398
T. Dewa et al.
decay of the negative A647 signal at 650 nm (Fig. 23.8c) is slower for the lipid bilayer
system. The DAS analysis exhibited four components: time constants (amplitude)
were 670 fs (0.15), 9.2 ps (0.47), 27 ps (0.35), and 490 ps (0.03). The second component with 9.2 ps lifetime corresponds to the energy transfer, which is longer than 6.4
± 0.1 ps, due to averaging of slower decay and faster rise of the negative signals at 650
and 855 nm, respectively, for the lipid bilayer system. The time constant in the lipid
bilayer system was equivalent to that in the micellar system (10 ps), suggesting that
energy transfer pathway are similar, A647 → B800 → B850. Note that the relative
amplitude of the 9.2 ps component (0.47) is smaller than the 10 ps component (0.70)
for the micellar system, which is responsible for the smaller energy transfer efficiency
of 42%. The third DAS at 27 ps likely represents non-radiative deactivation of A647.
A647 moieties located near the interface region of the lipid bilayer might be enhanced
to be deactivated in a non-radiative way. A LH2 complex tends to cluster in DOPG
lipid bilayer [13] due to which intercomplex energy transfer between neighboring
B850 rings is facilitated [14]. This results in faster deactivation kinetics of B850 in
the lipid bilayer (490 ps) than in the micellar system (1.0 ns). Similar deactivation
mechanism can be also taking place for A647 and such enhancement of deactivation
will lead to reduction of energy transfer efficiency. Nevertheless, energy transfer
dynamics of LH2-NC-A647 chiefly displayed monoexponential kinetics also in the
lipid bilayer system.
In order to rationalize the location of A647, the donor–acceptor distances, R calcd s,
were calculated based on the Förster mechanism. The R calcd s are depicted in the
structure of the LH2 complex as the colored circles (green and red) centered at B800
and B850 (Fig. 23.9; Here, the strucutre of Rps. acidophila 10050 (PDB 1NKZ)
was adopted because of the lack of crystallographic structure of LH2 from Rba.
sphaeroides. R calcd s were 19 and 22 Å from B800 to B850 (Fig. 23.9a), respectively,
in the micellar system. The latter distance was obviously insufficient for the A647
attached to the N-terminal domain, making direct energy transfer impossible. The
R calcd 19 Å from B800 suggested that the energy donor A647 (with the linkage length
~16 Å) was placed near the end of LH2β-polypeptide α-helix (N-terminal side). In
the micellar solution, the hydrophobic region of LH2 was covered with detergent
molecules as depicted in a and c (the top view from the N-terminal side) [15]. Even
if the N-terminal domain were flexible enough to approach B800, detergents bound
to LH2 would prevent A647 moieties from being localized at 19 Å external to the
hydrophobic region. Concerning the N-terminal domain, the β-polypeptide of LH2NC was longer than that of LH2 from Rps. acidophila 10050; the A647-attached
C6 was away from the N-terminal of LH2β-polypeptide of Rps. acidophila 10050
by seven amino acids toward the N-terminal (Fig. 23.2). The C6 position could
be presumed to be near the cytoplasmic surface of LH2-NC due to the P12 that
would induce a bend in the conformation of the β-polypeptide chain. Considering
the crystallographic structure of LH2 from Rps. acidophila 10050 (PDB: 1NKZ), the
N-terminal domain shows a bend at A5 heading toward the LH2α-polypeptide (the
direction of the center of circular structure, Fig. 23.9). Taken together, the energy
transfer distance of 19 Å from B800 can be explained by the location of A647 (with
~16 Å of the linkage length) positioned near the end of the α-helix of the N-terminal
T. Dewa et al.
decay of the negative A647 signal at 650 nm (Fig. 23.8c) is slower for the lipid bilayer
system. The DAS analysis exhibited four components: time constants (amplitude)
were 670 fs (0.15), 9.2 ps (0.47), 27 ps (0.35), and 490 ps (0.03). The second component with 9.2 ps lifetime corresponds to the energy transfer, which is longer than 6.4
± 0.1 ps, due to averaging of slower decay and faster rise of the negative signals at 650
and 855 nm, respectively, for the lipid bilayer system. The time constant in the lipid
bilayer system was equivalent to that in the micellar system (10 ps), suggesting that
energy transfer pathway are similar, A647 → B800 → B850. Note that the relative
amplitude of the 9.2 ps component (0.47) is smaller than the 10 ps component (0.70)
for the micellar system, which is responsible for the smaller energy transfer efficiency
of 42%. The third DAS at 27 ps likely represents non-radiative deactivation of A647.
A647 moieties located near the interface region of the lipid bilayer might be enhanced
to be deactivated in a non-radiative way. A LH2 complex tends to cluster in DOPG
lipid bilayer [13] due to which intercomplex energy transfer between neighboring
B850 rings is facilitated [14]. This results in faster deactivation kinetics of B850 in
the lipid bilayer (490 ps) than in the micellar system (1.0 ns). Similar deactivation
mechanism can be also taking place for A647 and such enhancement of deactivation
will lead to reduction of energy transfer efficiency. Nevertheless, energy transfer
dynamics of LH2-NC-A647 chiefly displayed monoexponential kinetics also in the
lipid bilayer system.
In order to rationalize the location of A647, the donor–acceptor distances, R calcd s,
were calculated based on the Förster mechanism. The R calcd s are depicted in the
structure of the LH2 complex as the colored circles (green and red) centered at B800
and B850 (Fig. 23.9; Here, the strucutre of Rps. acidophila 10050 (PDB 1NKZ)
was adopted because of the lack of crystallographic structure of LH2 from Rba.
sphaeroides. R calcd s were 19 and 22 Å from B800 to B850 (Fig. 23.9a), respectively,
in the micellar system. The latter distance was obviously insufficient for the A647
attached to the N-terminal domain, making direct energy transfer impossible. The
R calcd 19 Å from B800 suggested that the energy donor A647 (with the linkage length
~16 Å) was placed near the end of LH2β-polypeptide α-helix (N-terminal side). In
the micellar solution, the hydrophobic region of LH2 was covered with detergent
molecules as depicted in a and c (the top view from the N-terminal side) [15]. Even
if the N-terminal domain were flexible enough to approach B800, detergents bound
to LH2 would prevent A647 moieties from being localized at 19 Å external to the
hydrophobic region. Concerning the N-terminal domain, the β-polypeptide of LH2NC was longer than that of LH2 from Rps. acidophila 10050; the A647-attached
C6 was away from the N-terminal of LH2β-polypeptide of Rps. acidophila 10050
by seven amino acids toward the N-terminal (Fig. 23.2). The C6 position could
be presumed to be near the cytoplasmic surface of LH2-NC due to the P12 that
would induce a bend in the conformation of the β-polypeptide chain. Considering
the crystallographic structure of LH2 from Rps. acidophila 10050 (PDB: 1NKZ), the
N-terminal domain shows a bend at A5 heading toward the LH2α-polypeptide (the
direction of the center of circular structure, Fig. 23.9). Taken together, the energy
transfer distance of 19 Å from B800 can be explained by the location of A647 (with
~16 Å of the linkage length) positioned near the end of the α-helix of the N-terminal