Understanding the Dynamic Behavior of an Anticancer Drug, Doxorubicin, on a Lipid Membrane Using Multiple Spectroscopic Techniques

Probing the structural evolution on the surface of cardiolipin vesicles with an amphiphilic second harmonic generation and fluorescence probe

Investigating the influence of the ambient chemical environment on molecular behaviors in liposomes is crucial for understanding and manipulating cellular vitality as well as the capabilities of lipid drug carriers in various environments. Here, we designed and synthesized a second harmonic generation (SHG) and fluorescence probe molecule called Pyr-Py+-N+ (PPN), which possesses membrane-targeting capability. We employed PPN to investigate the response of lipid vesicles composed of cardiolipin to the presence of exogenous salt. The kinetic behaviors, including the adsorption and embedding of PPN on the surface of small unilamellar vesicles (SUVs) composed of cardiolipin, were analyzed. The response of the SUVs to the addition of NaCl was also monitored. A rapid decrease in vesicle size can be evidenced through the rapid drop in SHG emission originating from PPN located on the vesicle surface.

Monitoring the response of a model protocell to dye and surfactant molecules through second harmonic generation and fluorescence imaging

Probing the interaction between molecules and protocells is crucial for understanding the passive transport of functional molecules in and out of artificial and real cells. Second-harmonic generation (SHG) has been proven to be a powerful method for analyzing the adsorption and cross-membrane transport of molecules on lipid bilayers. In this study, we used SHG and two-photon fluorescence (TPF) imaging to study the interaction of charged dye molecules (D289) with a lipid vesicle. Unexpectedly, it was observed that the transport of D289 at a relatively high concentration is not as efficient as that at a lower dye concentration. Periodic shrinking of the model protocell and discharging of D289 out from the vesicle were revealed by combined analyses of SHG and TPF images. The response of the vesicle to a surfactant was also analyzed with D289 as a probe. This work demonstrates that the combined SHG and TPF imaging method is a unique approach that can provide detailed information on the interaction of molecules and lipids (both morphology and molecular kinetics). Determining these subtle interfacial kinetics in molecules is important for understanding the mechanism of many biophysical processes occurring on lipids.

Differences in Lipid Order and Dynamics in Plasma Membranes Assessed by Nonlinear Optical Microscopy

When amphiphilic polar dyes were added to the cells, they intercalated predominantly in the outer leaf of the plasma membrane, making them active for second harmonic generation (SHG). The fluorescence of the dye enabled simultaneous 3D imaging of SHG and two-photon excited fluorescence (TPF). Because SHG intensity is sensitive to the alignment of the dyes, which reflects lipid ordering in the plasma membrane, we assessed the difference in lipid ordering by comparing the SHG intensity normalized to the TPF intensity. Together with an enzyme release assay that detects pore formation in the plasma membrane, our SHG assay revealed how polycations affect lipid ordering at low concentrations, where membrane damage has not yet been examined. By scaling the results of the assays with the charge concentration of the two polycations, polyethylenimine (PEI) and poly-l-lysine (PLL), we found that PEI reduced the lipid order more than PLL, and PLL formed more pores than PEI. A comparison of the SHG and TPF images of the wounded cells revealed that one of the lipid dynamics (flip-flop) was significantly enhanced in the bleb membrane. Moreover, the SHG assay indicated that the biocompatible polymer, poly(N-(2-hydroxypropyl)methacrylamide), did not affect the lipid order. Thus, our technique allows the assessment of the plasma membrane structure at the molecular level.

Comprehensive NMR Investigation of Imidazolium-Based Ionic Liquids [BMIM][OSU] and [BMIM][Cl] Impact on Binding and Dynamics of the Anticancer Drug Doxorubicin Hydrochloride

For the design of an efficient drug delivery system utilizing an ionic liquid (IL) as a carrier, it is prudent to gain molecular/atomistic level insights of a drug with IL in terms of binding and dynamics. In this scenario, the influence of anionic counterpart of imidazolium-based ILs, namely, 1-butyl-3-methyl-imidazolium octyl sulfate [BMIM][OSU] and 1-butyl-3-methyl-imidazolium chloride [BMIM][Cl] in their submicellar region ([IL] = 20 mM) on the model water-soluble anticancer drug doxorubicin hydrochloride (DOX) was probed by employing an arsenal of nuclear magnetic resonance (NMR) approaches. The salient feature of the present study includes the significant interaction of DOX with [BMIM][OSU], whereas the lack of such an interaction with [BMIM][Cl] is gauged by 1H NMR translation self-diffusometry and is further corroborated by 13C chemical shift perturbation. The two-step model was utilized to estimate the bound fraction (pb) and equivalent partition coefficient (K) of DOX with [BMIM][OSU]. A combination of selective and nonselective spin-lattice relaxation rates (R1SEL and R1NS, respectively) enables to gauze the significant interaction of DOX with [BMIM][OSU] over [BMIM][Cl]. Furthermore, 1D transient and truncated driven nuclear Overhauser enhancement (NOE) data analyses in the initial rate limit permits the evaluation of the cross-relaxation efficacy of DOX with the investigated ILs. An Arrhenius-type temperature dependence of the drug's self-diffusion was observed for DOX, DOX-[BMIM][OSU], and DOX-[BMIM][Cl] aqueous mixtures and the corresponding activation energies were evaluated.

The Transport of Charged Molecules across Three Lipid Membranes Investigated with Second Harmonic Generation

Subtle variations in the structure and composition of lipid membranes can have a profound impact on their transport of functional molecules and relevant cell functions. Here, we present a comparison of the permeability of bilayers composed of three lipids: cardiolipin, DOPG (1,2-dioleoyl-sn-glycero-3-phospho-(1'-rac-glycerol), and POPG (1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol)). The adsorption and cross-membrane transport of a charged molecule, D289 (4-(4-diethylaminostyry)-1-methyl-pyridinium iodide), on vesicles composed of the three lipids were monitored by second harmonic generation (SHG) scattering from the vesicle surface. It is revealed that structural mismatching between the saturated and unsaturated alkane chains in POPG leads to relatively loose packing structure in the lipid bilayers, thus providing better permeability compared to unsaturated lipid bilayers (DOPG). This mismatching also weakens the efficiency of cholesterol in rigidifying the lipid bilayers. It is also revealed that the bilayer structure is somewhat disturbed by the surface curvature in small unilamellar vesicles (SUVs) composed of POPG and the conical structured cardiolipin. Such subtle information on the relationship between the lipid structure and the molecular transport capability of the bilayers may provide clues for drug development and other medical and biological studies.

Facilitating flip-flop: Structural tuning of molecule-membrane interactions in living bacteria

The first barrier that a small molecule must overcome before trespassing into a living cell is the lipid bilayer surrounding the intracellular content. It is imperative, therefore, to understand how the structure of a small molecule influences its fate in this region. Through the use of second harmonic generation, we show how the differing degrees of ionic headgroups, conjugated system, and branched hydrocarbon tail disparities of a series of four styryl dye molecules influence the propensity to "flip-flop" or to be further organized in the outer leaflet by the membrane. We show here that initial adsorption experiments match previous studies on model systems; however, more complex dynamics are observed over time. Aside from probe molecule structure, these dynamics also vary between cell species and can deviate from trends reported based on model membranes. Specifically, we show here that the membrane composition is an important factor to consider for headgroup-mediated small-molecule dynamics. Overall, the findings presented here on how structural variability of small molecules impacts their initial adsorption and eventual destinations within membranes in the context of living cells could have practical applications in antibiotic and drug adjuvant design.

Combined Second Harmonic Generation and Fluorescence Analyses of the Structures and Dynamics of Molecules on Lipids Using Dual-Probes: A Review

Revealing the structures and dynamic behaviors of molecules on lipids is crucial for understanding the mechanism behind the biophysical processes, such as the preparation and application of drug delivery vesicles. Second harmonic generation (SHG) has been developed as a powerful tool to investigate the molecules on various lipid membranes, benefiting from its natural property of interface selectivity, which comes from the principle of even order nonlinear optics. Fluorescence emission, which is in principle not interface selective but varies with the chemical environment where the chromophores locate, can reveal the dynamics of molecules on lipids. In this contribution, we review some examples, which are mainly from our recent works focusing on the application of combined spectroscopic methods, i.e., SHG and two-photon fluorescence (TPF), in studying the dynamic behaviors of several dyes or drugs on lipids and surfactants. This review demonstrates that molecules with both SHG and TPF efficiencies may be used as intrinsic dual-probes in plotting a clear physical picture of their own behaviors, as well as the dynamics of other molecules, on lipid membranes.

Indole Facilitates Antimicrobial Uptake in Bacteria

Indole signaling in bacteria plays an important role in antibiotic resistance, persistence, and tolerance. Here, we used the nonlinear optical technique, second-harmonic light scattering (SHS), to examine the influence of exogenous indole on the bacterial uptake of the antimicrobial quaternary ammonium cation (qac), malachite green. The transport rates of the antimicrobial qac across the individual membranes of Escherichia coli and Pseudomonas aeruginosa, as well as liposomes composed of the polar lipid extract of E. coli, were directly measured using time-resolved SHS. Whereas exogenous indole was shown to induce a 2-fold increase in the transport rate of the qac across the cytoplasmic membranes of the wild-type bacteria, it had no influence on a knockout strain of E. coli lacking the tryptophan-specific transport protein (Δmtr). Likewise, indole did not affect the transport rate of the qac diffusing across the liposome membrane. Our findings suggest that indole increases the bacterial uptake of antimicrobials through an interaction with the Mtr permease.

Understanding the different cross-membrane transport kinetics of two charged molecules on the DOPG lipid surface with second harmonic generation and MD simulation

A clear physical picture of the dynamic behavior of molecules on the surface of the lipid membrane is highly desired and has attracted great attention from researchers. In this study, a step forward in this direction based on previous studies was presented with second harmonic generation (SHG) and molecular dynamic (MD) simulation. Specifically, details on the orientation flipping and cross-membrane transport of two charged molecules, 4-(4-diethylaminostyry)-1-methyl-pyridinium iodide (D289) and malachite green (MG), on the surface of 2-dioleoyl-sn-glycero-3-phospho-rac-(1-glycerol) sodium salt (DOPG) lipids were presented. Firstly, the orientation flipping of the two molecules on the surface of lipids before their cross-membrane transport was confirmed by the MD simulation. Then, the concentration dependent rate of the cross membrane transport for MG/D289 was analyzed. It was found that a simplified model could satisfactorily interpret the faster cross-membrane transport of MG under higher bulk concentrations. A different concentration dependent dynamics was observed with D289 and the reason behind it was also discussed. With this investigation, the surface structures and dynamics of D289 and MG on the DOPG lipid surface were clearly presented.

Revealing the mechanisms of vesicle formation with multiple spectral methods

The investigation of the self-assembly of amphiphilic molecules and the formation of micelles/vesicles has attracted significant attention. However, in situ and real-time methods for such studies are rare. Here, a surface-sensitive second harmonic generation (SHG) technique was applied to study the formation of vesicles in solutions of an anti-cancer drug, doxorubicin (DOX), and a generally used surfactant (sodium bis (2-ethylhexyl) sulfosuccinate, AOT). With the aid of two-photon fluorescence (TPF), Rayleigh scattering and TEM, we revealed the structural evolution of the aggregated micelles/vesicles. It was found that AOT and DOX molecules rapidly aggregated and formed micelles in the solution. The residual DOX then acted as a "glue" that induced the aggregating/growing of the micelles and the transformation from aggregates to vesicles. The existence of lipid films, which was considered as the necessary intermediate state for vesicle formation, was excluded via the SHG observations, indicating that hollow shells may be directly transformed from solid aggregated micelles in the self-assembly formation of complex vesicles. The combined spectroscopic methods were also used to investigate the formation of vesicles from a commonly used lipid (i.e., 1,2-dioleoyl-sn-glycero-3-phospho-rac-(1-glycerol) sodium salt, DOPG) from its stacked bilayers. The swelling, curving and sealing of the DOPG bilayers for vesicle formation was monitored and clear dynamics were revealed. This work shows that the vesicle formation mechanism varies with the initial state of the surfactant/lipid molecules. It not only demonstrates the capability of the combined spectroscopic methods in investigating the aggregated systems but also provides new insight for understanding the formation of vesicles.

Evaluating the cross-membrane dynamics of a charged molecule on lipid films with different surface curvature

Does the curvature of a phospholipid membrane influence the permeability of the lipid bilayers? This is a question of great importance yet hard to answer. In this work the permeability of a positively charged rod like probing molecule (D289 dye) on the bilayers of DOPG lipid vesicles was investigated using angle resolved second harmonic generation method. It was revealed that the permeability of D289 on the surface of small vesicles with ∼ 100 nm diameter was notably lower than that on giant vesicles with ∼ 1000 nm diameter. With the increasing of temperature or the introducing of dimethyl sulfoxide (DMSO) in the solutions, the D289 permeability of the lipid bilayers was notably enhanced as expected, on both the small and the giant vesicles. Still, the D289 permeability of the lipid film with more curvature is lower than the relatively flat film in all these cases. This work demonstrated a general protocol for the investigating of surface permeability of lipid films with various curvature.

Unveiling the Molecular Dynamics in a Living Cell to the Subcellular Organelle Level Using Second-Harmonic Generation Spectroscopy and Microscopy

Second-harmonic generation (SHG) microscopy has been proved to be a powerful method for investigating the structures of biomaterials. SHG spectra were also generally used to probe the adsorption and cross-membrane transport of molecules on lipid bilayers in situ and in real time. In this work, we applied SHG and two-photon fluorescence (TPF) spectra to investigate the dynamics of an amphiphilic ion with an SHG and TPF chromophore, D289 (4-(4-diethylaminostyry)-1-methyl-pyridinium iodide), on the surface of human chronic myelogenous leukemia (K562) cells and the subcellular structures inside the cells. The adsorption and cross-membrane transport of D289 into the cells and then into the organelles such as mitochondria were revealed. SHG images were also recorded and used to demonstrate their capability of probing molecular dynamics in organelles in K562 cells. This work demonstrated the first SHG investigation of the cross-membrane transport dynamics on the surface of subcellular organelles. It may also shed light on the differentiation of different types of subcellular structures in cells.

Photocatalytic redox on the surface of colloidal silver nanoparticles revealed by second harmonic generation and two-photon luminescence

The redox of silver on the surface of Ag nanoparticles (AgNPs) has received extensive attention because of its significant impact on the biological, physical and chemical properties of AgNPs and their applications. Here we demonstrate that the surface redox reaction of AgNPs in colloids may be investigated by the second harmonic generation (SHG) and two-photon luminescence (TPL) emission from the AgNPs. It was revealed that the oxidation of silver on the surface of AgNPs was accelerated upon femtosecond laser excitation, accompanied by a decrease in the SHG and TPL emissions from the AgNPs. The photon-induced reduction of oxidized silver on AgNPs and the formation of surface defects were also revealed by the changes in the SHG and TPL emissions. Size and morphology changes have not been detected by dynamic light scattering and TEM measurements. The changes in the UV-vis extinction spectra were also very weak compared with previous reports. However, the occurrence of redox reactions on the Ag surface upon femtosecond laser irradiation has been confirmed by multiple control experiments. This work demonstrates that SHG and TPL can sensitively probe the subtle structural change on the surface of AgNPs.

Determination of bacterial surface charge density via saturation of adsorbed ions

Bacterial surface charge is a critical characteristic of the cell's interfacial physiology that influences how the cell interacts with the local environment. A direct, sensitive, and accurate experimental technique capable of quantifying bacterial surface charge is needed to better understand molecular adaptations in interfacial physiology in response to environmental changes. We introduce here the method of second-harmonic light scattering (SHS), which is capable of detecting the number of molecular ions adsorbed as counter charges on the exterior bacterial surface, thereby providing a measure of the surface charge. In this first demonstration, we detect the small molecular cation, malachite green, electrostatically adsorbed on the surface of representative strains of Gram-positive and Gram-negative bacteria. Surprisingly, the SHS-deduced molecular transport rates through the different cellular ultrastructures are revealed to be nearly identical. However, the adsorption saturation densities on the exterior surfaces of the two bacteria were shown to be characteristically distinct. The negative charge density of the lipopolysaccharide coated outer surface of Gram-negative Escherichia coli (6.6 ± 1.3 nm^-2) was deduced to be seven times larger than that of the protein surface layer of Gram-positive Lactobacillus rhamnosus (1.0 ± 0.2 nm^-2). The feasibility of SHS-deduced bacterial surface charge density for Gram-type differentiation is presented.

Drastically modulating the structure, fluorescence, and functionality of doxorubicin in lipid membrane by interfacial density control

In this work, we report on the observation of a drastic modulation of the fluorescence emission of an anticancer drug, doxorubicin, at the lipid interface during the variation of its molecular density at the interface. The emission efficiency of doxorubicin in the lipid membrane was modulated in the range of less than 10% to above 300% that in the aqueous solution. The corresponding changes in the structure and functionality of doxorubicin on the lipid surface were analyzed with the aid of second harmonic generation and theoretical calculation. It was observed that doxorubicin molecules aggregated on the lipid membrane at a relatively high interfacial density. However, this aggregation may not cause interfacial domain large enough to alter the permeability of the lipid bilayer. At an even higher doxorubicin density, the domain of the aggregated doxorubicin molecules induced a cross-membrane transportation.

Surface curvature-dependent adsorption and aggregation of fluorescein isothiocyanate on gold nanoparticles

The interaction between metallic nanoparticles and fluorescent molecules and its influence on the optical properties of the particles/molecules have been intensively investigated because of their biology and sensing applications. Here, we studied the adsorption and aggregation of a commonly used dye, fluorescein isothiocyanate (FITC), on gold nanoparticles of various diameters. It was observed that the adsorption of FITC on relatively large gold nanoparticles (≥15 nm in diameter) induced quenching in the two-photon fluorescence (TPF) emission from the FITC molecules, while smaller-sized gold nanoparticles (1.6 nm) had no such effect. This difference was interpreted by the fluorescence resonance energy transfer (FRET) between the FITC molecules and the larger gold nanoparticles. At the same time, it was observed that the ratio of TPF quenching was notably higher than the ratio of the FITC molecules chemically adsorbed on the large gold particles. This unexpected observation revealed that the aggregation-induced fluorescence quenching also contributed significantly to the attenuation of the TPF emission. Time-dependent TPF attenuation during the interaction of FITC and the larger gold nanoparticles was recorded and used to confirm this interpretation. With this experimental evidence, a clear picture of the interaction of the FITC molecules on the gold surface was presented: FITC molecules chemically adsorbed on the small gold nanoparticles. However, the relatively larger surface curvature hindered the aggregation of the FITC molecules on the small gold nanoparticles. On the surface of the larger gold nanoparticles, both adsorption and aggregation occured. The influence of the surface curvature on the interfacial structure of the adsorbed molecules on nanoparticles was discussed.

Molecule-Membrane Interactions in Biological Cells Studied with Second Harmonic Light Scattering

The nonlinear optical phenomenon second harmonic light scattering (SHS) can be used for detecting molecules at the membrane surfaces of living biological cells. Over the last decade, SHS has been developed for quantitatively monitoring the adsorption and transport of small and medium size molecules (both neutral and ionic) across membranes in living cells. SHS can be operated with both time and spatial resolution and is even capable of isolating molecule-membrane interactions at specific membrane surfaces in multi-membrane cells, such as bacteria. In this review, we discuss select examples from our lab employing time-resolved SHS to study real-time molecular interactions at the plasma membranes of biological cells. We first demonstrate the utility of this method for determining the transport rates at each membrane/interface in a Gram-negative bacterial cell. Next, we show how SHS can be used to characterize the molecular mechanism of the century old Gram stain protocol for classifying bacteria. Additionally, we examine how membrane structures and molecular charge and polarity affect adsorption and transport, as well as how antimicrobial compounds alter bacteria membrane permeability. Finally, we discuss adaptation of SHS as an imaging modality to quantify molecular adsorption and transport in sub-cellular regions of individual living cells.

Particle adsorption at the oil-water interface studied with second harmonic generation

In this work, energetics of the adsorption of polystyrene nanoparticles at the hexadecane-water interface was studied with second harmonic generation. The adsorption of positively and negatively charged nanoparticles at the oil-water interface induced a decrease and an increase in the SHG emission from the interface, respectively. This change in the SHG emission, which is similar to that upon the adsorption of ionic surfactants at the hexadecane-water interface, which we reported previously, was then used as an indicator of particle adsorption at the interface. The adsorption free energies of the particles with a diameter of 20 nm at the hexadecane-water interface were found to be -14.7 ± 0.5 kcal mol^-1, -14.4 ± 0.4 kcal mol^-1 and -15.1 ± 0.3 kcal mol^-1 for the amidine, carboxyl and sulfate latex beads, respectively. This result implied that the van der Waals interaction between the oil phase and the polystyrene particles is capable of driving negatively charged particles to the negatively charged hexadecane-water interface. The principle of like dissolves like played a major role in the adsorption of polystyrene particles from the aqueous phase to the oil-water interface. The origin of the SHG emission from the oil-water interface was also discussed.