We have developed a new unnatural amino acid based on the solvatochromic fluorophore 4-N,Ndimethylamino-1,8-naphthalimide (4-DMN) for application in the study of protein-protein interactions. The fluorescence quantum yield of this chromophore is highly sensitive to changes in the local solvent environment, demonstrating "switch-like" emission properties characteristic of the dimethylaminophthalimide family of fluorophores. In particular, this new species possesses a number of significant advantages over related fluorophores including greater chemical stability under a wide range of conditions, a longer wavelength of excitation (408 nm), and improved synthetic accessibility. This amino acid has been prepared as an Fmoc-protected building block and may readily be incorporated into peptides via standard solid-phase peptide synthesis (SPPS). A series of comparative studies are presented to demonstrate the advantageous properties of the 4-DMN amino acid relative to that of the previously reported 4-N,N-dimethylaminophthalimidoalanine (4-DAPA) and 6-N,Ndimethylamino-2,3-naphthalimidoalanine (6-DMNA) amino acids. Other commercially available solvatochromic fluorophores are also include in these studies. The potential of this new probe as a tool for the study of protein-protein interactions is demonstrated by introducing it into a peptide that is recognized by calcium-activated calmodulin. The binding interaction between these two components yields an increase in fluorescence emission greater than 900-fold.

Solvatochromic fluorophores possess emission properties that are sensitive to the nature of the local microenvironment. These dyes have been exploited in applications ranging from the study of protein structural dynamics to the detection of protein-binding interactions. While the solvatochromic indole fluorophore of tryptophan has been utilized extensively for in vitro studies to advance our understanding of basic protein biochemistry, the emergence of new extrinsic synthetic dyes with improved properties in conjunction with recent developments in site-selective methods to incorporate these chemical tools into proteins now open the way for studies in more complex systems. Herein we discuss recent technological advancements and their application in the design of powerful reporters, which serve critical roles in modern cell biology and assay development.

Here we report a simple Mn coordination complex with utility as a redox-sensitive MR probe. The HBET ligand stabilizes both the Mn2+ and Mn3+ oxidation states. In the presence of glutathione (GSH), low relaxivity MnIII-HBET is converted to high relaxivity MnII-HBET with a 3-fold increase in relaxivity, and concomitant increase in MR signal. Alternately, hydrogen peroxide can convert MnII-HBET to MnIII-HBET with a reduction in MR signal.