Proceedings of the IConSSE FSM SWCU 2015, pp. AZ.12–22 ISBN: 978-602-1047-21-7
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AZ.12
Excited state dynamics of carotenoids free and bound to pigment- protein complexes
Ferdy S. Rondonuwu
a
and Yasushi Koyama
b a
Faculty of Science and Mathematics, Satya Wacana Christian University, Jl. Diponegoro 52 60, Salatiga 50711, Indonesia
b
Faculty of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda 669-1337, Japan
Abstract
Excited state dynamics and energetics of carotenoids having a different number of conjugated double bonds
n, which include spheroidene n = 9 and lycopene n = 10 free and bound to LH2 complexes from
Rhodobacter spaheroides 2.4.1 and Rhodospirillum moliscianum have been evaluated with the use of time-resolved
absoprtion spectroscopy. A relaxation path, including a 1
1
B
u +
1
1
B
u
2
1
A
g
1
1
A
g
singlet internal conversion and a 1
1
B
u
T triplet has been identified for Car free in a solution. In antenna complexes, an additional two paths were identified as Car-to-BChl
singlet-energy transfer channels; first starting from the 1
1
B
u +
andor 1
1
B
u
states to the Q
x
state, and second starting from the 2
1
A
g
state to the Q
y
state. The second channel may be deactivated depending on the number of conjugated double bonds
n. The 2
1
A
g
donor state of spheroidene n = 9 promotoes the singlet-energy transfer to the Q
y
aceptor state of BChl. In contrast to spheroidene, the 2
1
A
g
state of lycopene n = 10
prevents the energy transfer process.
Keywords spheroidene, lycopene, singlet-energy transfer, LH2 complexes
1. Introduction
Pigments are molecules that absorb light and give color to living matter. Carotenoids Cars are one of the most abundant pigments available in nature. Their presence can be
easily recognized by bright yellow to red colors due to the lack of an absorption band in this region. Photosynthetic organisms, including green plants, photosynthetic bacteria, and algae
sensitize Cars in nature. In green plants, Cars are mainly located in leaves, fruits, and roots. The strong orange color in carrots and yellowish red color in tomatoes, for example, basically
originate from -carotene and lycopene, respectively. -carotene and lycopene are a few examples of more than 600 Cars that are naturally and chemically synthesized Britton et al.
2004. In leaves, Cars are present in a relatively large amount but they are indistinct due to the strong green color given by chlorophylls. However, as the leaves become old and
chlorophylls Chls degrade, the color gradually changes from green to yellow. Although only plants and microorganisms can synthesize Cars, they are also found accumulated in humans
and animals through the nutrients they consume. Besides their color-giving ability, Cars perform various functions. In photosynthesis, Cars play two important roles: for light-
harvesting Magdoang et al., 2014 and photo-protection Ruben et al., 2007; Staleva et al., 2015. As light-harvesters, Cars absorb light in the spectral region left over by
bacteriochlorophylls BChls, thereby the unique configuration of carotenoids and BChl in photosystem antenna complexes, for example, enhance the absorption of sunlight energy
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
