(A) Orthogonality of the yield, , and the energy storage efficiency, , of QSS formation by the PCT.

<p>For each point, and are set to the values that maximizes within the limits set by and as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0036065#pone-0036065-g005" target="_blank">Figure 5</a>. is strongly sensitive to the separation distance, , and is primarily sensitive to . (B) The decrease in the maximal values of with increasing plotted for different values of and . At large values of the optimized yield is primarily dependent on .</p

Structure and function of the photosynthetic triad.

<p>(A) Molecular detail of an idealized artificial charge separation construct, a self-assembling de novo designed protein. (B) Discrete steps in the formation of the charge separated state: The primary-donor molecule P in the ground state configuration DPA absorbs a photon of the correct frequency to form DA, where is the photoexcited state of P. The excited electron transfers to the acceptor cofactor, A, forming the intermediate state DP⁺A⁻. The donor cofactor, D, then transfers an electron into P, resulting in the charge separated state D⁺PA⁻. (C) Energy level diagram of the states in B. The <i>k</i>-variables denote the corresponding microscopic single-electron ET rates. In this scheme, the direct long range tunneling between D and A (i.e., ) and the ‘thermal back reaction’ <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0036065#pone.0036065-Xu1" target="_blank">[33]</a> between P and A (i.e., ) are not considered. As explained in the main text, their magnitudes can be significantly suppressed without affecting the efficiency and yield.</p

The evolution of the charge separated state derived in eqn:ct.

<p> is normalized by , which we take to be unity. Rate constants are chosen as , and s. Relevant timescales are labeled on the upper axis and are marked by vertical lines (see eqn:kpm for definitions of ). A central quasi-steadystate (QSS) plateau region is formed when these timescales are well separated. We define the decay time of the QSS, , as the lifetime of the charge separated state. The horizontal line marks the yield, , defined as the value of <i>C</i> in QSS. Analytical expressions for and are derived in Equations 14 and 17, respectively.</p

The optimal range for and are shown on the Marcus curve.

<p>A high yield, high efficiency QSS formation in a triad requires that back electron transfer from A to P be so downhill as to be well into the Marcus inverted region. To see this, we substitute in Equation 27b so that , from which it immediately follows that the condition is satisfied if the mean value . The condition was derived in Equation 20 to be a necessary condition for the formation of a QSS.</p

Predicted long-wavelength limit or red-edge for efficient solar energy conversion.

<p>Photon energies smaller than cause a loss in either yield or energy storage efficiciency. For each point, the value of used maximizes within the constraints set by and as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0036065#pone-0036065-g005" target="_blank">Figure 5</a>. The values are calculated as where and 0.4 eV. Wavelength limits of natural systems depicted above the axis are taken from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0036065#pone.0036065-Kiang1" target="_blank">[34]</a>.</p