One Retinoid Makes Many Crayons
Image from Flickr.com, via a Creative Commons License by Pink_Sherbet_Photography.
Light is essential for many biological processes including vision, photosynthesis, vitamin D synthesis, and it is useful for treating many skin diseases. The role of the chemical absorbing a specific wavelength of light is always emphasized in elementary photobiology. Sakmar’s recent Science article and the accompanying commentary demonstrate that the chemical environment of a light-absorbing molecule can remarkably affect the wavelength that is absorbed. Not a little difference -- but a very large difference -- in peak absorption; the difference between the same chromophore appearing yellow, with an absorption of 420nm and appearing greenish-blue, with a wavelength of 644 nm. The absorption of the chromophore, all-trans-retinal, a vitamin A-related molecule, was modulated by its interactions with genetically engineered variants of the human cellular retinol binding protein II (CRPII). The approach and chemistry are elegant, and the take-home message is that the protein changed the electrostatic density along the polyene portion of the retinoid without affecting the configuration of the retinoid itself.
For those investigating molecules important for photobiologic processes, e.g., sunburn, this article reinforces the concept that it may be necessary to investigate associated molecules as the chromophore of interest. The pure isolated chromophore may not be yielding the information necessary to understand the process. In this interesting example, the classical scientific approach of isolating and characterizing the pure molecule may actually divert the investigator from the in vivo biological process being studied. This finding has profound implications for those studying and strategizing on the synthesis of new photoactive molecules. Nature may not yield its secrets to those looking for simple answers.