Differences among coherent dynamics in evolutionary related light-harvesting complexes: evidence for subtle quantum-mechanical strategies for energy transfer optimization

In this work we exploit two-dimensional photon echo experiments (2DPE) to observe quantum coherence dynamics in energy transfer in light-harvesting proteins isolated from marine cryptophyte algae. Previous data, recorded on two complexes (PC645 and PE545) at room temperature, revealed exceptionally long lasting oscillations with distinct correlations and anti-correlations even at ambient temperature. These observations provided compelling evidence for quantum-coherent sharing of electronic excitation across the 5-nm-wide proteins under biologically relevant conditions, suggesting that distant molecules within the photosynthetic proteins are 'wired' together by quantum coherence for more efficient light-harvesting. In this work measurements performed on a different evolutionary related complex (PE555) at two excitation wavelengths are presented. The new results highlight different lifetimes for electronic coherences. Although preliminary, these evidences can be tentatively interpreted considering the difference in the protein structures.