An atomic force microscope to observe molecular charge transitions

The atomic force microscope allowed researchers to image the structure of four different molecules, depending on their state of charge. Credits: IBM

Molecular charge transition and fundamental biological processes

Atomic force microscopy images of each type of molecule, for four different states of electrical charge. Credits: Shadi Fatayer et al. 2019

Bibliography:



Molecular structure elucidation with charge-state control

Shadi Fatayer1,*, Florian Albrecht1, Yunlong Zhang2, Darius Urbonas1, Diego Peña3, Nikolaj Moll1, Leo Gross1,*

Science 12 Jul 2019:

Vol. 365, Issue 6449, pp. 142-145

DOI: 10.1126/science.aax5895

Just like an atom, a molecule can lose or gain electrons , thus altering its overall electrical charge. These phenomena of modification of the electric charge play a crucial role in the transfer of molecular energy governing certain catalytic and biochemical processes. For the first time, chemists have been able to observe in real time the structural modification of molecules due to electric charge transitions. Results that should help to better understand various essential biological processes.Using some of the most advanced microscopy technologies in the world, chemists have captured images of molecules that change their electrical charge in real time. To do this, they added and removed electrons , directly observing the changes in the structure of four molecules. The results were published in the journal Science.Molecular changes in electrical charge have been known for a long time, but this is the first direct observation of the phenomenon. This could help us better understand several molecular processes, including chemical reactions, catalysis and charge transport, and even biological processes." We were able to solve the structural changes of individual molecules with unprecedented resolution, " says chemist Leo Gross of IBM Research-Zurich. " This new understanding unveils some of the mysteries of molecular charge-function relationships in how biology converts and transports energy ."The team used atomic force microscopy. The laser tip scans the surface of the structures to be studied, detecting all structural changes, even the weakest ones. These are recorded to create an image of what the probe is scanning. In this way, scientists can get an image of the elements too small to be seen by optical means.Thus, four types of molecules - azobenzene, pentacene, tetracyanoquinodimethane (TCNQ) and porphine - were examined under a microscope in a cold vacuum chamber to ensure that no external influence would alter the results. A single molecule was placed on a sodium chloride film, and then a small voltage was sent through the probe to transfer electrons to the molecule, one at a time.Gross and his colleagues had already developed this load control technique and described it in a study in 2015. They also described their imaging technique in 2009. In this new work, however, the team found a way to combine the two techniques to image the molecules and control the charge at the same time.They imaged the four molecules in at least two of these four states: positive (minus one electron), neutral (the same number of protons and electrons), negative (plus one electron) and double negative (plus two electrons). The four molecules reacted differently to changes in charge.This video shows how the porphine molecule transforms as it loses electrons under these controlled conditions:The azobenzene molecule has become physically twisted. With pentacene, the areas of the molecule became more reactive because of the extra electrons. The change in charge resulted in a change in the type of bond between the TCNQ atoms, which moved physically on the film. And in the porphine, it was not only the type of links, but also their length that changed.These results will help to better understand the molecular energy transfer. Specifically, examining porphine molecules so closely may help us better understand some fundamental biological processes, because porphine is the parent compound of porphyrins, a group of organic compounds that make up both chlorophyll and hemoglobin." The charge transitions of these molecules are essential to life. Thanks to our new technique, we can better understand how the charge modifies the structure and function of molecules, which play an essential role in many ways, such as photoconversion and the transport of energy in living organisms "concludes Gross.