Excited state dynamics of carotenoids free and bound to pigment-protein complexes
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from near ultraviolet to near infrared. As a photo-protector, Cars manage excessive light energy by quenching singlet and triplet states of Cars and BChls in antenna complexes
Kirilovsky et al., 2015. Photon is absorbed by Cars in the form of excited-state energy. Later this excited-state energy is converted into redox energy, which is then used as fuel to drive
the sophisticated photosynthetic machines in photosynthetic bacteria and green plants; those machines are immensely important to sustain life. Through a series of energy-
transducing reactions, the photosynthetic machinery transforms light energy into a stable form of chemical energy of organic compounds, which can sometimes last as long as millions
of years. The fossil fuels are burnt to support human activity, and the food we eat every day is the result of photosynthesis. Other than photosynthesis, Cars quench harmful singlet
oxygen and free radicals by interrupting a sequence of oxidative reactions. The antioxidant ability of Cars makes them important molecules to prevent various diseases such as cancer
or macular degeneration Akufo et al., 2015. Although various functions of Cars have been well documented, a deeper understanding concerning the details mechanism of such
functions at the molecular level is very limited.
Recently Cars researches focused on structures and dynamics to widen their applications not only for natural colorants but also for medicine and artificial antenna such
as biosolar cells Xiang et al., 2005; Hug et al., 2015. This paper outlines early events in photosynthesis; how Cars capture sunlight energy and transfer it to neighboring BChl
through an S1-state will be discussed. Advancements of time-resolved absorption spectroscopies up to femtosecond and subpicosecond time-resolution bring up the
opportunity to explore excited state dynamics of low-lying singlet excited states including 1
1
B
u +
, 1
1
B
u
, and 2
1
A
g
of Cars.
2. Carotenoid structure
The basic structure of carotenoids consists of the C
40
hydrocarbon skeleton constructed from eight isoprenoid units in a sequence forming a conjugated chain Britton et
al., 2004. The optical properties of Cars are determined by the number of conjugated C=C bonds
n. The left-hand side of Figure 1 depicts the molecular structures of five different kinds of Cars with
n = 9 13 that are often found in bacterial photosynthetic.
Figure 1. Molecular structures of carotenoids left and their electronic absorption
spectra in n-hexane solution right.
The conjugated chain of Cars has approximately C
2h
symmetry see the structure of all -
trans-polyene, for example, which gives rise to the singlet states classified as k
1
A
g
, l
1
B
u
,
F.S. Rondonuwu, Y. Koyama
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m
1
A
g +
, and n
1
B
u +
groups. Here, the superscript 1 signifies the singlet states; + and are
Pariser signs showing the symmetry of electronic configuration Pariser, 1956; g gerade and u ungerade represent the even and odd parity of the total wave function; and k, l, m,
and n are labels of a series of electronic states having the same symmetry from lower to higher energy note, n is different from the symbol
n that is being used to specify the number of conjugated C=C bonds. Concerning the one-photon optical transition from the 1
1
A
g
state, the m
1
A
g +
and n
1
B
u +
states can be classified as the optically-allowed states, whereas the k
1
A
g
and l
1
B
u
states can be classified as the optically-forbidden states. The singlet states of carotenoids in the order: 1
1
B
u +
2
1
A
g
1
1
A
g
the ground state, have been well-documented. In all-
trans-carotenoids, the optically-allowed 1
1
B
u +
1
1
A
g
transitions can be clearly observed in the near-ultraviolet to the visible spectral regions. Fig. 1 right panel shows the
absorption spectra due to this transition for the above-mentioned five different kinds of carotenoids in
n-hexane solution. The spectral pattern of each 1
1
B
u +
1
1
A
g
absorption consists of a pair of vibrational progressions due to the C=C and C C stretching vibrations
with frequencies of ~1520 cm
1
and ~1150 cm
1
Sashima et al., 1999. The pairs of progressions are overlapped with each other to form the apparent vibrational progressions.
The 1
1
B
u +
1
1
A
g
transitions strongly depend on intramolecular interactions with the surroundings. For a nonpolar carotenoid in a nonpolar solvent, a dispersion interaction
Shulten et al., 1980 becomes predominant, which gives rise to the red shift in the transition. The solvent effect is particularly important to comprehend how the 1
1
B
u +
energy is modified when the carotenoid molecule is embedded in proteins such as the antenna complexes and
the reaction centers. Unlike the energies of the optically-allowed 1
1
B
u +
state, which can be readily determined by the use of a conventional spectrometer, the energy of the optically-forbidden
2
1
A
g
state, for example, needs to be determined by high-sensitivity fluorescence spectroscopy overcoming the low quantum yield of the 2
1
A
g
1
1
A
g
fluorescence originating from a state mixed with the strongly optically-allowed 1
1
B
u +
state Christensen, 1999. As the length of the conjugated chain increases, the energy gap between the 2
1
A
g
and the 1
1
B
u +
states widens, and therefore, reduces the mixed state, leading to weaker fluorescence. It implies that an accurate determination of the 2
1
A
g
-state energy of carotenoids is extremely difficult. Since knowledge of a 2
1
A
g
state is important in understanding the mechanisms of singlet-energy transfer in photosynthesis, substantial
efforts have been made to determine the 2
1
A
g
-state experimentally. The methods include: i measurements of resonance-Raman excitation profiles RREPs for crystalline lycopene, -
carotene, and mini-9- -carotene Sashima et al. 2000; ii fluorescence spectroscopy for all -
trans-neurosporene and spheroidene Fujii et al., 1998, -carotene Onaka et al., 1999, violaxanthin, and zeaxanthin Frank et al., 2000 in
n-hexane solution; and iii transient absorption spectroscopy using the 1
1
B
u +
2
1
A
g
absorption in the infrared region for zeaxanthin and violaxanthin in methanol Polívka et al., 1999, spheroidene bound to the
pigment-protein LH2 complexes from Rhodobacter sphaeroides 2.4.1, and rhodophin
glucoside bound to the LH2 complex from Rhodospseudomonas acidophila Polívka, 2002.
It took almost three decades to completely understand the dynamics of the Car singlet states in terms of the state ordering of 1
1
B
u +
2
1
A
g
1
1
A
g
. Based on this particular scheme, the Car-to-BChl singlet-energy transfer reactions were explained in terms of channels starting
from these two low-lying singlet states, i.e., 1
1
B
u +
and 2
1
A
g
. However, the pseudoparity selection rule Callis et al., 1983 using Pariser s label Pariser, 1956, which is defined within
the framework of the Pariser-Parr-Pople PPP approximation Pople, 1953, suggests that
Excited state dynamics of carotenoids free and bound to pigment-protein complexes
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vibronic coupling is not allowed between those electronic states having different Pariser signs. Therefore, the direct 1
1
B
u +
2
1
A
g
internal conversion is symmetrically forbidden. Through the extrapolation of the low-lying singlet states of shorter polyenes that were
calculated on the basis of the Pariser-Parr-Pople multi-reference double-excitation configuration-interaction PPP MRD-CI method, it predicted that the 1
1
B
u
state should be present in-between the 1
1
B
u +
and the 2
1
A
g
states for polyenes having 11 n 6, while the
3
1
A
g
state should be present in-between the 1
1
B
u +
and 1
1
B
u
for polyenes having 9 n 10
Tavan Sculten, 1986, 1987. The optically-forbidden 1
1
B
u
state was first identified experimentally in crystalline spheroidene Sashima, 1999, lycopene, -carotene, and mini-9- -carotene Sashima et al.,
2000 by the use of resonance-Raman excitation profiles RREPs. Continuous efforts have been put into this work, leading to the very recent spectroscopic identification of the 3
1
A
g
state Furuichi et al., 2002. for a series of carotenoids, i.e., mini-9- -carotene, spheroidene, lycopene, anhydrorhodovibrin, and spirilloxanthin, and for further detection of the 1
1
B
u
state for anhydrorhodovibrin and spirilloxanthin Furuichi et al., 2002. The energy states determined by the measurements of the RREPs for carotenoids having the number of
conjugated C=C bonds, n = 9 13, are shown in Figure 2; the energy level of the Q
x
and the Q
y
states of BChls are also shown in the same figure for comparison.
Figure 2. An energy diagram, determined by the measurement of RREPs, for the 1
1
B
u +
, 3
1
A
g
, 1
1
B
u
, and 2
1
A
g
states of crystalline carotenoids including mini-9- -carotene n =
9, spheroidene n = 10, lycopene n = 11, anhydrorhodovibrin n = 12, and
spirilloxanthin n = 13. Broken lines indicate the Q
x
and Q
y
energy levels of BChls in the LH2 complexes. The carotenoids data in this figure was taken from Figure 5a of Furuichi
et al. 2002 .
3. Materials and methods 3.1 Materials
