Science - USA (2020-08-21)

pair is even a factor of ~80 slower. The reason
forthisisthatanexcitationin,e.g.,anLHCII
monomer spends only 1.3% of its time on one
of its Chlbmolecules (see Box 2), leading to an
effective“hopping”time of 1 to 2 ns. Because
this is far longer than the average hopping

time between complexes, which was estimated

to be ~25 ps based on time-resolved fluores-

cence studies ( 49 ),itcansafelybeconcluded

that EET between complexes does not proceed

through Chlsb, and transfer pathways that

involve interacting Chlamolecules are the

most prominent. Only after direct excitation

might one or two Chlsbpreferably transfer

their excitation energy to a neighboring com-

plex, but even these Chls do not function as

efficient bridges for EET between complexes.

A timeline of the different EET processes oc-

curring within the complexes and between

them is shown in Fig. 3.

Light harvesting of PSI supercomplexes

PSI is considered to be the most efficient

photon-to-electron converter in nature ( 50 ). In

the PSI core (Fig. 4) of organisms as different

as cyanobacteria and plants, it takes only ~20

ps after the absorption of a photon by one of

the ~100 core Chls to extract an electron from

the RC. Therefore, the quantum efficiency of

CS is close to 1. In cyanobacteria, the core is

usually present as a trimer and sometimes as a

tetramer ( 51 ). In plants, it is instead mono-

meric, and four antenna complexes are as-

sociated with it as Lhca1-4 and Lhca2-3 dimers

(PSI-LHCI; Fig. 4), increasing the absorption

cross section of the PSI core by ~60%. The

Croceet al.,Science 369 , eaay2058 (2020) 21 August 2020 3of9

Apoproteins +carotenoids

Comparisonbetween LHCIIand Lhca4

Comparisonbetween LHCIIand FCP

Chlorophylls Absorption/fluorescence spectra

Fig. 2. Comparison of the structures and the absorption and fluorescence spectra of LHCII, Lhca4 (a PSI subunit), and FCP.[For original data, please see
( 107 ), ( 25 ), and ( 24 ), respectively.] (Left) Comparison of apoprotein structure and carotenoid organization (LHCII in orange, Lhca4 in yellow, FCP in dark yellow).
(Center) Chlorophyll organization; the inset shows the overlay of a Chlaof LHCII including its phytol chain and a Chlcand fucoxanthin occupying the same position in
FCP. (Right) 77K absorption (solid) and fluorescence (dashed) spectra.

Box 2. Estimating rates of EET between different pigment-protein complexes.

The rate of EET from LHC A to B (kAB) can be estimated as follows. When this transfer is dominated

by transfer between two pigmentsi(in A) andj(in B) (hopping ratekij), then the overall transfer rate

can be approximated bykAB=pikij, wherepiis the probability that the excitation resides on donor

pigmentiwhen complex A is excited. Using the numbers provided in Fig. 1, it can be calculated that the

average probabilitypbin monomeric LHCII is ~0.013 and that one of the six Chlbmolecules is in the

excited state, whereaspa= ~0.115 for one of the eight Chlamolecules. The total rate of transfer from

complex A to B should in general be summed over all pigment pairsiandjof both complexes, but only

one or a few pigment pairs dominate in most cases. The effective transfer time from complex A to B is

thusthop/pi(thop=kij−^1 ), and this time has to be augmented with the migration timetmig, which is the

average time that it takes for an excitation within the complex to reach the pigment that is transferring

to the neighboring complex:teff=thop/pi+tmig( 76 ). The value oftmigfor an LHCII trimer is ~25 ps

( 105 ), and for the monomeric complexes, we use an approximate value of 10 ps.

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