 13:l
14-methy lret inal
all-transf 9-cis 11-cis 13-cis
0.26 0.51
9-cis/l3-cis trans/g-cis,1 3 4 s trans trans/9-cis,l3-cis
1:25 6:5
0.05 0.07 0.79 t 0.06 0.47 t 0.06 i
t
187
.-
35:l
a he = 350 nm, 7-nm bandpass, 2 X t O - 5 - l 0 - 4 N [ in 3-methylpentane, aerated solutions, room temperature, ferrioxalate High pressure LC analysis; Standard deviation of at least six trials. actinometer, generally less than 3% conversion. Data from ref 26. e Data from ref 21. f Data from ref 29. g Based upon high pressure LC retention data it is i 10%. thought that ?-cis, 9-cis-, and 7-cis-,ll~cis-l3-deme1thylretinal are formed; however, these photoproducts were not character. Since the retention of this peak is similar to but on the long retention side of that of 11-cisized by NMR spectroscopy. 2, it is thought that this peak may represent 9-cis,ll-cis-l3-demethylretinal.
I
t
a 3
A
P
I
B
/
1
I I 2 ,
I
IO0
200
IRRADlATlOh T l l l E seconds
Figure 4. A plot of the number of molecule$ of photoproduct formed vs. irradiation time for the corresponding data used to determine the quantum yield data presented in Figure 3. The triangles represent data obtained for a/l-P~ans-l4-~nethyIretinal and the circles represent those of 9-cis, 13-cis-14-methylretinal. Shaded and unshaded symbols represent data obtained on separate solutions. The least-squares fit of the data for all-trans-:) (-)and 9-cis,13-cis-3 (---) is ipresented.
photoproducts vs. irradiation time is shown in Figure 4, along with the least-squares fit of the data. A product ratio of 6:5 is thus obtained for the primary photoproducts all-trans-3 and 9-cis,13-cis-3, respectively. At percentage conversions of approximately 3 % and higher, 13-cis-14methylretinal is present in the reaction mixture. It probably arises via the photoisomerization of all-trans-3 in a secondar:y photochemical reaction and is not present in those reactions whlere lower photoproduct conversions were attained.
-
-
Discussion Quantum yields of the trans cis or cis trans photoisomcbrization of the isomeric retinals21 and of the trans cis photoisomerization of the all-trans isomers of 13demethylretinal and 14-methylretir~a129 have previously been examined. These values along with the primary photoproducts and relative product ratios are reported in Table IV, which also summarizes the data on the cis trans photoiciomerization of the cis isomers of 13-demethylretinal and 14-methylretinal. The quantum yields of trans cis or cis trans photoisomerization of the isomers of retinal are notably different than those quantum yields of photoisomerization of the corresponding
-
-
-
-+
isomers of 13-demethylretinal and 14-methyiretinal. Photochemistry of the All-Trans Isomers of Polyenes 1-3. The all-trans isomers of polyenes 1-3 have 4p1 values that range from approximately 2 to 26%, and vary with alkyl substitutional differences that were introduced to the C-13-C-14 carbon-carbon double bond,29 whereby increased methyl substitution leads to an increase in the measured quantum yield of photoisomerization. When 4p1 is partitioned into its component isomerization processes (for example all-trans-retinal yields 9 4 s - and 13-cis-retinal upon direct excitation in a nonpolar and the 9-cis and trans quantum efficiencies of the trans 13-cis processes can be obtained from the data in the Table IV), a direct correlation of with methyl substitution is even more apparent. The all-trans isomers of retinals 2, 1, and 3 have zero, one, and two methyl groups attached to the C-13-C-14 carbon-carbon double bond position of the polyene, and the quantum yields for the all-trans 13-cis photoisomerization processes in these compounds are 0.00,0.065, and 0.25, respectively. Thus, it appears that the isomerization efficiency increases with increaiaing methyl substitution. The quantum yields for the all-tirans 9-cis processes in polyenes 1-3 are essentially unaff e ~ t e d .Keep ~ ~ in mind that the chemical modifications did not affect the C-9-C-10 carbon-carbon double bond of these polyenes. Furthermore, quantum yields of the trans cis photoisomerization of the all-trans isomers of 10-methylretinal (4) and 10,14-dimethylretinal (5)) retinal analogues that have a methyl substituent incorporated at the C-10 position of the polyene chain, indicate that the efficiencies of the trans 9-cis processes are similar to each other but are much greater than the trans 9-cis efficiencies of polyenes 1-3. The quantum efficiencies of the trans -* 9-cis processes of all-trans-4 and all-trans-5 are 0.09 and 0.10, respectively. Photochemistry of the 9-Cis and 13-Cis Isomers of Polyenes 1-3. The 9-cis and 13-cis isomers of polyenes 1-3 have 4pI values that may be similarly rationalized since isomerizations may occur about the C-13-C-14 double bond position of these polyenes to yield a dicis or all-trans photoproduct, respectively. Upon absorption of light, 13-cis-retinal is isomerized to all-trans-1 with a quantum yield of 0.21, and 13-cis-14-methylretinal yields all-tra,m-3 with an efficiency of 0.46. 9-cis,l3-cis-3 is formed as a minor, primary photoproduct of 13-cis-14-methylretinal. That dPI for 13-cis-3 is a higher than that of 13-cis-1 is consistent with having replaced the C-14 hydrogen atom
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-
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The Journal of Physical Chemisfty, Vol. 84, No. 2, 1980
Waddell and West
of 13-cis-1 with a methyl group. The more highly subTriplet sensitized isomerization is observed for 1l-cisstitut,ed position then has a corresponding higher quantum retinal,20~21~23,24 and since the intersystem crossing quantum yield of photoisomerization about that carbon-carbon yield is approximately 0.520~22~50952 a triplet isomerization double bond. mechanism is possible upon direct excitation. Weiss and co-workers22concluded that the isomerization of ll-cisThe 9-cis isomers of retinals 2, 1, and 3 again have zero, retinal proceeds predominantly via an internally excited one, and two methyl groups at the C-13-C-14 double bond, first triplet state, an idea first suggested by Ottolenghi and and quantum yields for the 9-cis 9-cis,l3-cis processes of these retinals are 0.00, 0.06, and 0.28, r e ~ p e c t i v e l y . ~ ~ co-workers20based upon the absence of an oxygen effect upon $PI. Bensasson and Land52 have reached similar Note that quantum efficiencies of these photoisomerizat,ion conclusions for the photoisomerization of all-trans-, 9-cis-, processes are essentially identical with those measured for 11-cis-, and 13-cis-retinal. During our examination of their corresponding all-trans isomers. This is not an several of these retinal analogues we found that the unexpected result. In the isomerization of 9-cis-retinal to quantum yields of photoisomerization showed no variation 9-cis,l3-cis-retinal the C-9-(3-10 carbon-carbon double in aerated, nitrogen bubbled, or vacuum degassed solution. bond undergoes no configurational change, hence the However, since quantitative data on the triplet sensitized photoisomerization can be reduced to a 13-trans 13-cis isomerizations and intersystem crossing efficiencies of the process, the same configurational change that occurs for cis isomers of retinals 2 and 3 are not available the all-trans-retinal. Thus, the efficiencies of the two isommechanism of their rearrangement is uncertain. erizations may be expected to be comparable. Quantum Product Studies. In the nonpolar solvent 3-methylefficiencies for the 9-cis all-trans photoisomerization pentane, all-trans-1 and all-trans-3 form their correprocess of the 9-cis isomers of polyenes 1-3 vary considsponding 9-cis and 13-cis isomers upon direct excitation. erably with no obvious trend. all-trans-2 yields 9-cis- and 11-cis-13-demethylretinal as Photochemistry of the 11-Cis Isomers of Polyenes 1-3. primary photoproducts. 13-cis-13-Demethylretinal is not The (PPI values of the biologically important 11-cis isomers f ~ r m e d . Irradiation ~ ~ , ~ ~ of the cis isomers of polyenes 1-3 of retinals 1-3 also encompass a large range, approximately results in the formation of the corresponding all-trans 25-75%, The C-11-C-12 carbon-carbon double bond of isomers; however, dicis products are observed, Table IV. polyenes 1-3 have not been chemically modified, hence the Without exception, it is found that upon direct excitation variation in (bpi cannot be understood on that basis. We the isomers of retinals 1-3 yield primary photoproducts have suggested that the differences in these values may that correspond to configurational changes that result from result from the ground-state conformational differences one carbon-carbon double bond isomerization per photon. of these 11-cis polyenes.4211-cis-Retinal is thought to exist Zechmeister’s assumption53thus applies to this class of in solution in an equilibrium mixture of distorted 12-s-cis polyenals related to the retinals, independent of alkyl and 12-s-trans conformation^,^^ with the former predomsubstitution and geometric configuration. It may be inating in an approximate two-to-one ratio in nonpolar possible to extend this conclusion to polar solvents by solvents at room temperature.44 11-cis-2has an essentially examining the photoequilibrium product distributions planar 12-s-trans configuration@‘ since the steric interaction obtained upon irradiation of the all-trans isomers of between the C-10 hydrogen atom and C-13 methyl group polyenes 1-3 in ethanol and acetonitrile (Tables I and 111). that exists in the 12-s-trans conformation of 11-cis-retinal Although dicis isomers are present a t photoequilibrium, has been eliminated. 11-cis-3 is thought to have a highly time-dependent high pressure LC analysis of the phototwisted 12-s-trans geornetry.l4J6 The high values are product distributions indicate that they are formed via one then associated with the 12-s-trans configurations. It is carbon-carbon double bond isomerizations of a correpossible that a spectroscopic basis for this variation in sponding monocis isomer. As an example see Figure 4. exists. Birge, Schulten, and K a r p l u ~have ~ ~ performed A notable difference in the primary photoproduct discalculations on 11-cis-retinal and find that a crossing of tribution is observed when the all-trans isomers of polyenes the two lowest singlet A,A* excited states (Ag-like and 1-3 are irradiated in polar vs. nonpolar solvents. For Bu-like) occurs as the geometry of the 11-cis polyene’s example, upon irradiation in a 3-methylpentane solution, C-12-C-13 single bond is altered by twisting from a 12-s-cis all-trans-1 yields 9-cis- and 13-cis-retinal in a 1:4 ratio, to a 12-s-trans conformation. Recently, Andrews and whereas all four monocis isomers (7-cis-,9-cis-, 11-cis-,and Hudson47reported on two trienes having different ground 13-cis-retJinal)are f ~ r m e d as ~ ~primary J~ photoproducts state geometries by virtue of a conformational difference when ethanol or acetonitrile is the solvent. Similar phoabout a key single bond and provided experimental evitochemical behavior is observed for all-trans-2 and alldence that they have different T,T* states as the lowest trans-3. Although the reason for the solvent effect is not excited singlet states. Such an excited state reordering yet clear, it may be the result of a reordering of the n,a* with ground-state conformation could serve to account for and two lowest P,P* excited singlet states which are of the observed differences in the photoisomerization effisimilar e n e r g i e ~ . ~ Becker ~ $ ~ ’ and c o - w ~ r k e r shave ~ ~ sugciencies of the 11-cis isomers of retinals 1-3. gested that the n,T* singlet state is of lower energy in free Mechanistic Considerations. The mechanism of the retinal, but when a hydrogen bonding species is present, trans cis or cis trans photoisomerization of all-transin solution a T,T*singlet state is of lowest energy. Howand 11-cis-retinal has been the subject of much c o n ~ e r n . ~ ~ever, ~ ~ hydrogen bonding may not account for the similarIn spite of a high value for the intersystem crossing ities in the photochemistry of the all-trans isomers of quantum yield (41sc),20922,50,51 it was concluded that allretinals 1-3 in the two polar solvents studied. In order to trans-retinal rearranges via the singlet excited state since gain further insight into this aspect of the photochemistry no photoisomerization was observed upon triplet sensitiof the retinals, the effect of solvent on the quantum yield zation.20r21 Although preliminary results indicate that of trans cis or cis trans photoisomerization is being all-trans-13-demethylretinal and all-trans-14-methylretinal investigated. do not undergo significant isomerization upon triplet Summary sensitization, it is not yet possible to conclude that only an excited singlet pathway is responsible for the reactivity Upon irradiation the all-trans isomers of polyenes 1-3 of the all-trans isomers of polyenes 2 and 3. yield a number of configuration isomers as photoproducts.
-
-
4
-
4
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Photochemistry of Linear iPolyenes
The exact composition of the photochemical reaction mixture is dependent upon the irradiation time, excitation wavelength, and the nature of the solvent. Quantum yields of trans .+ cis or cis trans photoisomerization for several isomers of retinal, 13-demethylretinal, and 14-methylretinal were determined and were found to be notably different. The variation in quantum yields of the corresponding all-trans, 9-cis, and 13-cisisomers of retinals 1-3 appears to have a direct correlation with the allkyl substitutional differences of the C-13-C-14 carbon-carbon double bond of these polyenes whereby increased methyl substitution leads to an increase in the observed photoisomerization at the position. The 11-cis isomers of' retinals 1-3 have 4pIvalues that also vary with alkyl substitution, the probable result of a change in thLeground-state conformation of these polyenes. High pressure liquid chromatographic analysis of the primary photoproducts and their relative distributions indicates that for all compounds studied, one carbon-carbon double bond is isomerized per unit photon absorbed.
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Acknowledgment. We acknowledge Dr. Motokazu Uemura and Dr. Usanna Younes for their assistance in the synthesis of 13-demetliylretinal and 14methylretinal. The authors thank the National Eye Institute, National Institutes of Health (Grrants EY0177T and EY01930), for their generous support of this research and the l'ennsylvania Lions Sight Conservation and Eye Research Foundation for fellowship support (J.L.W.). References and Notes (1) R. Hubbard and G. Waid, J . Gen. Physbl., 36, 269 (1EE2). (2) A. Lewis, R. Fager, and E. W. Abrahamson, J . Raman Spectrosc., 1, 465 (1973). (3) A. f?. Oseroff and R. H. Caiiender, Biochemistry, 13, 4243 (1974). (4) D. i3ownds, Nature (London), 216, 1178 (1967). (5) M. Akhtar, P. T. Biosse, and P. B. Dewhurst, Blochem. J., 110, 693 (1968). (6) R. $. Fager, P. Sejnowski, and E. W. Abrahamson, Biochen?.Biophys. Res. Commun., 47, 1244 (1973). (7) For a review see S. E. Ostroy, Biochim. Biophys. Acta, 463, 91 (1977). (8) For a review see 8. Honig, Annu. Rev. Pl7ys. Chem., 29, 31 (1978). (9) F. I.Harosi, J. Favrot, J. M. Leclercq, D. Vocelie, and C Sandorfy, Rev. Can. Bioi., 37, 257 (1978). (10) H. Dartnail, "Handbook Sensory Physiology", Vol. VII/l, SpringerVerlag, Berlin, 1972, pp 122-145. (1 1) W. H. Waddeii, A. P. Yudd, and K. Nakanishi, J. Am. Chem. Soc., 98, 238 (1976). (12) W. J. DeGrip, R. S. H. Liu, V. Ramamurthy, and A. Aseto, Nature (bondonl. 262. 416 11976). (13) k. Crouch, V. Purvin,'K. Nakanishi, and T. Ebrey, Proc. Natl. Acad. Sci. U . S . A . , 72, 1538 (1975). (14) W. K. Chan, K. Nakanishi, T. G. Ebrey, and B. Honig, J. Am, Chem. Sot:., 96, 3642 (1974). (15) T. Ebrey, E. Govindjee, B. Honig, E. Pollock, W. Chan, R. Crouch, A. Yudd, and K. Nakanishi, Biochemistry, 14, 3933 (1075).
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