Some bacterial proteins involved in cell division and oxidative phosphorylation are tightly bound to cardiolipin. Cardiolipin is a non-bilayer anionic phospholipid found in bacterial inner membrane. It forms lipid microdomains located at the cell poles and division plane. Mechanisms by which microdomains are affected by membrane-acting antibiotics and the impact of these alterations on membrane properties and protein functions remain unclear. In this study, we demonstrated cardiolipin relocation and clustering as a result of exposure to a cardiolipin-acting amphiphilic aminoglycoside antibiotic, the 3′,6-dinonyl neamine. Changes in the biophysical properties of the bacterial membrane of P. aeruginosa, including decreased fluidity and increased permeability, were observed. Cardiolipin-interacting proteins and functions regulated by cardiolipin were impacted by the amphiphilic aminoglycoside as we demonstrated an inhibition of respiratory chain and changes in bacterial shape. The latter effect was characterized by the loss of bacterial rod shape through a decrease in length and increase in curvature. It resulted from the effect on MreB, a cardiolipin dependent cytoskeleton protein as well as a direct effect of 3′,6-dinonyl neamine on cardiolipin. These results shed light on how targeting cardiolipin microdomains may be of great interest for developing new antibacterial therapies.

Capsaicin is an ingredient of a wide variety of red peppers, and it has various pharmacological and biological applications. The present study explores the interaction of capsaicin with dimyristoylphosphatidylcholine (DMPC) lipid bilayer membrane by monitoring various photophysical parameters using its intrinsic fluorescence. In order to have a clearer understanding of the photophysical responses of capsaicin, studies involving (i) its solvation behavior in different solvents, (ii) the partition coefficient of capsaicin in different thermotropic phase states of lipid bilayer membrane, and (iii) its location inside lipid bilayer membrane have been carried out. Capsaicin has a reasonably high partition coefficient for DMPC liposome membrane, in both solid gel (2.8 ± 0.1 × 10(5)) and liquid crystalline (2.6 ± 0.1 × 10(5)) phases. Fluorescence quenching study using cetylpyridinium chloride (CPC) as quencher suggests that the phenolic group of capsaicin molecule is generally present near the headgroup region and hydrophobic tail present inside hydrophobic core region of the lipid bilayer membrane. The intrinsic fluorescence intensity and lifetime of capsaicin sensitively respond to the temperature dependent phase changes of liposome membrane. Above 15 mol %, capsaicin in the aqueous liposome suspension medium lowers the thermotropic phase transition temperature by about 3 °C, and above 30 mol %, the integrity of the membrane is significantly lost.

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Thermotropic microenvironmental changes and the level of hydration in different microenvironments of pluronic F127 (PF127), (PEO106 PPO70 PEO106, average molar mass 13 000) in aqueous media have been studied using 1-naphthol, which is an ESPT fluorescent molecular probe. The appearance of 1-naphthol neutral form fluorescence in aqueous PF127 (10% w/v) solution indicates the ability of 1-naphthol to sense hydrophobic domains in micellar aggregations. There is a marked enhancement of the neutral form fluorescence at and above the gelation temperature (20 °C), which shows that the probe can accurately sense the sol-gel transition. In the temperature range of 10-40 °C, with increase in temperature there is a progressive enhancement of the neutral form fluorescence and the blue shift of the neutral and anionic form fluorescence; a decrease in the deprotonation rate constant (kpt) indicates that the water-polymer interfacial region is progressively dehydrated. Because kpt is related to the availability of proton-accepting water in the microenvironment of 1-naphthol, the reduction of kpt indicates progressive dehydration. The thermotropic response of the I1/I3 vibronic band ratio of pyrene-1-butyric acid fluorescence shows a progressive increase in the non-polarity of the interfacial domain with increasing temperature. The increase in non-polarity and the decrease of the hydration level are strongly correlated.

The photophysical behaviour and excited state decay kinetics of the fluorescent probe Nile red were used for quantitative monitoring of micropolarity, microviscosity and the sol-gel transition temperature of a copolymer hydrogel, pluronic F127. There was considerable enhancement of the emission intensity with a large blue shift in emission and an absorption maximum at and above the sol-gel transition temperature (20 °C), showing the sensitivity of Nile red fluorescence to the sol-gel transition. The estimation of micropolarity by comparing the Nile red emission maximum in dioxane-water mixtures suggested a considerable decrease in the polarity of the PF127 microenvironment from less polar (20% dioxane-water) in its sol phase to almost non-polar (90% dioxane-water) microenvironments in the gel phase. The thermotropic response of the wavelength dependent fluorescence lifetime of the probe with a rise time in the longer wavelength region has enabled monitoring of the microheterogeneity of the gel medium. With an increase in temperature, the microviscosity progressively increases from ∼10 mPa s (sol state) to ∼23 mPa s (gel state). The mismatch between microviscosity as estimated by the Nile red and the corresponding bulk viscosity reflected the microheterogeneity of the pluronic medium and its sensitivity towards PF127 microenvironments.

This work focuses on the study of aqueous phase aggregation of the recently FDA approved oral drug molecule FTY720 (fingolimod hydrochloride) and its effect on dimyristoylphosphatidylcholine (DMPC) liposomes using different fluorescent molecular probes and fluorescence parameters. The variation of the steady state fluorescence intensity of 8-anilino-1-naphthalene sulfonic acid (ANS) with FTY720 in water shows an efficient micellar aggregation with the critical micellar concentration (CMC) at ~75 μM. The aggregation number calculation from steady state fluorescence quenching of pyrene shows the formation of small micellar aggregates in aqueous solution having an aggregation number of 42 ± 3 with the free energy of micellization ~-23 kJ mol(-1). Fluorescence intensity and lifetime decay analysis of the molecular probe 1-naphthol indicate that the interaction of FTY720 with the DMPC lipid bilayer membrane prevents partitioning of small molecules such as 1-naphthol to the membrane in both solid gel (SG) and liquid crystalline (LC) phases. Temperature dependent fluorescence intensity studies of 1-naphthol and fluorescence anisotropy measurements of 1,6-diphenyl-1,3,5-hexatriene (DPH) have shown that above the CMC of FTY720, the SG to LC main phase transition temperature (T(M)) of the lipid bilayer membrane decreases from 23 °C to 21 °C in the aqueous medium.

Amphiphilic aminoglycoside derivatives are promising new antibacterials active against Gram-negative bacteria such as Pseudomonas aeruginosa, including colistin resistant strains. In this study, we demonstrated that addition of cardiolipin to the culture medium delayed growth of P. aeruginosa, favored asymmetrical growth and enhanced the efficiency of a new amphiphilic aminoglycoside derivative, the 3’,6-dinonylneamine. By using membrane models mimicking P. aeruginosa plasma membrane composition (POPE:POPG:CL), we demonstrated the ability of 3’6-dinonylneamine to induce changes in the biophysical properties of membrane model lipid systems in a cardiolipin dependent manner. These changes include an increased membrane permeability associated with a reduced hydration and a decreased ability of membrane to mix and fuse as shown by monitoring calcein release, Generalized Polarization of Laurdan and fluorescence dequenching of octadecyl rhodamine B, respectively. Altogether, results shed light on how cardiolipin may be critical for improving antibacterial action of new amphiphilic aminoglycoside derivatives.

The thermoreversible sol-gel transition of pluronic F127 is markedly altered even with addition of submicellar concentration of sodium dodecyl sulfate (SDS) surfactant. Multiple fluorescence parameters like fluorescence intensity, fluorescence anisotropy and fluorescence lifetime of both the prototropic forms (anion (A*) and phototautomer FT*) of the photoprototropic fluorescent probe fisetin has been efficiently used to understand the molecular level properties like polarity and microviscosity of the PF127-SDS system as a function of temperature. The SDS-induced increase in the interfacial hydrophobicity level is seen to affect the sol-gel phase transition of PF127 (21-18 °C). The E(30) polarity parameter value of anionic emission of fisetin suggests that there is a considerable decrease in the polarity of the PF127 medium with increase in temperature and with the addition of SDS. The microviscosity progressively increases from ∼5 mPa s (sol state, 10 °C) to ∼22.01 mPa s (gel state 35 °C) in aqueous solution of PF127. The variation in microviscosity with addition of SDS in PF127-SDS mixed system is significant in sol phase whereas in gel phase this variation is significantly less. Temperature dependent fluorescence lifetime of FT* indicates that there is heterogeneity in distribution of fisetin molecules at different domains of PF127. This work also show-cases the sensitivity of fisetin toward change in polarity and change in sol-gel transition temperature of copolymer PF127 with variation in temperature (both forward and reverse directions) and SDS.