2R-D-2PF6 Dipole = 28.0 D
2R-D-2F Dipole = 14.8 D
a)
b)
2R-D-2PF6 Dipole = 24.9 D
2R-D-2F Dipole = 5.8 D
Figure 4.8: Dipole moments (µ) from ESP charges obtained for the (a)RFamily (2R-D-2PF6 and 2R-D-2F) and for the (b)R’family (2R-Dp-2PF6 and 2R-Dp-2F)
The rotaxane-rotaxane distance in theRfamily is of 4.0 ˚A, this is close to the optimal value of 3.6 ˚A.
On the other hand, this distance is 5.0 ˚A for theR’family making this interaction to be negligible.
Thus the efficiency for the template directed synthesis is tuned by controlling the distance between recognition sites.
Most importantly, we found that the role of the counter anion is to tune the charge population on the -NH+2- recognition site so that the larger (softer) the counter anion the more charge on the recognition site and the more interaction with the rotaxane ring is obtained. The interaction with the recognition site serves as the first directed template mechanism (clipping) for the formation of the rotaxane rings. This has many implications for the future synthesis of rotaxanes because we predict that we can control the positive cooperativity by changing the charge population on the recognition site by tuning the softness of the counter anion. Thus we can control the efficiency of future directed synthesis of rotaxanes for the clipping mechanism by using different degree of softness for the counter anion.
Chapter 5
Design of New Models for the Oxygen Evolving Complex
Jose L. Mendoza-Cortes, Robert Nielsen, Jacob Kanady, Emily Y. Tsui, Theodor Agapie, William A. Goddard III
5.1 Introduction
Photosystem II is a homodimer where photosynthetic water oxidation occurs.[41] Each monomer contains 20 subunits with a total molecular mass of 350 kDa. Besides the protein subunits, there are other cofactors such as four manganese atoms, three to four calcium atoms (one of which is in the Mn4Ca cluster) and others such as chlorophylls andβ-carotenes.[41] Water oxidation is catalyzed by a center containing the Mn4Ca cluster, which is known as the Oxygen Evolution Complex (OEC).
The OEC is one of nature’s capacitors. It couples successive one-electron reductions of an adjacent chlorophyll center (known as P680) to four-electron oxidation of water to dioxygen.[42] At a funda- mental level, the chemical reactions taking place are shown in Figure 5.1. First, the most reduced OEC state is oxidized four units by P+680, giving a state that can be characterized as OEC(+4) or S4. The S4 state reacts with water to produce dioxygen (Figure 5.1a). However, the path that the OEC undergoes to reach this virtual OEC(+4) is not well understood and it has been a matter of debate.[42] On the other hand, there is a consensus that the oxidation of the OEC is stepwise;
each of the oxidation steps are shown in Figure 5.1b. Each oxidation state of the OEC is known as S-state or Storage-state (Sn) with S4 being the most oxidized and S0 the most reduced state. The transition from S4 to S0 is the most important and more controversial step because it is when the water is converted to dioxygen.
Many mechanisms have been proposed for the transition from S4 to S0 but almost all of them proposed the need for high oxidation state for Mn, such as Mn(IV) or even Mn(V).[43] This is consistent with the necessary transfer of 4e−to the OEC from the water oxidation. This transition,
however, has proven to be challenging to observe because it happens very quickly, but some progress have been made in observing S3 to S0but not the S4 itself.[44, 45, 46]
P680
hν=1.82 eV
e- + P680+
OEC + 4 P680+ OEC+4 + 4 P680 4
OEC+4 + 2H2O OEC + O2 + 4H+
2H2O O2 + 4H+ + 4e-
OEC + 4 P680+ OEC+4 + 4 P680
S0
S1
S2 S3
S4
P680
P680+ hν
P680
P680+ hν P680
P680+ P680
P680+
hν hν
2H2O O2
His332O O
Ca
O
Mn Mn
O O Mn
O H2O
O O O
O O
O O O
H2O
Asp342
Glu354
Ala344
Mn OHx
OHx
Glu333 O
Asp170
O Glu189
O
OEC
a) b)
Figure 5.1: (a) Fundamental chemical reactions that take place in the production of O2 that start with the conversion of solar energy (hν) to an electron and a hole in the chlorophyll center called P680. (b) The catalytic cycle of the Oxygen-evolving complex (OEC) is shown where every oxidation is defined as a S-state (Sn). (Inset) The full description of the OEC is shown.
N O O Ca
O
Mn Mn
O Mn O
O O
O
O Me O
Me O
O Me
O N N
O
N N
N
HO N HO OH
N
N N
N O O Mn
O
Mn Mn
O Mn O
O O
O Me O
Me O
O Me
O N N
O
N N
N N
N
TAB-H3
Figure 5.2: Complexes synthesized in the Agapie group containing (left) a Mn3Ca and (right) a Mn4
cubane. Notice how the Ca in Mn3 at the top have been substituted by one Mn to give Mn4. A molecular model of the Mn4Ca cluster would allow study of the electronic structures involved in all the oxidation states of the OEC. The synthesis of such a biomimetic compound can be attained by designing ancillary ligands. One such approach performed by the Apapie group was able to obtain
a Mn3Ca complex. It is the first example where the Ca atom has been incorporated with Mn into a cubane that resembles the OEC (Figure 5.2). They were also able to synthesize an all Mn cubane Mn4. This opens the possibility of studying the role of the Ca in the oxidation states of the Mn.
Thus in this work we validate a methodology to reproduce and predict the reduction potential of the biomimetic model for the OEC using the rigid ligand 1,3,5-triarylbenzene spacer which incor- porates six pyridine and three alcohol groups (TAB-H3) shown in Figure 5.2. We then can use this method to design new compounds that structurally and electronically resemble to a high degree the most oxidized state of the OEC.