23 Ultrafast Energy Transfer of Biohybrid Photosynthetic Antenna …
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the LH2-NC-A647. For comparison the lifetime of the excited state of free A647 was
1.2 ns [9]. With careful inspection at < 2 ps, a small negative band at 805 nm ascribed
to GSB of B800 was observed with the appearance (<1 ps) and disappearance (>1 ps)
of GSB of B800. This is a clear evidence demonstrating the mediation of B800 in
the energy transfer from A647 to B850. Figure 23.8c and d shows the time traces
of A at 650 (c) and 855 nm (d). The former exhibited bi-exponential dynamics
(τ av = 7.3 ps), on the other hand, the dynamics of the latter could be appropriately described by monoexponential energy transfer, providing the time constant of
9.1 ± 0.1 ps.
Detailed dynamics were inspected by decay-associated spectra (DAS) obtained
through global analysis. The DAS consists of three components: The time constants
(amplitude ratio of GSB and SE of A647) were 770 fs (0.24), 10 ps (0.70), and
1.0 ns (0.06). The first component (770 fs) can be assigned to the A647 relaxation in
the excited state, such as solvation or structural relaxation [9]. In addition, a small
positive signal at 805 nm was observed, indicating energy transfer to B800 in LH2.
The time constant of 770 fs was in the same time scale to that for the energy transfer
from B800 to B850. The second DAS indicates the energy transfer component with
the time constant of 10 ps, which was in good agreement with that evaluated from
the time trace at 855 nm, 9.1 ps. The third DAS with 1.0 ns represents the decay in
the excited state of B850 [9]. The energy transfer dynamics was also analyzed based
on a sequential kinetic model, suggesting that energy transfer from A647 to B800
takes place through single-phase dynamics.
We attempted to modulate environments surrounding A647 moieties through
reconstitution of LH2-NC-A647 into a lipid bilayer composed of an anionic phospholipid DOPG, because A647, which is a highly hydrophilic fluorophore, has been
reported to be preferentially partitioned in aqueous phase and less soluble in lipid
bilayer phase [12]. Therefore, we hypothesized that the A647 moieties should be
positioned proximate to or in the exterior region of the interface of the anionic lipid
bilayer, away from B800 through the reconstitution. As a result, energy transfer efficiency significantly decreased to 42% (evaluated from an excitation spectrum). This
result suggested that part of A647 moieties in the LH2-NC-A647 disengaged from
the energy transfer system. The energy transfer dynamics in the lipid bilayer system
was observed by femtosecond absorption spectroscopy (Fig. 23.8b). Basic spectral
features were same as those for the micellar solution system. Although the appearance of GSB of B800 was not clearly observed in lipid bilayer system, the signal
at 805 nm was negative until ~4.5 ps. For comparison, a B800-removed conjugate
exhibited very small A intensity, which became positive at ~3.5 ps. These results
supported the fact that B800 is also involved in the energy transfer pathway in the
lipid bilayer system. The time traces of transient absorbance at 650 and 855 nm for
the membrane systems are shown in Fig. 23.8c and d. The rates of energy transfer
evaluated from the formation of excited state of B850 were slightly faster than those
in the micellar system. Interestingly, the B850 dynamics were again described by
monoexponential kinetics, giving a time constant of 6.4 ± 0.1 ps, which is unexpectedly shorter than 9.1 ± 0.1 ps for the micellar system. Note that the rise of the
negative signal of B850 (Fig. 23.8d) is faster for the lipid bilayer system, while the
397
the LH2-NC-A647. For comparison the lifetime of the excited state of free A647 was
1.2 ns [9]. With careful inspection at < 2 ps, a small negative band at 805 nm ascribed
to GSB of B800 was observed with the appearance (<1 ps) and disappearance (>1 ps)
of GSB of B800. This is a clear evidence demonstrating the mediation of B800 in
the energy transfer from A647 to B850. Figure 23.8c and d shows the time traces
of A at 650 (c) and 855 nm (d). The former exhibited bi-exponential dynamics
(τ av = 7.3 ps), on the other hand, the dynamics of the latter could be appropriately described by monoexponential energy transfer, providing the time constant of
9.1 ± 0.1 ps.
Detailed dynamics were inspected by decay-associated spectra (DAS) obtained
through global analysis. The DAS consists of three components: The time constants
(amplitude ratio of GSB and SE of A647) were 770 fs (0.24), 10 ps (0.70), and
1.0 ns (0.06). The first component (770 fs) can be assigned to the A647 relaxation in
the excited state, such as solvation or structural relaxation [9]. In addition, a small
positive signal at 805 nm was observed, indicating energy transfer to B800 in LH2.
The time constant of 770 fs was in the same time scale to that for the energy transfer
from B800 to B850. The second DAS indicates the energy transfer component with
the time constant of 10 ps, which was in good agreement with that evaluated from
the time trace at 855 nm, 9.1 ps. The third DAS with 1.0 ns represents the decay in
the excited state of B850 [9]. The energy transfer dynamics was also analyzed based
on a sequential kinetic model, suggesting that energy transfer from A647 to B800
takes place through single-phase dynamics.
We attempted to modulate environments surrounding A647 moieties through
reconstitution of LH2-NC-A647 into a lipid bilayer composed of an anionic phospholipid DOPG, because A647, which is a highly hydrophilic fluorophore, has been
reported to be preferentially partitioned in aqueous phase and less soluble in lipid
bilayer phase [12]. Therefore, we hypothesized that the A647 moieties should be
positioned proximate to or in the exterior region of the interface of the anionic lipid
bilayer, away from B800 through the reconstitution. As a result, energy transfer efficiency significantly decreased to 42% (evaluated from an excitation spectrum). This
result suggested that part of A647 moieties in the LH2-NC-A647 disengaged from
the energy transfer system. The energy transfer dynamics in the lipid bilayer system
was observed by femtosecond absorption spectroscopy (Fig. 23.8b). Basic spectral
features were same as those for the micellar solution system. Although the appearance of GSB of B800 was not clearly observed in lipid bilayer system, the signal
at 805 nm was negative until ~4.5 ps. For comparison, a B800-removed conjugate
exhibited very small A intensity, which became positive at ~3.5 ps. These results
supported the fact that B800 is also involved in the energy transfer pathway in the
lipid bilayer system. The time traces of transient absorbance at 650 and 855 nm for
the membrane systems are shown in Fig. 23.8c and d. The rates of energy transfer
evaluated from the formation of excited state of B850 were slightly faster than those
in the micellar system. Interestingly, the B850 dynamics were again described by
monoexponential kinetics, giving a time constant of 6.4 ± 0.1 ps, which is unexpectedly shorter than 9.1 ± 0.1 ps for the micellar system. Note that the rise of the
negative signal of B850 (Fig. 23.8d) is faster for the lipid bilayer system, while the