Cell-Penetrating Metal Complex Optical Probes: Targeted and Responsive Systems Based on Lanthanide Luminescence

Abstract

To understand better the structure and function of biological systems, cell biologists and biochemists would like to have methods that minimally perturb living systems. The development of emissive optical probes is essential for improving our observation of intracellular signaling and recognition processes. Following excitation of the probe, photons emitted from the probe may be observed by spectroscopy or microscopy and encode information about their environments in their energy, lifetime, and polarization. Such optical probes may be based on organic fluorophores, quantum dots, recombinant proteins, or emissive metal complexes. In this Account, we trace the emergence of lanthanide coordination complexes as emissive optical probes. These probes benefit from sharp emission bands and long lifetimes. We can design these complexes to report on the concentration of key biochemical variables by modulation of spectral form, lifetime, or circular polarization. These properties allow us to apply ratiometric methods of analysis in spectroscopy or microscopy to report on local pH, pM (M = Ca, Zn), or the concentration of certain anionic metabolites, such as citrate, lactate, bicarbonate, or urate. For optical microscopy studies in living cells, these probes must be cell-permeable and, ideally, should localize in a given cell organelle. We undertook systematic studies of more than 60 emissive complexes, examining the time dependence of cellular uptake and compartmentalization, cellular toxicity, protein affinity, and quenching sensitivity. These results and their relationship to probe structure have allowed us to identify certain structure-activity relationships. The nature and linkage mode of the integral sensitizing group-introduced to harvest incident light efficiently-is of primary importance in determining protein affinity and cellular uptake and trafficking. In many cases, uptake may occur via macropinocytosis. We have defined three main classes of behavior: complexes exhibit predominant localization profiles in protein-rich regions (nucleoli/ribosomes), in cellular mitochondria, or in endosomes/lysosomes. Therefore, these systems offer considerable promise as intracellular optical probes, amenable to single- or two-photon excitation, that may report on the local ionic composition of living cells subjected to differing environmental stresses.

Microscopic Visualization of Metabotropic Glutamate Receptors on the Surface of Living Cells Using Bifunctional Magnetic Resonance Imaging Probes

“…The lanthanide luminescence microscopy studies had shown the well-defined punctate vesicular distribution profile, possibly associated with lysosomal localization, as seen previously with Ln(III) complexes of related structure and overall complex charge. 23 Therefore, it is reasonable to speculate that confinement within vesicles restricts the exchange of water molecules that can access the paramagnetic metal center. Such a "relaxation quenching" effect has already been reported by Aime and co- workers 30,31 and has been interpreted in terms of a three compartment relaxation model.…”

Recent Advances on Functionalized Upconversion Nanoparticles for Detection of Small Molecules and Ions in Biosystems

“…Among all sensing technologies, photon‐signal‐based biosensors such as organic dyes,1, 2, 3, 4 metal complexes,5, 6, 7, 8, 9, 10 and semiconductor nanocrystals11 are the most commonly used ones. However, these probes have several limitations in detection or sensing.…”

Fluorescent Labeling of Plasmid DNA and mRNA: Gains and Losses of Current Labeling Strategies

“…These polynucleotides are of interest for any 21 biological application in which the expression of proteins is beneficial. The main obstacle to 22 the wide-spread use of these pDNA and mRNA based therapies is the low efficiencies obtained 23 with the current transfection methods. Negatively charged nucleic acids are mostly complexed 24 with positively charged carriers to enhance cellular uptake of the complexes which are formed.…”

“…Also, the often cationic fluorophores lower the overall negative charge of the 20 polynucleotides which might interfere with the intracellular migration behavior of nucleic acids, 21 complexation with and dissociation from cationic carrier. 22 Incorporation of modified nucleotides 23 Apart from the labeling strategies mentioned above, another popular commercially available 24 option is the in vitro incorporation of modified nucleotides into the polynucleotide backbone. 25 The term "modified" stands for a nucleotide that differs from the naturally occurring 26 nucleotides through the addition of a functional moiety.…”