Visualizing the Chemistry of Vision

In an effort to see how we see, scientists in Japan have imaged the cis-to-trans isomerization of retinal at the atomic level by using high-resolution transmission electron microscopy (Nat. Nanotechnol., DOI: 10.1038/nnano.2007.187). To prepare retinal, a chromophore found in the human eye, for its molecular close-up, Kazu Suenaga and colleagues at the National Institute of Advanced Industrial Science & Technology covalently attached the molecule to a C₆₀ fullerene and placed the resulting complex into a single-walled carbon nanotube. The nanotube serves as a specimen cell, separating the individual molecules from one another and restricting their molecular motion within the narrow interior. The fullerene provides a marker for the researchers, helping them distinguish retinal molecules from carbon contaminants. Upon irradiating the nanotube with an electron beam, the team observed cis-retinal isomerize to trans-retinal—the same isomerization that plays a key role in vision. According to the researchers, this is the first time anyone has directly observed the dynamic behavior of a conjugated carbon chain. The molecular imaging technique, they say, can be applied to a wide range of systems.