Biology Reference
In-Depth Information
4.4.3 Light-Harvesing Systems
Light-harvesting systems in nature allow the conversion of light into energy
(Nelson & Ben-Shem, 2004). In plants, this process is called
photosynthesis
.
The energy of sunlight is harvested and converted into glucose (see Eq. 4.2).
Sunlight + 6H
O + 6CO
2
C
6
H
O
6
(glucose) + 6O
(4.2)
2
12
2
In plants, the green pigments (chromophores) called chlorophylls
collect energy from light. Chromophores transport energy through a
mechanism referred to as
(FRET). When
a photon is absorbed by a chromophore, it causes electron excitation,
which means the excited electron is raised to a higher energy level. The
electron eventually returns to the non-activated lower energy level and
light is emitted. In FRET, two chromophores are coupled, and the excitation
energy is transferred from one chromophore to another, from a donor to an
acceptor, that is when the energy levels of absorption and emission match.
The distance between the chromophores plays an important role. Nature
has developed precisely spaced arrays of chromophores that facilitate light
harvesting and conversion into energy (Nelson & Ben-Shem, 2004).
Artificial light-harvesting systems are of great interest as they could
potentially be used in solar cells and allow the conversion of sunlight into
electrical energy. Protein scaffolds, such as the coat proteins of VNPs allow
the precise positioning of chromophores through chemical bioconjugation.
A range of systems based on the TMV platform, all based on similar design
principles, have been developed; here we will highlight one of these studies
(Endo
Forster resonance energy transfer
, 2007).
TMV coat proteins were modified with different chromophores that
function as donor and acceptor sites, and then assembled into disks and rods.
The chromophores are brought in proximity through self-assembly of the
coat proteins and enable FRET to occur. In brief, TMV coat protein monomers
with genetically introduced Cys residues were modified with three different
chromophores through maleimide coupling. Oregon Green served as the
primary donor, rhodamine was used as an intermediate donor, and Alexa
Fluor 594 was used as the acceptor. The different labeled coat proteins
were then self-assembled into disk and rod structures making use of the
well-understood
et al.
, 2006, 2007; Ma
et al.
, 2008; Miller
et al.
self-assembly mechanism of TMV. TMV rod
assemblies contain 700 chromophores per 100-nm rod; the spacing
between the chromophores lies within the distance range in which FRET
occurs. Horizontal spacing was 1.8 nm and vertical spacing 2.3 nm. Rod-
assemblies with varying donor:acceptor were assembled and evaluated
using fluorescence spectroscopy. A complex system containing the three
in vitro
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