Green tea is traditionally associated with good health and long life - benefits linked to chemicals known as catechins, which act as antioxidants. These polyphenolic flavonoid compounds disrupt the damaging chain reaction between free radicals and lipids.
But no one understood how catechins work at micromolar concentrations in the body. Now, Àngels González-Lafont and colleagues at the Autonomous University of Barcelona in Spain have modeled the chemical reaction that allows green tea catechins to zap antioxidants (See Abstract at the end of this synopsis).
The reaction involves the catechin losing a hydrogen atom to a reactive free radical. The scientists found that in this process the radical and catechin were bound together tightly, leading to very small energy changes as the reaction proceeds.
The compact structures and narrow energy profile revealed by González-Lafont's calculations allows for a huge tunneling effect in the hydrogen transfer step. Tunneling can help quantum particles overcome otherwise insurmountable energy barriers. It relies on that fact that particles can behave like waves. If this waveform extends to the other side of the energy barrier, there is a significant probability that the particle will pop up on the other side of the barrier, as if it had tunneled through a hillside.
Tunneling makes the hydrogen transfer much faster than the free radical's reaction with the body's vulnerable lipids so the radicals are trapped before they can do harm.
'Tunneling is a ubiquitous phenomenon in nature', González-Lafont told Chemistry World. 'Our finding could also be useful to understand the molecular basis for the antioxidant activity of other compounds,' she said. Joe Vinson, who studies antioxidants at the University of Scranton, Pennsylvania, US, welcomed the findings.
'It's really astounding what this [research] has done for the rest of us,' said Vinson. 'We had a problem understanding how polyphenols work at such low concentrations. This paper gives theoretical credence to a large amount of experimental evidence of polyphenols as in vitro and in vivo antioxidants.
Source: Chemistry World (Tom Westgate)
Tunneling in Green Tea: Understanding the Antioxidant Activity of Catechol-Containing Compounds. A Variational Transition-State Theory Study
Ismael Tejero, Núria Gonz lez-García, Àngels Gonz lez-Lafont,* and José M. Lluch
Departament de Química and Institut de Biotecnologia i de Biomedicina, Universitat Aut noma de Barcelona, 08193 Bellaterra (Barcelona), Spain
The catechol functionality present in the catechins is responsible for the protective effects exerted by green tea against a wide range of human diseases. High-level electronic structure calculations and canonical variational transition-state theory including multidimensional tunneling corrections have allowed us to understand the key factors of the antioxidant effectiveness of the catechol group. This catechol group forms two hydrogen bonds with the two oxygen atoms of the lipid peroxyl radical, leading to a very compact reactant complex. This fact produces an extremely narrow adiabatic potential-energy profile corresponding to the hydrogen abstraction by the peroxyl radical, which makes it possible for a huge tunneling contribution to take place. So, quantum-mechanical tunneling highly increases the corresponding rate constant value, in such a way that catechins become able to trap the lipid peroxyl radicals in a dominant competition with the very damaging free-radical chain-lipid peroxidation reaction.