How Quantum Coherence Assists Photosynthetic Light Harvesting

Abstract

This perspective examines how hundreds of pigment molecules in purple bacteria cooperate through quantum coherence to achieve remarkable light harvesting efficiency. Quantum coherent sharing of excitation, which modifies excited state energy levels and combines transition dipole moments, enables rapid transfer of excitation over large distances. Purple bacteria exploit the resulting excitation transfer to engage many antenna proteins in light harvesting, thereby increasing the rate of photon absorption and energy conversion. We highlight here how quantum coherence comes about and plays a key role in the photosynthetic apparatus of purple bacteria.

Figure 1

(a) Cartoon representation of the photosynthetic reaction center protein with surface outline. (b) Surface outline of the reaction center showing bacteriochlorophylls (Chl₁, Chl₂, Chl₃ and Chl₄) in green, bacteriopheophytins (Ph₁ and Ph₂) in orange and quinones (Q₁ and Q₂) in red. The central bacteriochlorophylls, Chl₁ and Chl₂, form the so-called special pair (SP). (c) Atomic structure of a BChl.

Figure 2

When excitation is transferred from a donor bacteriochlorophyll (BChl) to an acceptor BChl, the Stokes shift between the emission and absorption spectra causes an imperfect energy overlap, as shown in Case 1 (see filled area illustrating overlap J_DA). This results in a reduced rate of excitation transfer. The reaction center can counter the reduced overlap by introducing a second acceptor BChl that is strongly coupled to the first, forming the special pair (SP). Strong coupling, accounted for by an interaction energy of V = 500 cm⁻¹ (V is determined in Ref. 50), coherently spreads excitation between the two acceptor SP BChls, shifting also the SP exciton energies from the single BChl excited state energy E to energies ε₋ and ε₊. This shift alters the absorption spectrum, as shown in Case 2, and accordingly increases the overlap, J_DA, between emission (green line) and absorption (blue line) spectra (see filled area). Furthermore, due to the anti-parallel orientation of the SP BChl’s transition dipole moments, the lower energy exciton state $\widetilde{|-\rangle }$ attains a transition dipole moment of ${\mathbf{d}}_A=\sqrt{2}\mathbf{d}$. The combination of better spectral overlap J_DA and a stronger d_A value increases the rate of excitation transfer for Case 2 over that of Case 1.

Figure 3

(a) Light harvesting complex 1 (LH1) surrounding the photosynthetic reaction center. The 32 LH1 bacteriochlorophylls forming the B875 ring are shown in green. (b) Exciton spectrum and oscillator strengths of the B875 ring. The transition dipole moment of a single bacteriochlorophyll is given by δ = 6.3 Debye. The gray bar indicates the amount of thermal energy at 300 Kelvin.

Figure 4

One LH1-RC complex with three LH2 complexes nearby. The upper and lower rings of BChls in LH2 are the B800 and B850 rings, respectively.

Figure 5

Spherical chromatophore from Rhodobacter sphaeroides showing (a) proteins and (b) bacteriochlorophylls. Reaction center (RC) is shown in red, light harvesting complex 1 (LH1) in blue and light harvesting complex 2 (LH2) in green. LH1-RC complexes form figure-8 shaped dimers in Rhodobacter sphaeroides.