6,7-dimethoxy-coumarin as a probe of hydration dynamics in biologically relevant systems

Dissecting the mechanisms of environment sensitivity of smart probes for quantitative assessment of membrane properties

The plasma membrane, as a highly complex cell organelle, serves as a crucial platform for a multitude of cellular processes. Its collective biophysical properties are largely determined by the structural diversity of the different lipid species it accommodates. Therefore, a detailed investigation of biophysical properties of the plasma membrane is of utmost importance for a comprehensive understanding of biological processes occurring therein. During the past two decades, several environment-sensitive probes have been developed and become popular tools to investigate membrane properties. Although these probes are assumed to report on membrane order in similar ways, their individual mechanisms remain to be elucidated. In this study, using model membrane systems, we characterized the probes Pro12A, NR12S and NR12A in depth and examined their sensitivity to parameters with potential biological implications, such as the degree of lipid saturation, double bond position and configuration (cis versus trans), phospholipid headgroup and cholesterol content. Applying spectral imaging together with atomistic molecular dynamics simulations and time-dependent fluorescent shift analyses, we unravelled individual sensitivities of these probes to different biophysical properties, their distinct localizations and specific relaxation processes in membranes. Overall, Pro12A, NR12S and NR12A serve together as a toolbox with a wide range of applications allowing to select the most appropriate probe for each specific research question.

Polarity-based fluorescence probes: properties and applications

Local polarity can affect the physical or chemical behaviors of surrounding molecules, especially in organisms. Cell polarity is the ultimate feedback of cellular status and regulation mechanisms. Hence, the abnormal alteration of polarity in organisms is closely linked with functional disorders and many diseases. It is incredibly significant to monitor and detect local polarity to explain the biological processes and diagnoses of some diseases. Because of their in vivo safe and real-time monitoring, several polarity-sensitive fluorophores and fluorescent probes have gradually emerged and been used in modern research. This review summarizes the fluorescence properties and applications of several representative polarity-sensitive fluorescent probes.

Cysteine 159 delineates a hinge region of the alternating access monocarboxylate transporter 1 and is targeted by cysteine-modifying inhibitors

Monocarboxylate transporter isoforms 1-4, MCT, of the solute carrier SLC16A family facilitate proton-coupled transport of l-lactate. Growth of tumors that exhibit the Warburg effect, that is, high rates of anaerobic glycolysis despite availability of oxygen, relies on swift l-lactate export, whereas oxygenic cancer cells import circulating l-lactate as a fuel. Currently, MCTs are viewed as promising anticancer targets. Small-molecule inhibitors have been found, and, recently, high-resolution protein structures have been obtained. Key questions, however, regarding the exact binding sites of cysteine-modifying inhibitors and the substrate translocation cycle lack a conclusive experimental basis. Here, we report Cys159 of the ubiquitous human MCT1 to reside in a critical hinge region of the alternating access-type transporter. We identified Cys159 as the binding site of the organomercurial pCMBS. The inhibitory effect of pCMBS was proposed to be indirect via modification of the chaperone basigin. We provide evidence that pCMBS locks MCT1 in its outward open conformation in a wedge-like fashion. We corroborated this finding using smaller cysteine-modifying reagents that size-dependently inhibited l-lactate transport. The smallest modifiers targeted additional cysteines as shown by a C159S mutant. We found a Cys399/Cys400 pair to constitute the second hinge of the transporter that tolerated only individual replacement by serine. The hinge cysteines, in particular the selectively addressable Cys159, provide natural anchors for placing probes into MCTs to report, for instance, on the electrostatics or hydration upon binding of the transported l-lactate substrate and the proton cosubstrate.

Computational and spectroscopic studies of biologically active coumarin-based fluorophores

We used density functional theory (DFT) calculations to examine the various molecular properties of two coumarin derivatives, namely 4-(5-amino-[1,3,4]thiadiazol-2-ylsulfanylmethyl)-7-methyl-chromen-2-one and 4-(5-amino-[1,3,4]thiadiazol-2-ylsulfanylmethyl)-7-methoxy-chromen-2-one at different levels of theory and basis sets. The calculated highest occupied molecular orbital and lowest unoccupied molecular orbital energies revealed that the investigated molecules were chemically active with a tendency for molecular interactions. The theoretical vibrational frequencies of these molecules were found to be consistent with the experimentally obtained frequencies. Moreover, solvatochromic measurements indicated no significant change in absorption spectral peak by varying the polarity of solvent. Under the same conditions we found that there was a red shift of 39 nm in the fluorescence spectral peak with increase in solvent polarity. The solvatochromic data were used to estimate excited dipole moments and the change in dipole moment was interpreted based on resonance structure of molecules.