Photosynthetic complexes are exquisitely tuned to capture solar light efficiently, and then transmit the excitation energy to reaction centres, where long term energy storage is initiated. The energy transfer mechanism is often described by semiclassical models that invoke 'hopping' of excited-state populations along discrete energy levels. Two-dimensional Fourier transform electronic spectroscopy has mapped these energy levels and their coupling in the Fenna-Matthews-Olson (FMO) bacteriochlorophyll complex, which is found in green sulphur bacteria and acts as an energy 'wire' connecting a large peripheral light-harvesting antenna, the chlorosome, to the reaction centre. The spectroscopic data clearly document the dependence of the dominant energy transport pathways on the spatial properties of the excited-state wavefunctions of the whole bacteriochlorophyll complex. But the intricate dynamics of quantum coherence, which has no classical analogue, was largely neglected in the analyses-even though electronic energy transfer involving oscillatory populations of donors and acceptors was first discussed more than 70 years ago, and electronic quantum beats arising from quantum coherence in photosynthetic complexes have been predicted and indirectly observed. Here we extend previous two-dimensional electronic spectroscopy investigations of the FMO bacteriochlorophyll complex, and obtain direct evidence for remarkably long-lived electronic quantum coherence playing an important part in energy transfer processes within this system. The quantum coherence manifests itself in characteristic, directly observable quantum beating signals among the excitons within the Chlorobium tepidum FMO complex at 77 K. This wavelike characteristic of the energy transfer within the photosynthetic complex can explain its extreme efficiency, in that it allows the complexes to sample vast areas of phase space to find the most efficient path.

Photosynthetic light-harvesting complexes absorb energy and guide photoexcitations to reaction centers with speed and efficacy that produce near-perfect efficiency. Light harvesting complex II (LHCII) is the most abundant light-harvesting complex and is responsible for absorbing the majority of light energy in plants. We apply two-dimensional electronic spectroscopy to examine energy flow in LHCII. This technique allows for direct mapping of excitation energy pathways as a function of absorption and emission wavelength. The experimental and theoretical results reveal that excitation energy transfers through the complex on three time scales: previously unobserved sub-100 fs relaxation through spatially overlapping states, several hundred femtosecond transfer between nearby chlorophylls, and picosecond energy transfer steps between layers of pigments. All energy is observed to collect into the energetically lowest and most delocalized states, which serve as exit sites. We examine the angular distribution of optimal energy transfer produced by this delocalized electronic structure and discuss how it facilitates the exit step in which the energy moves from LHCII to other complexes toward the reaction center.

Without therapy, most persons infected with the human immunodeficiency virus (HIV) ultimately progress to AIDS. Rare individuals (“elite controllers”) maintain very low levels of HIV RNA without therapy, thereby making disease progression and transmission unlikely. Certain HLA Class I alleles are markedly enriched in elite controllers, with the highest association observed for HLA-B571. Since HLA molecules present viral peptides that activate CD8+ T cells, an immune mediated mechanism is likely responsible for superior control of HIV. We report that the peptide binding characteristics of HLA-B57 molecules impact thymic development such that, compared to other HLA-restricted T cells, a larger fraction of the naïve repertoire of B57-restricted clones recognizes a viral epitope and these T cells are more cross-reactive to mutants of targeted epitopes. Our calculations predict that such a T cell repertoire imposes strong immune pressure on immunodominant HIV epitopes and emergent mutants, thereby promoting efficient control of virus. Supporting these predictions, in a large cohort of HLA-typed individuals, our experiments show that the relative ability of HLA-B alleles to control HIV correlates with their peptide binding characteristics that impact thymic development. Our results provide a conceptual framework that unifies diverse empirical observations, with implications for vaccination strategies.