The existence of photoinduced charge separation was confirmed
by the transient absorption of the N,N-NDI•− radical anion at
510 and above 670 nm following excitation with a 50 fs laser pulse
at 610 nm.16 According to the decay kinetics of the N,N-NDI•−
radical anion, the lifetime of the photoinduced charge-separated
species of push–pull system 1 (MeOH: s = 50 ps, TFE: s = 40 ps,
Fig. 3B) was quite similar to that of the blue control 2 (MeOH: s =
65 ps,16 TFE: s = 50 ps) but clearly, about 10-times shorter than
that of the red control 3 (MeOH: s ≈ 500 ps18). This suggested that,
in contrast to accelerated charge separation, scaffold asymmetry
in push–pull photosystem 1 did not affect charge recombination
much.
The delivery of functional nanoarchitecture to bilayer mem-
branes is an underrecognized and underexplored process that
often determines the finally observed apparent activities.29,30 For
membrane delivery, concentrated solutions in various solvents
(and eventual additives) are simply added to the vesicle sus-
pension, and the functional compounds are hoped to reach
the membrane before precipitation. To determine the solvent
that best delivers photosystem 1, large egg yolk phosphatidylcho-
line vesicles (EYPC LUVs) were labeled with N-(7-nitrobenz-2-
oxa-1,3-diazol-4-yl)-1,2-dihexadecanoyl-sn-glycero-3-phospho-
ethanolamine (NBD-PE: kex 463 nm, kem 535 nm). Quenching of
lipid-bound NBD by FRET to N,N-NDIs indicates the relative
quantity of delivered photosystem 1. With the disassembling TFE
as best (᭹) and the assembling MeOH as worst transporter
(᭺), significant differences were found in the ability of solvent
additives to deliver photosystem 1 to EYPC LUVs. Controls with
monomeric N,N-NDIs 9a (ꢀ)16,18 confirmed that poor delivery by
MeOH is compound-specific and not intrinsic (Fig. 4A).
4 ↔ 5 exhibited no cooperativity under the present conditions.17
Different to both symmetric controls, this finding suggested that
the introduced asymmetry has a negative effect on the formation
of active self-assembly. Facilitated antiparallel self-assembly into
the hydrophobic p-helix 5 by dipole–dipole attraction before
reaching the membrane could explain both poor activity and
lack of cooperativity. Alternatively, these results could also be
explained by hindrance of parallel self-assembly into 4 by dipole–
dipole repulsion in the membrane. Moreover, vectorial control
of partitioning by the terminal carboxylate might be lacking or
opposite to expectations. Overall, we caution that a number of
pathways to the functional system are conceivable to account
for the reduced activity and cooperativity found with push–pull
photosystems.
In summary, the introduction of asymmetry in photoactive
rigid-rod p-stack nanoarchitecture is found to reduce fluorescence,
to accelerate photoinduced charge separation, but to have little
effect on charge recombination. These overall favorable properties,
however, were not reflected in higher photosynthetic activity
in lipid bilayer membranes. Hindered formation of the active
suprastructure might account at least in part for these overall
complex and surprisingly small effects. We conclude that vectorial
self-assembly of asymmetric photosystems in lipid bilayers will be
very challenging.
We thank D. Jeannerat, A. Pinto and S. Grass for NMR mea-
surements, P. Perrottet, the group of F. Gu¨lac¸ar and C. A. Schalley
for MS measurements, H. Hagemann for access to the Xe lamp,
one referee for helpful suggestions, and the Swiss NSF for financial
support. S. B. is a fellow of the Roche Research Foundation.
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2562 | Org. Biomol. Chem., 2007, 5, 2560–2563
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