Robustness of electronic coherence in the Fenna-Matthews-Olson complex to vibronic and structural modifications

We present the first two-dimensional electronic spectra of photosynthetic antenna complexes bearing modifications to the protein and the chromophores. The vibronic structure of the Fenna-Matthews-Olson complex was altered by near-complete substitution of 13C for naturally abundant carbon and separately by randomly distributed partial deuteration. The structure and arrangement of the bacteriochlorophyll a chromophores were modified by deletion of the gene encoding the enzyme responsible for reducing the isoprenoid tail of the bacteriochlorophylls. Analysis of the time-dependent amplitude of the crosspeak corresponding to excitons 1 and 2 indicates that these modifications do not affect the frequency or dephasing of the beating observed in this particular peak. This result leads us to conclude that this beating indeed arises from electronic coherence and not vibrational wavepacket motion. We further conclude that the protection of zero-quantum coherences afforded by the protein matrix of this photosynthetic complex is not the result of a finely-tuned series of system-bath interactions perfected by billions of years of evolution but rather a simple downstream property of a close arrangement of chromophores within a phonon bath. We conclude with a brief discussion of the outstanding questions and possible applications of this phenomenon.