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A. Biophotosensor Composed of Cyanobacterial Photosystem I, Molecular Wire, Gold Nanoparticle, and Transistor

A. Biophotosensor Composed of Cyanobacterial Photosystem I, Molecular Wire, Gold Nanoparticle, and Transistor

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406



Redox and Photo Functions of Metal Complex Wires

Photon



PSI



Quinone

pocket





Molecular

wire



Gold

nanoparticle





Gate of FET



FIGURE 14. The concept of the biophotosensor composed of PSI and a transistor via

molecular wire. (Reprinted with permission from Ref. 15.)



electron transfer. When PSI is irradiated, the special pair of chlorophylls (P700)

is excited, followed by a stepwise electron transfer from P700 to FB via A0, A1

(vitamin K1; VK1) (Fig. 16), Fx, and FA without any backward transfer. Such

events induce a through-membrane charge separation, with the resulting efficiency of electron transfer being B100%.47,48

The reconstitution process of PSI with a molecular wire is illustrated in

Figure 16. After the extraction of VK1 from the isolated intact PSI (Fig. 16a and b)

using 50% water-saturated diethyl ether, the VK1-free PSI was reconstituted with

a specifically designed molecular wire of 1-[15-(3-methyl-1,4-naphthoquinon2-yl)]pentadecyl thiolate-protected gold nanoparticle, NQC15S-AuNP (Fig. 17).

After the removal of VK1 from intact PSI, the naphthoquinone-sulfur-linked

molecular wire equipped with a gold nanoparticle (NQC15S-AuNP, particle size:

B1.6 nm)68 was treated with VK1-free PSI in an MES buffer solution to give

a reconstituted PSI (NQC15S-AuNP@PSI) (Fig. 16c). The NQC15S-AuNP

possesses three advantageous characteristics:

(1) A naphthoquinone unit is located at the end of a molecular wire to fit into

the pocket (so-called quinone pocket) from which VK1 has been extracted.

(2) The redox potential of naphthoquinone is suitable for the output of

electrons from the A0 site to a molecular wire.

(3) A sufficient molecular length of the wire is provided for the output of

electrons from the pocket lacking VK1 to the surface external to PSI, since

the molecular wire has the same length of C15 alkyl-chain as that of VK1

itself (Fig. 17).



Biophotosensor and Biophotoelectrode

(a)



(b)



407



PsaC



PsaE



PsaD



(c)

FB



FB



12.3

FA



22.0

14.9



FX



FX

14.2

QK-A



14.1

QK-B



8.6



8.6



8.8



8.2



eC-B3



eC-A3

eC-B2

11.7

eC-A1



eC-A2

12.0

eC-B1



FIGURE 15. X-ray structure of cyanobacterial PSI: (a) top view of PSI trimer, (b) side

view (view direction indicated by the arrow at monomer II in (a)), (c) view of the

cofactors of the electron transfer chain in PSI monomer. (Reprinted with permission

from Ref. 67.)



Reconstitution of PSI with NQC15S-AuNP was identified by TEM

observation. Figure 18 shows the image of NQC15S-AuNP@PSI, in which can

be seen both a large gray circle (10 nm-o.d.), attributed to PSI, and a small,

clear and black dot (2 nm-o.d.), attributed to NQC15S-AuNP. Every gray

circle (PSI) has one or two black dots (NQC15S-AuNP). Considering that one

PSI has two VK1 pockets, this stoichiometry indicates no random aggregations



408



Redox and Photo Functions of Metal Complex Wires



(a)



(b)

P700



h␯



A0

e–



A1

Fx



(c)



Quinone

pocket



e–

Reconstitution



Extraction



Au

FA

FB



Au

VK1 (ϭA1)



NQC15S-AuNP



FIGURE 16. The process of the reconstitution of PSI with a molecular wire: (a) intact

PSI, (b) VK1-free PSI, and (c) NQC15S-AuNP@PSI. (Reprinted with permission from

Ref. 15.)

(a)

O



O

VK1



(b)



O



S

O

NQC15S-AuNP



FIGURE 17. Molecular structure of (a) VK1 and (b) NQC15S-AuNP.



and only connections via reconstitution at the VK1 sites. A common photoactivity test of reconstituted PSI with NQC15S-AuNP was also useful in

confirming that the 2-methyl-1,4-naphthoquinone moiety of NQC15S-AuNP

was fitted to the quinone pocket of PSI and worked as a mediator of the

electron transport system in PSI.15

A photocurrent measurement of NQC15S-AuNP@PSI chemically connected to a 1,4-benzenedimethanethiol SAM-modified gold electrode was carried

out in the presence of sodium L-ascorbate (NaAs) as a sacrificial reagent and

2,6-dichloroindophenol sodium hydrate (DCIP) as a mediator in a MES-NaOH

(pH 6.4) buffer solution containing NaClO4 as an electrolyte at 0 V vs. Ag/AgCl.

A clear peak at 680 nm was observed, consistent with the absorption spectrum

of PSI and indicating that these photocurrent responses were due to the

photoexcitation of PSI. Finally, this system was applied to the gate of an FET,

demonstrating the potential of the system as an electronic imaging device.15



Biophotosensor and Biophotoelectrode



Aun



409



PSI



Aun



PSI



Aun



FIGURE 18. TEM images of NQC15S-AuNP@PSI. The scale bar is 10 nm. (Reprinted with permission from Ref. 15.)



B. Biophotoelectrode Composed of Cyanobacterial

Photosystem I and Molecular Wires

The previous section described the connection of a biocomponent PSI to

a gold nanoparticle-attached molecular wire with a methylnaphthoquinone

moiety to construct a photosensor. In this section, we present a combination of

this technique, to connect PSI to an artificial molecular wire, with the technique

used for ITO modification with molecular wires described earlier for the

development of a biophotoelectrode.

The new molecular wire (tpy-C15NQ) designed for the biophotoelectrode is a compound with a chemical structure similar to VK1, possessing

a naphthoquinone moiety for insertion into the quinone pocket of PSI

and a terpyridine moiety for connecting to a Co(II) ion at the surface of a

modified ITO.16 Reconstitution of PSI with tpy-C15NQ in a similar manner

to that shown earlier afforded tpy-C15NQ@PSI, for which photoactivity

was confirmed by the absorption change of P700,15 thus indicating that the

2-methyl-1,4-naphthoquinone moiety of tpy-C15NQ fitted into the quinone

pocket of PSI and worked as a mediator of the electron transport system

in PSI.

We fabricated a PSI-immobilized ITO electrode, PSI_tpy-C15NQ_ITO,

by a combination of SAM formation with a terpyridine derivative and stepwise

metal-terpyridine coordination reactions in a similar manner as given earlier

(Fig. 19). First, a cleaned ITO electrode was immersed in a chloroform solution

of 0.1 mM p-terpyridylbenzoic acid for 12 h at 25 C. Next, the modified

ITO electrode was immersed in 0.1 M CoCl2aq for 0.5 h at 25 C, which formed



410



Redox and Photo Functions of Metal Complex Wires

P700



P700



A0



A0



A1



FX

FA



FB



HO

O



N

N

O

O



N



0.1 mM CHCI3

2 days



ITO



N

N

N



Ϫ



Br



O



0.1 mMCH3OH

5 min



O

O



3 days



CoCI2



N



0.1 M H2O

5h



N L

N Co L

N L



O

O



O







tpy-C15NQ



N

N



O

O



N N

N Co N

N N



N N

N Co N

N N



Br



Ϫ



O







Ϫ



Br





O



O

O



PSI_tpy-C15NQ_ITO



FIGURE 19. The procedure for the immobilization of VK1-free PSI onto tpy-C15NQmodified ITO electrode.



metal complexation. The modified ITO electrode was then immersed in a

0.1 mM methanol solution of tpy-C15NQ for 5 min. Finally, the modified ITO

electrode was immersed in an aqueous solution of 1 μg/mL VK1-free PSI

containing an MES buffer, thereby yielding the reconstituted ITO electrode

(PSI_tpy-C15NQ_ITO).

The absorption spectrum of PSI_tpy-C15NQ_ITO was nearly identical to

that of the native PSI solution (Fig. 20); however, the λmax value of the Q band

of chlorophylls (B690 nm) was B11 nm red shifted due to the oriented

aggregation of PSI onto the ITO electrode.

Figure 21 displays photocurrent responses and a photocurrent action

spectrum of PSI_tpy-C15NQ_ITO at À0.05 V vs. Ag/AgCl in the presence of

NaAs as a sacrificial reagent and DCIP as a mediator in a MES-NaOH (pH

6.4) buffer solution containing NaClO4 as an electrolyte. The action spectrum

shows a maximum around 660À680 nm, approximately corresponding with the

absorption spectrum of PSI and indicating that photocurrent responses were

due to the photoexcitation of PSI.

In this section, we have shown two types of biophotonic systems using

PSI and molecular wires. The first type uses a combination of PSI, molecular



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