Light-harvesting at ambient temperature

85262_largeCoherently wired light-harvesting in photosynthetic marine algae at ambient temperatures

Elisabetta Collini, Cathy Wong, Krystyna Wilk, Paul Curmi, Paul Brumer & Gregory Scholes

Universities of Toronto, New South Wales and Padua

Nature, 463, pp. 644-7, 4 February 2010  doi:10.1038/nature08811

This low-key paper may in time come to be seen as one of the decisive studies of the 21st century.  The paper shows that room temperature quantum coherence can occur in biological matter. In 2007, Engel et al had shown that coherence was possible in organic matter, but this was only demonstrated at very low temperatures, whereas the Collini study demonstrates similar activity at ambient temperature. The paper and related commentaries makes no mention of consciousness, although a relevance to quantum computing is suggested, which is a possible step towards discussing consciousness. The main plank of the arguments against quantum consciousness relates to the speed of decoherence in biological matter being too quick for coherence to be relevant to processing, particularly neural processing, in such matter. This argument looks to have been substantially undermined by the recent study.

Antenna proteins are an essential part of the photosyntetic process, which absorbs light and transmits the resulting excitation between molecules to a reaction centre. Recent research has concentrated on determining the mechanisms that support a very high level of efficiency in this energy transport. Light-harvesting antennas are comprised of eight pigment-molecules, with different pigments absorbing different frequencies of light. The route the energy takes across the molecule is important in terms of energy efficiency.

Studies have documented the fact that light-absorbing molecules in some photosynthetic proteins transfer energy according to quantum mechanical rather than classical laws even at ambient temperature. This contradicts the 20th century dogma that long-range quantum coherence would always decohere in the temperatures found found in biological systems.

This paper by Collini et al describes X-ray crystallography studies of two types of marine cryptophyte algae that have long-lasting excitation oscillations and correlations and anti-correlations, symptomatic of quantum coherence even at ambient temperature. Distant molecules within the photosynthetic protein are thought to be connected to quantum coherence, and to produce efficient light-harvesting as a result. The cryptophytes can photosynthesise in low-light conditions suggesting a particularly efficient transfer of energy within protein. According to the traditional theory, this would imply only small separation between chromophores, whereas the actual separation is unusually large.

In this study, performed at room temperature, the antenna protein received a laser pulse, which results in a coherent superposition in the protein. The experimental data of the study shows that the superposition persists for 400 femtoseconds and over a distance of 2.5 nanometres. Quantum coherence occurs in a complex mix of quantum interference between electronic resonances, and decoherence caused by interaction with the environment. The authors think that long-lived quantum coherence facilitates efficient energy transfer across protein units. P. The authors remains uncertain, as to how quantum coherence can persist for hundreds of femtoseconds in biological matter. One suggestion is that the expected rate of decoherence is slowed by shared or correlated motions in the surrounding environment. Where light-harvesting chromophores are covalently bound to the protein backbone, it is suggested that this may strengthen correlated motions between the chromophores and the protein.

In the same issue of ‘Nature’ that published Collinis study, the ‘News and Views’ section of the journal also comments on her paper. It emphasises that this is the first study in which quantum coherence in photosynthetic proteins has been observed at room temperature. It comments on the remarkable efficiency of energy transfer, between the antennas that guide excitation energy from hundreds of light-absorbing pigment molecules towards the subsequent reaction centres that drive biochemical events. Collini is suggesting that quantum coherence could be a factor in this efficiency.

Earlier studies had observed coherent behaviour in green sulphur bacteria, but at very low temperatures. Collini et al observed quantum coherence in the antenna, and found that this persisted over 400 femtoseconds, in contrast to an expectation in traditional theory of only 100 femtoseconds. Coherence was observed between widely separated pigment molecules. This has also been observed in bacterial light-harvesting complexes. However, this was at very low temperatures, while the Collini study was at room temperature. Engel et al, who were responsible for some of the earlier studies, have speculated that quantum coherence allows antennas to search for the lowest energy state of the complex more efficiently, thus enhancing the energy transfer to the reaction centre. Coherence might help excitations to avoid local energy traps or minima, on their way to the reaction centre. Covalent binding to the protein backbone is speculated to make coherence longer lasting.

Perhaps the most surprising aspect of this latest paper on coherence in proteins is the speed with which news of the development has made its way to the level of more popular science, in the form of a useful full page summary by Kate McAlpine in the ‘New Scientist’. She mentions that Gregory Engel, who was respnsible for the earlier low temperature studies of coherence in bacterial proteins, is enthusiastic about the Collini result. Engel and his group have also performed a study at 4 degrees centigrade, much above previous levels, although below the 21 degrees of the Collini study. Engel is also quoted as saying that this work could have implications for quantum computing, where a core problem has been to operate at the very low temperatures that are usually thought necessary to prevent quantum decoherence. The speed with which this work has been picked up and given prominence in a popular science magazine suggests a background change of attitude to coherence in protein. The vexed question of quantum consciousness is not mentioned, but the suggestion of activities within protein as a model for quantum computing is moving is in that direction.

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