Captivated: in the cage, in the cell

Max Planck scientists invent and apply novel class of masked fluorescent dyes

12. April 2010

In life sciences, the term "caged" is often used for compounds whose function is temporally hampered by the chemical transformation of the initially active substances. However, the original properties can be restored by "uncaging". UV or visible light is often used for the uncaging, as it may be applied non-invasively, i.e. without cellular damage, with high spatial and temporal precision.

Figure 1. Structure and uncaging reaction of Rhodamine NN. (a) The nonfluorescent caged compound TMR-NN (left) is uncaged upon irradiation with UV light (≤ 420 nm), liberating a brightly fluorescent (uncaged) TMR derivative (right). (b) TMR-NN penetrates cellular membranes and can be used for imaging structures not only in fixed but also in living cells. Upon irradiation with low intensity UV light, the fluorescence signal increases by a factor of more than 40.

Caged fluorescent dyes are of great interest for biological imaging because they may be used for protein tracking, multi color applications, optical nanoscopy and beyond. They are kept in a non-fluorescent state by incorporating a labile chemical group. This photosensitive masking group or "molecular cage" may be destroyed to re-establish the fluorescent state. ο-Nitrobenzyl residues are frequently utilized as the masking groups. However, the complex synthesis and unwanted by-products liberated during the photolysis limit the use of these and other bulky caging groups.

To overcome these drawbacks, Vladimir Belov and colleagues from the Department of NanoBiophotonics at the Max Planck Institute for Biophysical Chemistry in Göttingen invented and studied a novel class of caged compounds called Rhodamines NN, with an incorporated 2-diazoketone (COCNN) caging group (Figure 1a).[1,2]

This very simple and small chemical group is the core element of a new class of masked fluorescent dyes with remarkable properties. The rhodamines NN may be easily prepared and conjugated with biomolecules. Upon irradiation with UV light (at wavelengths ≤ 420 nm), they undergo rapid uncaging to form highly fluorescent derivatives and liberate only non toxic nitrogen. Moreover, they may be photoactivated in aqueous buffers as well as in various embedding media normally used in imaging applications.

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