Depth-Dependent Heterogeneity in Membranes by Fluorescence Lifetime Distribution Analysis

Delving into Membrane Heterogeneity Utilizing Fluorescence Lifetime Distribution Analysis

Lipid bilayer membranes are indispensable parts of cellular architecture. One of the integral properties of bilayer membranes is the environmental heterogeneity over a wide range of spatiotemporal scales. The environmental heterogeneity is a manifestation of the dynamic and compositional anisotropy in the plane of the membrane as well as along the bilayer normal. Fluorescence lifetime distribution analysis provides a spectroscopic tool to quantitatively characterize such heterogeneities. The review discusses recent applications of fluorescence lifetime distribution analysis utilizing the maximum entropy method to characterize horizontal and vertical heterogeneities in membranes.

The role of fusion peptides in depth-dependent membrane organization and dynamics in promoting membrane fusion

Membrane fusion is an important event in the life of eukaryotes; occurs in several processes such as endocytosis, exocytosis, cellular trafficking, compartmentalization, import of nutrients and export of waste, vesiculation, inter cellular communication, and fertilization. The enveloped viruses as well utilize fusion between the viral envelope and host cell membrane for infection. The stretch of 20-25 amino acids located at the N-terminus of the fusion protein, known as fusion peptide, plays a decisive role in the fusion process. The stalk model of membrane fusion postulated a common route of bilayer transformation for stalk, transmembrane contact, and pore formation; and fusion peptide is believed to facilitate bilayer transformation to promote membrane fusion. The peptide-induced change in depth-dependent organization and dynamics could provide important information in understanding the role of fusion peptide in membrane fusion. In this review, we have discussed about three depth-dependent properties of the membrane such as rigidity, polarity and heterogeneity, and the impact of fusion peptide on these three membrane properties.

Prediction of paclitaxel pharmacokinetic based on in vitro studies: Interaction with membrane models and human serum albumin

Interactions of paclitaxel (PTX) with models mimicking biological interfaces (lipid membranes and serum albumin, HSA) were investigated to test the hypothesis that the set of in vitro assays proposed can be used to predict some aspects of drug pharmacokinetics (PK). PTX membrane partitioning was studied by derivative spectrophotometry; PTX effect on membrane biophysics was evaluated by dynamic light scattering, fluorescence anisotropy, atomic force microscopy and synchrotron small/wide-angle X-ray scattering; PTX distribution/molecular orientation in membranes was assessed by steady-state/time-resolved fluorescence and computer simulations. PTX binding to HSA was studied by fluorescence quenching, derivative spectrophotometry and dynamic/electrophoretic light scattering. PTX high membrane partitioning is consistent with its efficacy crossing cellular membranes and its off-target distribution. PTX is closely located in the membrane phospholipids headgroups, also interacting with the hydrophobic chains, and causes a major distortion of the alignment of the membrane phospholipids, which, together with its fluidizing effect, justifies some of its cellular toxic effects. PTX binds strongly to HSA, which is consistent with its reduced distribution in target tissues and toxicity by bioaccumulation. In conclusion, the described set of biomimetic models and techniques has the potential for early prediction of PK issues, alerting for the required drug optimizations, potentially minimizing the number of animal tests used in the drug development process.

Influence of Eugenol on the Organization and Dynamics of Lipid Membranes: A Phase-Dependent Study

Eugenol is known for its antimicrobial effects against microorganisms responsible for infectious diseases in humans, food-borne pathogens, and oral pathogens. In spite of several reports on the antimicrobial function of eugenol by modulating the structural properties of cell membranes, there is limited information on the influence of eugenol in the lipid membrane. In this work, we explored the effect of eugenol on the organization and dynamics of large unilamellar vesicles (LUVs) of DMPC using the intrinsic fluorescence of eugenol and an extrinsic hydrophobic probe, DPH, in varying phases. The organization and dynamics of the bilayers of DMPC vesicles were monitored by utilizing varieties of steady-state and time-resolved fluorescence measurements. Our results show that eugenol stabilizes the gel phase and elevates the phase-transition temperature of DMPC in a concentration-dependent fashion. Fluorescence lifetime measurements demonstrate that higher eugenol-induced water penetration was observed in fluid-phase membranes. Time-resolved anisotropy measurements demonstrate that eugenol reduces the semiangle of DPH wobbling-in-a-cone in gel-phase membranes, whereas the semiangle remains unaffected in fluid-phase membrane. This implies that the eugenol further orders the gel-phase membrane, and this could be a plausible reason for the eugenol-dependent elevation of the phase-transition temperature of DMPC. We envisage that these results will contribute important information to understand the interaction of eugenol with biological membranes.

Exploring the Mechanism of Viral Peptide-Induced Membrane Fusion

Membrane fusion is essential in several cellular processes in the existence of eukaryotic cells such as cellular trafficking, compartmentalization, intercellular communication, sexual reproduction, cell division, and endo- and exocytosis. Membrane fusion proceeds in model membranes as well as biological membranes through the rearrangement of lipids. The stalk hypothesis provides a picture of the general nature of lipid rearrangement based on mechanical properties and phase behavior of water-lipid mesomorphic systems. In spite of extensive research on exploring the mechanism of membrane fusion, a clear molecular understanding of intermediate and pore formation is lacking. In addition, the mechanism by which proteins and peptides reduce the activation energy for stalk and pore formation is not yet clear though there are several propositions on how they catalyze membrane fusion. In this review, we have discussed about various putative functions of fusion peptides by which they reduce activation barrier and thus promote membrane fusion. A careful analysis of the discussed effects of fusion peptides on membranes might open up new possibilities for better understanding of the membrane fusion mechanism.

Depth-Dependent Organization and Dynamics of Archaeal and Eukaryotic Membranes: Development of Membrane Anisotropy Gradient with Natural Evolution

The lipid composition of archaea is unique and has been correlated with increased stability under extreme environmental conditions. In this article, we have focused on the evolution of membrane organization and dynamics with natural evolution. Dynamic anisotropy along the membrane normal (i.e., gradients of mobility, polarity, and heterogeneity) is a hallmark of fluid phase diester or diether phospholipid membranes. We monitored gradients of mobility, polarity, and heterogeneity along the membrane normal in membranes made of a representative archaeal lipid using a series of membrane depth-dependent fluorescent probes, and compared them to membranes prepared from a typical diether lipid from higher organisms (eukaryotes). Our results show that the representative dynamic anisotropy gradient along the membrane normal is absent in membranes made from archaeal lipids. We hypothesize that the dynamic gradient observed in membranes of diester and diether phospholipids is a consequence of natural evolution of membrane lipids in response to the requirement of carrying out complex cellular functions by membrane proteins.

Interactions of a non-fluorescent fluoroquinolone with biological membrane models: A multi-technique approach

Fluoroquinolones are antibiotics which act by penetrating into bacterial cells and inhibiting enzymes related to DNA replication, and metal complexes of these drugs have recently been investigated as one approach to counteracting bacterial resistance. In this work, we apply a multi-technique approach to studying the partition coefficient (Kp) for the non-fluorescent third-generation fluoroquinolone sparfloxacin or its copper-complex with lipid membrane models of Gram-negative bacteria. The techniques investigated are UV-vis absorption and (19)F NMR spectroscopies together with quenching of a fluorescent probe present in the lipids (using steady-state and time-resolved methods). (19)F NMR spectroscopy has previously been used to determine the Kp values of fluorinated drugs but in the case of sparfloxacin did not yield useful data. However, similar Kp values for sparfloxacin or its copper-complex were obtained for the absorption and fluorescence quenching methods confirming the usefulness of a multi-technique approach. The Kp values measured for sparfloxacin were significantly higher than those found for other fluoroquinolones. In addition, similar Kp values were found for sparfloxacin and copper-complex suggesting that in contrast to other fluoroquinolones hydrophobic diffusion occurs readily for both of these molecules.

K(+) preference at the NaK channel entrance revealed by fluorescence lifetime and anisotropy analysis of site-specifically incorporated (7-hydroxycoumarin-4-yl)ethylglycine

The fluorescent unnatural amino acid, (7-hydroxycoumarin-4-yl)ethylglycine (HC), was site-specifically incorporated at the Phe69 site, close to the entrance of the selectivity filter of the NaK channel. Decreased fluorescence lifetime and elevated time-resolved anisotropy of NaK-F69HC in buffers with high K(+)/Na(+) molar ratios indicated the K(+) preference at the entrance of the NaK channel, consistent with previous crystal structure results of the NaK channel.

Synthesis and evaluation of a glucose attached pyrene, as a fluorescent molecular probe in sugar and non-sugar based micro-heterogeneous media

A new fluorescent pyrene–glucose conjugate (pyd-glc), 1-(4,6-O-butylidene-β-d-glucopyranosyl)-4-(1-pyrene)-butan-2-one, has been synthesized by attaching a pyrene molecule to acetal (butylidene) protected glucose via a butane-2-one linker.

Calibration of Distribution Analysis of the Depth of Membrane Penetration Using Simulations and Depth-Dependent Fluorescence Quenching

Determination of the depth of membrane penetration provides important information for studies of membrane protein folding and protein-lipid interactions. Here, we use a combination of molecular dynamics (MD) simulations and depth-dependent fluorescence quenching to calibrate the methodology for extracting quantitative information on membrane penetration. In order to investigate the immersion depth of the fluorescent label in lipid bilayer, we studied 7-nitrobenz-2-oxa-1,3-diazole (NBD) attached to the lipid headgroup in NBD-PE incorporated into POPC bilayer. The immersion depth of NBD was estimated by measuring steady-state and time-resolved fluorescence quenching with spin-labeled lipids co-incorporated into lipid vesicles. Six different spin-labeled lipids were utilized: one with headgroup-attached Tempo probe (Tempo-PC) and five with acyl chain-labeled n-Doxyl moieties (n-Doxyl-PC where n is a chain labeling position equal to 5, 7, 10, 12, and 14, respectively). The Stern-Volmer analysis revealed that NBD quenching in membranes occurs by both static and dynamic collisional quenching processes. Using the methodology of Distribution Analysis, the immersion depth and the apparent half-width of the transversal distributions of the NBD moiety were estimated to be 14.7 and 6.7 Å, respectively, from the bilayer center. This position is independently validated by atomistic MD simulations of NBD-PE lipids in a POPC bilayer (14.4 Å). In addition, we demonstrate that MD simulations of the transverse overlap integrals between dye and quencher distributions can be used for proper analysis of the depth-dependent quenching profile. Finally, we illustrate the application of this methodology by determining membrane penetration of site selectively labeled mutants of diphtheria toxin T-domain.

Importance of indole N-H hydrogen bonding in the organization and dynamics of gramicidin channels

The linear ion channel peptide gramicidin represents an excellent model for exploring the principles underlying membrane protein structure and function, especially with respect to tryptophan residues. The tryptophan residues in gramicidin channels are crucial for the structure and function of the channel. In order to test the importance of indole hydrogen bonding for the biophysical properties of gramicidin channels, we monitored the effect of N-methylation of gramicidin tryptophans, using a combination of steady state and time-resolved fluorescence approaches along with circular dichroism spectroscopy. We show here that in the absence of the hydrogen bonding ability of tryptophans, tetramethyltryptophan gramicidin (TM-gramicidin) is unable to maintain the single stranded, head-to-head dimeric channel conformation in membranes. Our results show that TM-gramicidin displays a red-shifted fluorescence emission maximum, lower red edge excitation shift (REES), and higher fluorescence intensity and lifetime, consistent with its nonchannel conformation. This is in agreement with the measured location (average depth) of the 1-methyltryptophans in TM-gramicidin using the parallax method. These results bring out the usefulness of 1-methyltryptophan as a fluorescent tool to examine the hydrogen bonding ability of tryptophans in proteins and peptides. We conclude that changes in the hydrogen bonding ability of tryptophans, along with coupled changes in peptide backbone structure induce the loss of single stranded β(6.3) helical dimer conformation. These results agree with earlier results from size-exclusion chromatography and single-channel measurements for TM-gramicidin, and confirm the importance of indole hydrogen bonding for the conformation and function of ion channels and membrane proteins.