The innards of the cell: studies of water dipolar relaxation using the ACDAN fluorescent probe

This article reviews the use of the 6-acetyl-2-(dimethylamino)naphthalene (ACDAN) fluorophore to study dipolar relaxation in cells, tissues, and biomimetic systems. As the most hydrophilic member of the 6-acyl-2-(dimethylamino)naphthalene series, ACDAN markedly partitions to aqueous environments. In contrast to 6-lauroyl-2-(dimethylamino)naphthalene (LAURDAN), the hydrophobic and best-known member of the series used to explore relaxation phenomena in biological (or biomimetic) membranes, ACDAN allows mapping of spatial and temporal water dipolar relaxation in cytosolic and intra-organelle environments of the cell. This is also true for the 6-propionyl-2-(dimethylamino)naphthalene (PRODAN) derivative which, unlike LAURDAN, partitions to both hydrophobic and aqueous environments. We will (i) summarize the mechanism which underlies the solvatochromic properties of the DAN probes, (ii) expound on the importance of water relaxation to understand the intracellular environment, (iii) discuss technical aspects of the use of ACDAN in eukaryotic cells and some specialized structures, including liquid condensates arising from processes leading to liquid immiscibility and, (iv) present some novel studies in plant cells and tissues which demonstrate the kinds of information that can be uncovered using this approach to study dipolar relaxation in living systems.