Study of the vibronic mixing in chlorophylls with multidimensional spectroscopy

Abstract

Chlorophylls comprise one of the most important families of molecules for the photosynthetic process. Chlorophyll-like molecules have been classically interpreted under the Gouterman model, being thought to feature two independent transitions, Qy(S1) and Qx(S2). Recent studies on the matter provided a new interpretation, assuming these states to feature strong vibronic coupling and to be inseparably mixed. Experimental study of vibronic mixing can be done with non-linear spectroscopy techniques that can create and record superpositions of states, the so-called "coherences". These coherences, when excited via vibronically coupled transitions, represent a direct observation of vibronic mixing phenomena. Polarization-controlled two-dimensional electronic spectroscopy is performed on chlorophyll a at cryogenic (77 K) temperature, followed by a complex analysis to the oscillatory signals, unveiling vibronic coupling in the most common member of the chlorophyll family.

Popular Abstract

Life needs energy to function, and living beings have developed different mechanisms to obtain it. Heterotrophic organisms, such as human beings, can nourish themselves with organic matter to get it. However, for autotrophic organisms, this task is not as straightforward, since they must generate their own complex organic compounds from inorganic matter. To do this, they incorporate a complex biological machinery in their cells. Like a power plant, they harvest sunlight as a source of energy, and they use it to generate, from carbon dioxide and water, chemical energy stored in the form of sugars, producing also oxygen in the process. For millions of years, plants, algae and some bacteria have been carrying out this process, generating their own molecular fuel.

The way in which these organisms conduct photosynthesis is very diverse, since the biochemical machineries they employ for this purpose is different. Nevertheless, they have one common feature: the presence of photosynthetic pigments known as chlorophylls inside their cells, in the thylakoid membranes. These are responsible for absorbing sunlight, and for transmitting it to the rest of the machinery for the production of chemical energy. Generally, these chlorophylls absorb light in the visible region of the spectrum. This is also the responsible for the vivid colours that these organisms have, such as the green of leaves.

Therefore, the study of these photosynthetic pigments is fundamental to the understanding of how these natural power plants function. Such studies have several complications associated to them, since the first steps in the processes of sunlight absorption occur on an ultra-fast time scale, in which energy is transferred from one pigment to another. This time scale is generally in the femtosecond to picosecond range, which is around 10 to the power of -15 or 10 to the power of -12 seconds.

To capture processes that happen so quickly, we need equally fast "cameras". A similar problem was encountered in the late 19th century, when some members of high society were trying to understand how the gallop of race horses worked. The animal's gallop was so fast that the human eye could not pick up what exactly the rapid movement of its legs looked like. Therefore, it was not known if the animal ever got completely off the ground, or if it always had one foot on it. Eadweard Muybridge, with the help of multiple cameras and high-speed automatic triggers, was able to recreate the sequence of the horse's gallop, demonstrating that it did indeed get completely off the ground, and resolving, in this way, the long standing question.

In our laboratory, the concept used is similar. Even though the process to be studied was extremely rapid, we were able to capture it by employing also really fast triggers, which consisted of short laser pulses. Their duration was in the range of just 15 femtoseconds. Therefore, by employing them, it was possible to research how these chlorophylls interacted with light in such a short time. Specifically, for this purpose, we used a technique known as "Two-Dimensional Electronic Spectroscopy" (2DES).

In the experiments developed, the objective consisted on the study of the electronic structure of chlorophyll a (Chl a). It is the most common of the chlorophylls, as it is present in each and every organism that carries out oxygenic photosynthesis. In particular, we seek to understand the relationship between the two main energy transitions that occur in Chl a in the visible region, which are traditionally denoted as Qy and Qx.

The earlier studies of their electronic structure had been carried out from the so-called "Gouterman model" perspective. This was originally applied to porphyrins in general, but was later applied to chlorophylls, since they exhibit a porphyrinic head in addition to a long phytol tail. This model proposed that the two transitions, Qy and Qx, were independent, with orientations perpendicular to each other. Thus, it did not consider any special coupling or mixing between them.

However, recent studies on the matter, pointed out that these states could be intimately coupled, by means of a phenomenon known as "vibronic mixing". Although there were evidences of the presence of vibronic mixing in chlorophylls, it was not until 2019, that this phenomenon was directly witnessed for the first time. It was observed in the chlorophyll c1, a chlorophyll present in a certain group of marine algae, such as those of the Chromista kingdom.

The way we were able to demonstrate the existence of this vibronic coupling is by detecting a series of signals known as quantum beats. Our ultrafast laser pulses interacted with the Chl a, producing a set of electronic transitions and, through these, superpositions of states called "coherences". In our experiments, these coherences manifested as oscillations (quantum beats), which serve as a probe of how the energetic states of a molecule interact with each other.

In order to selectively study the coherences of special interest, a "polarization control" has been used. With it, a multitude of unwanted signals that could arise in the experiments were suppressed, leaving, among the remaining signals, those that have a special relationship with the vibronic mixing phenomenon.

The result of the experiments turned out to be positive, identifying a multitude of coherences whose origin must involve transitions between vibronically mixed states. Therefore, we can conclude that the coupling between the Qy and Qx states is present in Chl a, in accordance with the most recent studies mentioned above. Moreover, due to the previous findings, and the fact that Chl a is the most common among chlorophylls, it is expected to be present, not only in this chlorophyll, but in every other member of the chlorophylls family.

Vibronic mixing phenomenon is, so far, a scarcely explored phenomenon in chlorophylls, and there is still uncertainty as to which extent this "vibronic mixing" may be involved in the photosynthetic process. The scope of its importance is yet to be discovered. Nonetheless, these results open the way to an understanding of chlorophylls from a new perspective that, from now on, will consider the vibronic mixing as a fundamental property when studying the properties of chlorophylls in regard to its photosynthetic functions.

• author: Martínez Casasús, Ignacio
• supervisor: Donatas Zigmantas
• organization: Department of Chemistry
• course: KEMP30 20202
• year: 2021
• type: L3 - Miscellaneous, Projetcs etc.
• subject: Chemistry
• keywords: Two-dimensional spectroscopy, Ultrafast spectroscopy, Chlorophyll a, Chlorophylls, Photosynthesis, Light harvesting, Vibronic mixing, Quantum coherence, Chemical physics
• language: English
• id: 9040765
• date added to LUP: 2021-03-01 08:30:48
• date last changed: 2021-03-01 08:30:48