Nothing is self-evident in the world of quantum phenomena. Already a couple of years ago a surprising fact has been discovered in the Institute of Physical Chemistry of the Polish Academy of Sciences: two hydrogen atoms in the central part of porphycene molecule move easier and faster together than each of them individually. Only recently, however, the researchers succeeded in finding the answer why it happens. The outcome of the study is not only of scientific importance but can also be used in advanced medical therapies, including cancer treatments.
Derivatives of porphyrin, a substance present, i.a., in human blood, have been studied at the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) in Warsaw for about a dozen years. One of them is porphycene C20H14N4. Porphycene molecules have a form of a flat carbon ring with hydrogen atoms outside and four nitrogen atoms inside the ring. The empty space in the centre of the molecule contains two hydrogen atoms, permanently tunnelling between nitrogens. The measurements using experimental techniques developed at the IPC PAS allowed to spot these ultrafast processes. To the researchers’ surprise it turned out that both internal hydrogen atoms tunnel easier and faster when they do it concertedly. „Another series of experiments, completed recently, allowed us to develop a theoretical model explaining the essence of the surprising phenomenon. And by the way we identified porphycene derivatives with properties important for medical applications” – says Prof. Jacek Waluk from the Institute of Physical Chemistry of the PAS.
The tunnelling mechanism explains why quantum objects are able to overcome potential barriers without the need to acquire energy. „Imagine we are standing in front of a wall. To get through we need first to accumulate energy that is needed to climb the wall. A quantum particle is able, however, just to disappear on one side of the wall and to reappear on the other” – describes Dr. Piotr Fita from the Faculty of Physics of the Warsaw University.
„Already the question alone of whether whole hydrogen atoms or only protons do tunnel in porphycene, excited a lot of emotions among scientists” – says Prof. Waluk. A hydrogen atom consists of a proton and an electron around the proton. When hydrogen becomes a part of a molecule, the electron joins molecular electron cloud (thus playing a role of an adhesive keeping the nuclei together). A tunnelling of proton would indicate that electron density in the electron cloud after the process is completed would remain unchanged. If the entire hydrogen atom had tunnelled, then the electron would have followed the proton. „The real life is usually neither this nor that” – explains Prof. Waluk. „When the proton tunnels in the molecule, only a part of electron density in the cloud is following it”.
A porphycene molecule can oscillate in 108 vibrational modes. The scientists from the ICP PAS have shown already earlier that proton tunnelling can proceed faster or slower, depending on the vibrational state of the molecule. Recent experimental studies and theoretical models have shown that when one proton is transferred, the energy of the molecule changes and it must appropriately modify its structure, whereas when two protons tunnel simultaneously, the energy remains unchanged. The atoms in the molecule do not need to be reorganised then and the tunnelling becomes easier.
Initially, tunnelling was considered to be a rare phenomenon to occur only under sophisticated conditions. It becomes obvious today for an increasingly large number of chemists that tunnelling is a common process occurring in most – if not in all – chemical reactions. Also the biologists change their approach along similar lines. It is assumed that tunnelling plays a significant role in hydrogen transfer in enzymes. It seems that a synchronous, simultaneous tunnelling of several protons is not a rare phenomenon, helping to avoid reorganisation of a massive skeleton of the molecule. Similar mechanism may be important in formation of DNA mutations.
In recent experiments carried out at the Institute of Physical Chemistry of the PAS and the Faculty of Physics of the Warsaw University porphycene was studied in solutions. The tunnelling was monitored with laser techniques allowing for measurements with time resolution down to 50 femtosecond (1 fs = 10-15 s, i.e., one quadrillionth part of a second). It has been shown that the way the tunnelling hydrogen behaves has a significant effect on the lifetime of fluorescence, i.e. light emission by the molecule. At room temperature in highly viscous solutions, the fluorescence lifetime increased even by thousand times: from picoseconds (1 ps = 10-12 s) to nanoseconds (1 ns = 10-9 s).
The excited porphycene molecule could be used as a probe for viscosity measurements in micro- and nanoscale. When used appropriately, it would allow to observe selected parts of cells and to detect their certain pathologic conditions. Because of strong dependence of oxidative properties on solution viscosity, porphycene derivatives described by the researchers from the IPC PAS open also interesting prospects for selective killing of cancer cells with photodynamic therapies.