NMR-based molecular ruler for determining the depth of intercalants within the lipid bilayer Part II. The preparation of a molecular ruler

Nanotechnology-based cell-mediated delivery systems for cancer therapy and diagnosis

The global prevalence of cancer is increasing, necessitating new additions to traditional treatments and diagnoses to address shortcomings such as ineffectiveness, complications, and high cost. In this context, nano and microparticulate carriers stand out due to their unique properties such as controlled release, higher bioavailability, and lower toxicity. Despite their popularity, they face several challenges including rapid liver uptake, low chemical stability in blood circulation, immunogenicity concerns, and acute adverse effects. Cell-mediated delivery systems are important topics to research because of their biocompatibility, biodegradability, prolonged delivery, high loading capacity, and targeted drug delivery capabilities. To date, a variety of cells including blood, immune, cancer, and stem cells, sperm, and bacteria have been combined with nanoparticles to develop efficient targeted cancer delivery or diagnosis systems. The review paper aimed to provide an overview of the potential applications of cell-based delivery systems in cancer therapy and diagnosis.

Polarity of Hydrated Phosphatidylcholine Headgroups

The headgroup (H) stratum (sometimes called the polar region) of membrane bilayers is a relevant yet poorly understood solvation phase for small molecules and macromolecules interacting with the membranes. Solvation of compounds in bilayer strata is characterized experimentally by wide- and small-angle X-ray scattering, neutron diffraction, and various NMR techniques. The quantification is tedious and only available for a limited set of small molecules. Our recently published model of liposome partitioning of small molecules shows that solvation of compounds in the H-stratum of fluid phosphatidylcholine (PC) bilayers correlates well with their solvation in hydrated diacetyl phosphatidylcholine (DAcPC), and solvation in the core (C) depends in a similar way on that in n-hexadecane. These two correlations became a basis for a model describing the location of compounds in the H- and C-strata and at the connecting interface as a nonlinear function of the fragment solvation characteristics of the compounds. In this study, refractivity of hydrated DAcPC phases with varying water contents was measured and polarity was determined using the steady-state fluorescence of indole and Nile Red. The results were compared with the published data obtained by other techniques for PC bilayers in liposomes or on solid supports. The demonstrated qualitative agreement, as well as the polarity and refractivity dependencies on the DAcPC concentration, supports the suitability of hydrated DAcPC as the H-stratum surrogate. Interestingly, depending on hydrations typical for the H-strata of fluid PC bilayers, the dielectric constant could decrease significantly from 31.0 to 7.3 for 16 and 8 water molecules per headgroup, respectively. Although additional experiments are needed for confirmation, this observation could help set proper dielectric constant magnitudes in continuum-based computational models of accumulation and crossing of the PC bilayers with varying hydration levels thanks to the temperature or the structure of fatty acid chains.

Locating intercalants within lipid bilayers using fluorescence quenching by bromophospholipids and iodophospholipids

In previous work, we have been able to determine the depth of intercalated molecules within the lipid bilayer using the solvent polarity sensitivity of three spectroscopic techniques: the ¹³C NMR chemical shift (δ); the fluorescence emission wavelength (λ_em), and the ESR β-H splitting constants (a_β-H). In the present paper, we use the quenching by a heavy atom (Br or I), situated at a known location along a phospholipid chain, as a probe of the location of a fluorescent moiety. We have synthesized various phospholipids with bromine (or iodine) atoms substituted at various locations along the lipid chain. The latter halolipids were intercalated in turn with various fluorophores into DMPC liposomes, biomembranes and erythrocyte ghosts. The most effective fluorescence quenching occurs when the heavy atom location corresponds to that of the fluorophore. The results show that generally speaking the fluorophore intercalates the same depth independent of which lipid bilayer is used. KBr (or KI) is the most effective quencher when the fluorophore resides in or at the aqueous phase. Presumably because of iodine's larger radius and spin coupling constant, the iodine analogs are far less discriminating in the depth range it quenches.

"Snorkelling" vs. "diving" in mixed micelles probed by means of a molecular bathymeter

A photoactive bathymeter based on a carboxylic acid moiety covalently linked to a signalling methoxynaphthalene (MNP) fluorophore has been designed to prove the concept of "snorkelling" vs. "diving" in mixed micelles (MM). The carboxylic acid "floats" on the MM surface, while the MNP unit sinks deep in MM. The rate constants of MNP fluorescence quenching by iodide, which remains basically in water, consistently decrease with increasing spacer length, revealing different regions. This is associated with the distance MNP should "dive" in MM to achieve protection from aqueous reactants. Unequivocal proof of the exergonic photoinduced electron transfer was obtained from the UV-visible spectral signature of I₃^- upon steady-state photolysis. The applicability of the bathymeter was examined upon testing a family of MNP derivatives. The obtained results were validated by comparison with different lipophilicity tests: (i) a modified version of the K_ow partition coefficient and (ii) the retention factor on thin layer chromatography. This concept could potentially be extended to test drugs or pharmacophores exhibiting any photoactive moiety.

Vitamin E Circular Dichroism Studies: Insights into Conformational Changes Induced by the Solvent's Polarity

We used circular dichroism (CD) to study differences in CD spectra between α-, δ-, and methylated-α-tocopherol in solvents with different polarities. CD spectra of the different tocopherol structures differ from each other in intensity and peak locations, which can be attributed to chromanol substitution and the ability to form hydrogen bonds. In addition, each structure was examined in different polarity solvents using the Reichardt index-a measure of the solvent's ionizing ability, and a direct measurement of solvent-solute interactions. Differences across solvents indicate that hydrogen bonding is a key contributor to CD spectra at 200 nm. These results are a first step in examining the hydrogen bonding abilities of vitamin E in a lipid bilayer.

Advances of blood cell-based drug delivery systems

Blood cells, including erythrocytes, leukocytes and platelets are used as drug carriers in a wide range of applications. They have many unique advantages such as long life-span in circulation (especially erythrocytes), target release capacities (especially platelets), and natural adhesive properties (leukocytes and platelets). These properties make blood cell based delivery systems, as well as their membrane-derived carriers, far superior to other drug delivery systems. Despite the advantages, the further development of blood cell-based delivery systems was hindered by limitations in the source, storage, and mass production. To overcome these problems, synthetic biomaterials that mimic blood cell and nanocrystallization of blood cells have been developed and may represent the future direction for blood cell membrane-based delivery systems. In this paper, we review recent progress of the rising blood cell-based drug delivery systems, and also discuss their challenges and future tendency of development.

Rationalization of reduced penetration of drugs through ceramide gel phase membrane

Since computing resources have advanced enough to allow routine molecular simulation studies of drug molecules interacting with biologically relevant membranes, a considerable amount of work has been carried out with fluid phospholipid systems. However, there is very little work in the literature on drug interactions with gel phase lipids. This poses a significant limitation for understanding permeation through the stratum corneum where the primary pathway is expected to be through a highly ordered lipid matrix. To address this point, we analyzed the interactions of p-aminobenzoic acid (PABA) and its ethyl (benzocaine) and butyl (butamben) esters with two membrane bilayers, which differ in their fluidity at ambient conditions. We considered a dioleoylphosphatidylcholine (DOPC) bilayer in a fluid state and a ceramide 2 (CER2, ceramide NS) bilayer in a gel phase. We carried out unbiased (100 ns long) and biased z-constraint molecular dynamics simulations and calculated the free energy profiles of all molecules along the bilayer normal. The free energy profiles converged significantly slower for the gel phase. While the compounds have comparable affinities for both membranes, they exhibit penetration barriers almost 3 times higher in the gel phase CER2 bilayer. This elevated barrier and slower diffusion in the CER2 bilayer, which are caused by the high ordering of CER2 lipid chains, explain the low permeability of the gel phase membranes. We also compared the free energy profiles from MD simulations with those obtained from COSMOmic. This method provided the same trends in behavior for the guest molecules in both bilayers; however, the penetration barriers calculated by COSMOmic did not differ between membranes. In conclusion, we show how membrane fluid properties affect the interaction of drug-like molecules with membranes.

NMR-based molecular ruler for determining the depth of intercalants within the lipid bilayer. Part IV: studies on ketophospholipids

In our companion paper, we described the preparation and intercalation of two homologous series of dicarbonyl compounds, methyl n-oxooctadecanoates and the corresponding n-oxooctadecanoic acids (n=4-16), into DMPC liposomes. (13)C NMR chemical shift of the various carbonyls was analyzed using an E(T)(30) solvent polarity-chemical shift correlation table and the corresponding calculated penetration depth (in Å). An iterative best fit analysis of the data points revealed an exponential correlation between E(T)(30) micropolarity and the penetration depth (in Å) into the liposomal bilayer. However, this study is still incomplete, since the plot lacks data points in the important area of moderately polarity, i.e., in the E(T)(30) range of 51-45.5 kcal/mol. To correct this lacuna, a family of ketophospholipids was prepared in which the above n-oxooctadecanoic acids were attached to the sn-2 position of a phosphatidylcholine with a palmitic acid chain at sn-1. To assist in assignment and detection several derivatives were prepared (13)C-enriched in both carbonyls. The various homologs were intercalated into DMPC liposomes and give points specifically in the missing area of the previous polarity-penetration correlation graph. Interestingly, the calculated exponential relationship of the complete graph was essentially the same as that calculated in the companion paper based on the methyl n-oxooctadecanoates and the corresponding n-oxooctadecanoic acids alone. The polarity at the midplane of such DMPC systems is ca. 33 kcal/mol and is not expected to change very much if we extend the lipid chains. This paper concludes with a chemical ruler that maps the changing polarity experienced by an intercalant as it penetrates the liposomal bilayer.

Structural determinants of drug partitioning in surrogates of phosphatidylcholine bilayer strata

The knowledge of drug concentrations in bilayer headgroups, core, and at the interface between them is a prerequisite for quantitative modeling of drug interactions with many membrane-bound transporters, metabolizing enzymes and receptors, which have the binding sites located in the bilayer. This knowledge also helps understand the rates of trans-bilayer transport because balanced interactions of drugs with the bilayer strata lead to high rates, while excessive affinities for any stratum cause a slowdown. Experimental determination of bilayer location is so tedious and costly that the data are only available for some fifty compounds. To extrapolate these valuable results to more compounds at a higher throughput, surrogate phases have been used to obtain correlates of the drug affinities for individual strata. We introduced a novel system, consisting of a diacetyl phosphatidylcholine (DAcPC) solution with the water content of the fluid bilayer as the headgroup surrogate and n-hexadecane (C16) representing the core. The C16/DAcPC partition coefficients were measured for 113 selected compounds, containing structural fragments that are frequently occurring in approved drugs. The data were deconvoluted into the ClogP-based fragment solvation characteristics and processed using a solvatochromic correlation. Increased H-bond donor ability and excess molar refractivity of compounds promote solvation in the DAcPC phase as compared to bulk water, contrary to H-bond acceptor ability, dipolarity/polarizability, and volume. The results show that aromates have more balanced distribution in bilayer strata, and thus faster trans-bilayer transport, than similar alkanes. This observation is in accordance with the frequent occurrence of aromatic rings in approved drugs and with the role of rigidity of drug molecules in promoting intestinal absorption. Bilayer locations, predicted using the C16/DAcPC system, are in excellent agreement with available experimental data, in contrast to other surrogate systems.

Utilizing NMR and EPR spectroscopy to probe the role of copper in prion diseases

Copper is an essential nutrient for the normal development of the brain and nervous system, although the hallmark of several neurological diseases is a change in copper concentrations in the brain and central nervous system. Prion protein (PrP) is a copper-binding, cell-surface glycoprotein that exists in two alternatively folded conformations: a normal isoform (PrP(C)) and a disease-associated isoform (PrP(Sc)). Prion diseases are a group of lethal neurodegenerative disorders that develop as a result of conformational conversion of PrP(C) into PrP(Sc). The pathogenic mechanism that triggers this conformational transformation with the subsequent development of prion diseases remains unclear. It has, however, been shown repeatedly that copper plays a significant functional role in the conformational conversion of prion proteins. In this review, we focus on current research that seeks to clarify the conformational changes associated with prion diseases and the role of copper in this mechanism, with emphasis on the latest applications of NMR and EPR spectroscopy to probe the interactions of copper with prion proteins.

Convergence of Free Energy Profile of Coumarin in Lipid Bilayer

Atomistic molecular dynamics (MD) simulations of druglike molecules embedded in lipid bilayers are of considerable interest as models for drug penetration and positioning in biological membranes. Here we analyze partitioning of coumarin in dioleoylphosphatidylcholine (DOPC) bilayer, based on both multiple, unbiased 3 μs MD simulations (total length) and free energy profiles along the bilayer normal calculated by biased MD simulations (∼7 μs in total). The convergences in time of free energy profiles calculated by both umbrella sampling and z-constraint techniques are thoroughly analyzed. Two sets of starting structures are also considered, one from unbiased MD simulation and the other from "pulling" coumarin along the bilayer normal. The structures obtained by pulling simulation contain water defects on the lipid bilayer surface, while those acquired from unbiased simulation have no membrane defects. The free energy profiles converge more rapidly when starting frames from unbiased simulations are used. In addition, z-constraint simulation leads to more rapid convergence than umbrella sampling, due to quicker relaxation of membrane defects. Furthermore, we show that the choice of RESP, PRODRG, or Mulliken charges considerably affects the resulting free energy profile of our model drug along the bilayer normal. We recommend using z-constraint biased MD simulations based on starting geometries acquired from unbiased MD simulations for efficient calculation of convergent free energy profiles of druglike molecules along bilayer normals. The calculation of free energy profile should start with an unbiased simulation, though the polar molecules might need a slow pulling afterward. Results obtained with the recommended simulation protocol agree well with available experimental data for two coumarin derivatives.

Morphological transition of the host-structure influences solvent-relaxation: a wavelength-selective fluorescence exploration through environment-sensitive intramolecular charge transfer photophysics

Here, we report the modulation of photo-induced intramolecular charge transfer (ICT) photophysics of N,N-dimethylaminonaphthyl-acrylo-nitrile (DMANAN) associated with sphere-to-rod structural transition of SDS micelles induced by increasing ionic strength of the medium. Emphasis is rendered on the exploration of solvent-relaxation associated with this transition on the basis of wavelength-selective fluorescence technique which includes monitoring of red-edge excitation shift (REES) and excitation/emission anisotropy profiles. Based on micropolarity determination and organization of solvent water around the probe microenvironment we argue that the present results advocate for rod-shaped micelles to be a better mimic for membrane bilayers than spherical micelles.

Using fluorescence to locate intercalants within the lipid bilayer of liposomes, bioliposomes and erythrocyte ghosts

In previous work, we have shown the utility of the "NMR technique" in locating intercalants within the lipid bilayer. We describe herein the development of a more sensitive and complementary "fluorescence technique" for this purpose and its application to liposomes, bioliposomes and erythrocyte ghosts. This technique is based on the observation in selected compounds of an excellent correlation between the emission wavelength (λ(em)) and Dimroth-Reichardt E(T)(30) polarity parameter for the solvent in which the fluorescence emission spectrum was obtained.

Anisotropic solvent model of the lipid bilayer. 1. Parameterization of long-range electrostatics and first solvation shell effects

A new implicit solvation model was developed for calculating free energies of transfer of molecules from water to any solvent with defined bulk properties. The transfer energy was calculated as a sum of the first solvation shell energy and the long-range electrostatic contribution. The first term was proportional to solvent accessible surface area and solvation parameters (σ(i)) for different atom types. The electrostatic term was computed as a product of group dipole moments and dipolar solvation parameter (η) for neutral molecules or using a modified Born equation for ions. The regression coefficients in linear dependencies of solvation parameters σ(i) and η on dielectric constant, solvatochromic polarizability parameter π*, and hydrogen-bonding donor and acceptor capacities of solvents were optimized using 1269 experimental transfer energies from 19 organic solvents to water. The root-mean-square errors for neutral compounds and ions were 0.82 and 1.61 kcal/mol, respectively. Quantification of energy components demonstrates the dominant roles of hydrophobic effect for nonpolar atoms and of hydrogen-bonding for polar atoms. The estimated first solvation shell energy outweighs the long-range electrostatics for most compounds including ions. The simplicity and computational efficiency of the model allows its application for modeling of macromolecules in anisotropic environments, such as biological membranes.

A reinvestigation of the reaction of coumarins with superoxide in the liposomal bilayer: correlation between depth and reactivity

Afri et al. reported in this journal (Free Radic. Biol. Med.32:605-618; 2002) that a direct relationship exists between the depth of alkanoylcoumarins 1 within the liposomal lipid bilayer and the rate at which they undergo superoxide-mediated saponification. These results were based on a correlation between the (13)C NMR chemical shift of polarizable carbonyl carbons and the E(T)(30) polarity they sense. Subsequent studies challenged these results, however, demonstrating that, in conjugated ketones and aldehydes, charge separation influences the E(T)(30) polarity measured. To elucidate whether this is true for conjugated esters such as coumarins as well, the nonconjugated analogs 3,4-dihydrocoumarins 11 and 15 were intercalated within DMPC liposomal bilayers and their relative locations within the liposomal bilayer were determined. The length of the alkyl chain substituted at C-4 and C-10 influences the depth of the substrates within the liposome. The location of these 3,4-dihydrocoumarins corresponds well with the conjugated analog coumarin 1-confirming the validity of the abovementioned results of Afri et al. The lack of substantial charge separation in the coumarin 1 system presumably results from the "swamping-out" effect of the ester oxygen. Instead of 1,3-delocalization of charge, typical of conjugated systems, delocalization of the nonbonding pair on the ester oxygen predominates.

Solid-state NMR paramagnetic relaxation enhancement immersion depth studies in phospholipid bilayers

A new approach for determining the membrane immersion depth of a spin-labeled probe has been developed using paramagnetic relaxation enhancement (PRE) in solid-state NMR spectroscopy. A DOXYL spin label was placed at different sites of 1-palmitoyl-2-stearoyl-sn-glycero-3-phosphocholine (PSPC) phospholipid bilayers as paramagnetic moieties and the resulting enhancements of the longitudinal relaxation (T₁) times of ³¹P nuclei on the surface of the bilayers were measured by a standard inversion recovery pulse sequence. The ³¹P NMR spin-lattice relaxation times decrease steadily as the DOXYL spin label moves closer to the surface as well as the concentration of the spin-labeled lipids increase. The enhanced relaxation vs. the position and concentration of spin-labels indicate that PRE induced by the DOXYL spin label are significant to determine longer distances over the whole range of the membrane depths. When these data were combined with estimated correlation times τ(c), the r⁻⁶-weighted, time-averaged distances between the spin-labels and the ³¹P nuclei on the membrane surface were estimated. The application of using this solid-state NMR PRE approach coupled with site-directed spin labeling (SDSL) may be a powerful method for measuring membrane protein immersion depth.