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Bioelectromagnetism

Ishizaki and Fleming (2009) have investigated theoretically the spatial and temporal dynamics of

the electronic energy transfer through the Fenna–Matthews–Olson (FMO) pigment-protein complex of

green sulfur bacteria Chlorobaculum tepidum in order to address the robustness and role of the quan­

tum coherence under physiological conditions. Tey suggested that the excitation energy of bacterio­

chlorophyll (BChl) in the FMO complex that works in the frst step of photosynthesis is transferred

while maintaining a “quantum superposition” that simultaneously realizes conficting states (Ishizaki

and Fleming, 2009). Until then, the photosynthetic electronic energy transfer was thought to classically

difuse between BChls according to the gradient of the energy level generated in the complex, but in

reality, the multiple pathways were superposed to simultaneously realize diferent energy states as a

whole (Ishizaki and Fleming, 2009). Consequently, it was clarifed that the electronic energy transfer

was observed from one BChl to another BChl like a vibrating wave (Ishizaki and Fleming, 2009). Te

transfer of this excitation energy towards the reaction center occurred with a near unity quantum yield

(Ishizaki and Fleming, 2009). Te electrons of the BChl were excited to change the energy state, which

changed the state of the adjacent BChl and transfers energy (Ishizaki and Fleming, 2009). Tus, the

experiments have confrmed that multiple pathways were entangled (Ishizaki and Fleming, 2009). Tey

speculated that the quantum phenomenon might be related to the role of the “rectifer” that controls the

fow of energy (Ishizaki and Fleming, 2009).

Ishizaki and Fleming (2009) presented and discussed the numerical results regarding the dynamics

of the electronic energy transfer in the FMO complex of C. tepidum (Ishizaki and Fleming, 2009). Te

complex is a trimer made of identical subunits, each containing seven BChls. Tey clarifed that the

energy is transferred through two primary transfer pathways in the FMO complex: baseplate → BChls

1 → 2 → 3 → 4 and baseplate → BChls 6 → 5, 7, 4 → 3. Here, seven bacteriochlorophyll (BChl) molecules

belonging to the monomeric subunit of the Fenna–Matthews–Olson (FMO) pigment-protein complex

(Ishizaki and Fleming, 2009). Te complex is oriented with BChl 1 and 6 towards the baseplate protein,

whereas BChl 3 and 4 defne the target region in contact with the reaction center complex. Te spiral

strands are α-helices that are part of protein environment. It is reasonable to think that these pathways

are not explored each time by quantum computation but are determined by the structure of proteins

and the arrangement of BChl molecules (Ishizaki and Fleming, 2009). Te energy received by BChl 1

transfers to BChls 2 and 3, but in reality there is almost no energy level diference between BChls 1 and

3, and normally the energy of BChl 3 easily returns to BChl 1 due to thermal fuctuation (Ishizaki and

Fleming, 2009). However, in reality, the energy level of BChl 2 is relatively high, and it can overcome

local energetic traps, so no backfow occurs (Ishizaki and Fleming, 2009). It may seem that the electronic

energy transfer from BChl 1 to BChl 2 does not occur for the same reason, but since BChls 1 and 2 are

strongly entangled, such a classical pathway does not occur (Ishizaki and Fleming, 2009). Te electronic

energy can overcome local energetic traps and transfer to BChl 2 through quantum superposition or

entanglement (Ishizaki and Fleming, 2009). On the other hand, the quantum entanglement between

BChls 2 and 3 is relatively weak and the pathway is almost classical, so the energy cannot overcome local

energetic traps (Ishizaki and Fleming, 2009). In order to elucidate the extremely high efciency of the

light capture system, it will be necessary to consider not only the electronic energy transfer speed but

also the rectifcation mechanism and one-way pathway by the combination of quantum and classical

physics (Ishizaki and Fleming, 2009). Tus, they suggested that quantum coherence allows the FMO

complex to work as a rectifer for unidirectional energy fow from the chlorosome antenna to the reac­

tion center complex (Ishizaki and Fleming, 2009). It is reasonable to suppose that quantum coherence

can overcome local energetic traps to aid the subsequent trapping of electronic energy by the BChl mol­

ecules in contact with the reaction center complex (Ishizaki and Fleming, 2009).

Tese multiple states seem to play a role in the efcient use of light energy. Electrons and pigments

in living organisms are always surrounded by macroscopic substances such as surrounding tissues and

proteins. Until now, it has been the “common sense” of physics that microscopic substances such as elec­

trons cannot maintain their state even if they are in multiple states once they come into contact with the

macroscopic world. However, in reality, quantum multiple states can be realized even with macroscopic