Y. Hu et al. / Spectrochimica Acta Part A 53 (1997) 913–926
925
explains that the larger amount of the 11-cis
isomer can be produced in C20% aldehyde than in
C20 aldehyde. Similarly, the parallel (antiparallel)
polarization of the 15Cꢀ15%C bond in C25 alde-
hyde (C25% aldehyde) explains the presence (ab-
sence) of the 15-cis isomer in C25 aldehyde (C25%
aldehyde). (3) The polarization of the 9Cꢀ10C
bond are almost the same in both C20 and C20%
aldehyde in the ground state. However, the direc-
tion of polarization in the neighboring 11Cꢀ12C
bond may affect a cooperative enhancement of
polarization upon excitation in the conjugated
double bonds. The 9Cꢀ10C and 11Cꢀ12C parallel
(antiparallel) polarization in C20% aldehyde (C20
aldehyde) may enhance (suppress) the excited-
state polarization, and as a result, it produces a
larger (smaller) amount of the 9-cis isomer in C20%
aldehyde (C20 aldehyde). Similarly, the parallel
(antiparallel) polarization in C25 aldehyde (C25%
aldehyde) should increase (decrease) the amount
of the 13-cis isomer produced. Thus, the effect of
the terminal-methyl position on the stationary-
state composition can be qualitatively explained
in terms of the enhancement of the ground-state
polarization upon excitation. However, theoreti-
cal calculations concerning the singlet-excited
state and more quantitative experimental evidence
are necessary to establish this hypothetical mecha-
nism.
of the particular methyl group closer to the car-
bonyl end caused the following systematic
changes in the isomeric composition: (1) increase
in the amount of the 9-cis isomer; (2) increase in
the amount of the 11-cis isomer; and (3) disap-
pearance of the 13-cis isomer on going from C20
to C20% aldehyde correspond, on the basis of the
structural similarity in the carbonyl side of the
conjugated chain, to (1%) increase in the amount of
the 13-cis isomer, (2%) appearance of the 15-cis
isomer, and (3%) disappearance of the 13%-cis iso-
mer on going from C25% to C25 aldehyde. These
changes can be explained in terms of enhancement
of the ground-state polarization upon excitation.
The position of the terminal-methyl group is
not relevant to the presence or absence of the
11-cis isomer in the set of aldehydes examined.
The key factor in the generation of the 11-cis
configuration (in a parallel-methyl fragment) must
be the length of the conjugated chain.
Acknowledgements
The authors thank Dr Stanislav L. Bondarev
for critical reading of the manuscript. This work
has been supported by grants from the Ministry
of Education, Science and Culture (c06239101)
and from Human Frontier Science Program.
4. Conclusion
References
[1] T. Yoshizawa and G. Wald, Nature, 197 (1963) 1279.
[2] R.S. Becker, Photochem. Photobiol., 48 (1988) 369.
[3] Y. Koyama, J. Photochem. Photobiol. B: Biol., 9 (1991)
265.
[4] G.E. Bialek-Bylka, T. Tomo, K. Satoh and Y. Koyama,
FEBS Lett., 363 (1995) 137.
Direct photo-isomerization of all-trans C20%
aldehyde in acetonitrile produced the 7-cis, 9-cis,
11-cis and 9,11-cis isomers, whereas I2-sensetized
photo- isomerization of all-trans C25% aldehyde in
n-hexane produced the 9-cis, 13-cis, 13%-cis, 9,13-
cis, 9,13%-cis and 13,13%-cis isomers. (In the latter
aldehyde, direct photo-isomerization in acetoni-
trile produced the same set of isomers.)
In order to determine the effects of the termi-
nal-methyl position on the types and the amounts
of cis isomers which are generated by direct
photo-isomerization of the all-trans isomer in ace-
tonitrile, the compositions of mono-cis isomers
were compared between C20 and C20% aldehydes as
well as between C25% and C25 aldehydes. The shift
[5] G.E. Bialek-Bylka, T. Hiyama, K. Yumoto and Y.
Koyama, Photosyn. Res., in press.
[6] Y. Koyama and Y. Mukai, Advances in spectroscopy, in
R.J.H. Clark and R.E. Hester (eds.), Biomolecular Spec-
troscopy, Part B, Vol. 21, Wiley, Chichester, 1993, p. 49.
[7] N. Ohashi, N. Ko-chi, M. Kuki, T. Shimamura, R.J.
Cogdell and Y. Koyama, Biospectroscopy, 2 (1996) 59.
[8] M. Kuki, Y. Koyama and H. Nagae, J. Phys. Chem., 95
(1991) 7171.
[9] L. Zechmeister, Cis–Trans Isomeric Carotenoids Vita-
mins A and Arylpolyenes, Academic Press, New York,
1962.