Membrane-buffer partition coefficients of tetracaine for liquid-crystal and solid-gel membranes estimated by direct ultraviolet spectrophotometry

Inclusion of fluorophores in cyclodextrins: a closer look at the fluorometric determination of association constants by linear and nonlinear fitting procedures

The author discusses methods for the fluorometric determination of affinity constants by linear and nonlinear fitting methods. This is outlined in particular for the interaction between cyclodextrins and several anesthetic drugs including benzocaine. Special emphasis is given to the limitations of certain fits, and the impact of such studies on enzyme-substrate interactions are demonstrated. Both the experimental part and methods of analysis are well suited for students in an advanced lab.

Study of procaine and tetracaine in the lipid bilayer using molecular dynamics simulation

Despite available experimental results, the molecular mechanism of action of local anesthetics upon the nervous system and contribution of the cell membrane to the process are still controversial. In this work, molecular dynamics simulations were performed to investigate the effect of two clinically used local anesthetics, procaine and tetracaine, on the structure and dynamics of a fully hydrated dimyristoylphosphatidylcholine lipid bilayer. We focused on comparing the main effects of uncharged and charged drugs on various properties of the lipid membrane: mass density distribution, diffusion coefficient, order parameter, radial distribution function, hydrogen bonding, electrostatic potential, headgroup angle, and water dipole orientation. To compare the diffusive nature of anesthetic through the lipid membrane quantitatively, we investigated the hexadecane/water partition coefficient using expanded ensemble simulation. We predicted the permeability coefficient of anesthetics in the following order: uncharged tetracaine > uncharged procaine > charged tetracaine > charged procaine. We also shown that the charged forms of drugs are more potent in hydrogen bonding, disturbing the lipid headgroups, changing the orientation of water dipoles, and increasing the headgroup electrostatic potential more than uncharged drugs, while the uncharged drugs make the lipid diffusion faster and increase the tail order parameter. The results of these simulation studies suggest that the different forms of anesthetics induce different structural modifications in the lipid bilayer, which provides new insights into their molecular mechanism.

Biophysical properties of ergosterol-enriched lipid rafts in yeast and tools for their study: characterization of ergosterol/phosphatidylcholine membranes with three fluorescent membrane probes

In this work, binary mixtures of phospholipid/ergosterol (erg) were studied using three fluorescent membrane probes. The phospholipid was either saturated (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC) or monounsaturated (1-palmitoyl-2-dioleoyl-sn-glycero-3-phosphocholine, POPC) phosphatidylcholine, to evaluate the fluorescence properties of the probes in gel, liquid ordered (l(o)) and liquid disordered (l(d)) phases. The probes have been used previously to study cholesterol-enriched domains, but their photophysical properties in erg-enriched membranes have not been characterized. N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (NBD-DPPE) presents modest blue-shifts upon erg addition, and the changes in the fluorescence lifetime are mainly due to differences in the efficiency of its fluorescence dynamic self-quenching. However, the steady-state fluorescence anisotropy of NBD-DPPE presents well-defined values in each lipid phase. N-(lissamine rhodamine B sulfonyl)-1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (Rhod-DOPE) presents a close to random distribution in erg-rich membranes. There are no appreciable spectral shifts and the steady-state fluorescence anisotropy presents complex behavior, as a result of different photophysical processes. The probe is mostly useful to label l(d) domains in yeast membranes. 4-(2-(6-(Dibutylamino)-2-naphthalenyl)ethenyl)-1-(3-sulfopropyl)-pyridinium (di-4-ANEPPS) is an electrochromic dye with excitation spectra largely insensitive to the presence of erg, but presenting a strong blue-shift of its emission with increasing concentrations of this sterol. Its partition coefficient is favorable to l(o) domains in POPC/erg mixtures. Although the fluorescence properties of di-4-ANEPPS are less sensitive to erg than to chol, in both cases the fluorescence lifetime responds monotonically to sterol mole fraction, becoming significantly longer in the presence of sterol as compared to pure POPC or DPPC bilayers. The probe displays a unique sensitivity to sterol-lipid interaction due to the influence of hydration and H-bonding patterns at the membrane/water interface on its fluorescence properties. This makes di-4-ANEPPS (and possibly similar probes) potentially useful in the study of erg-enriched domains in more complex lipid mixtures and in the membranes of living yeast cells.

The effect of pharmaceuticals on the nanoscale structure of PEO–PPO–PEO micelles

We present results on the effects of various hydrophobic drugs and additives on the micellar structure of Pluronic F127 solutions. Small-angle neutron scattering experiments on 5wt% F127 solutions were used to measure micelle core size (R(1)), micelle corona size (R(2)), intermicellar interaction distance (R(int)), polydispersity (sigma), and aggregation number (N(agg)); dynamic light scattering was used to measure critical micelle concentration (CMC); and ultraviolet spectroscopy was used to measure drug solubility and apparent micelle-water partition coefficient (K(mw)). The core and corona size were found to generally increase in the presence of the drugs, as did R(int). Both sigma and N(agg) were found to decrease in the presence of most of the drugs, and the CMC was found to vary considerably with no clear correlation. A design of experiments (DOE) approach was used to analyze the results and build empirical correlations. All of the parameters from the SANS experiments were found to depend strongly on drug solubility, with a weak dependence on K(mw) in most cases. The aggregation number, however, was found to depend strongly on both K(mw) and solubility. The correlations can be used to roughly predict the structural parameters of F127 micelles for other hydrophobic drugs.

Quantifying molecular partition into model systems of biomembranes: an emphasis on optical spectroscopic methods

Optical spectroscopies have been intensively used to determine partition coefficients by a plethora of methodologies. The present review is intended to give detailed and useful information for the determination of partition coefficients and addresses several relevant aspects, namely: (i) definition and calculation of the partition coefficient between aqueous and lipidic phases; (ii) partition coefficients vs. "binding" formalisms; (iii) advantages of spectroscopic methodologies over separation techniques; (iv) formalisms for various experimental approaches based on UV-Vis absorption or fluorescence parameters (fluorescence intensity, lifetime, anisotropy and quenching); (v) experimental hints, artifacts and model limitations; and (vi) a brief survey of nonoptical techniques.

Spectroscopic evidence for a preferential location of lidocaine inside phospholipid bilayers

We examined the effect of uncharged lidocaine on the structure and dynamics of egg phosphatidylcholine (EPC) membranes at pH 10.5 in order to assess the location of this local anesthetic in the bilayer. Changes in the organization of small unilamellar vesicles were monitored either by electron paramagnetic resonance (EPR)-in the spectra of doxyl derivatives of stearic acid methyl esters labeled at different positions in the acyl chain (5-, 7-, 12- and 16-MeSL)-or by fluorescence, with pyrene fatty-acid (4-, 6-, 10- and 16-Py) probes. The largest effects were observed with labels located at the upper positions of the fatty-acid acyl-chain. Dynamic information was obtained by 1H-NMR. Lidocaine protons presented shorter longitudinal relaxation times (T(1)) values due to their binding, and consequent immobilization to the membrane. In the presence of lidocaine the mobility of all glycerol protons of EPC decreased, while the choline protons revealed a higher degree of mobility, indicating a reduced participation in lipid-lipid interactions. Two-dimensional Nuclear Overhauser Effect experiments detected contacts between aromatic lidocaine protons and the phospholipid-choline methyl group. Fourier-transform infrared spectroscopy spectra revealed that lidocaine changes the access of water to the glycerol region of the bilayer. A "transient site" model for lidocaine preferential location in EPC bilayers is proposed. The model is based on the consideration that insertion of the bulky aromatic ring of the anesthetic into the glycerol backbone region causes a decrease in the mobility of that EPC region (T(1) data) and an increased mobility of the acyl chains (EPR and fluorescence data).

Localization of flunitrazepam in artificial membranes. A spectrophotometric study about the effect the polarity of the medium exerts on flunitrazepam acid-base equilibrium

In the present paper we tried to test the hypothesis that nonspecific flunitrazepam-membrane interactions are consistent with drug molecules accommodated between lipid molecules, becoming an integral part of the bilayer. We developed a spectrophotometric method to determine FNTZH+ equilibrium dissociation constant and applied it to the study of the acid-base equilibria of this drug in homogeneous media of different polarity. In these conditions, pK decreased with the decrement in the dielectric constant (D) of the media. These results, analyzed under the light of the theory developed by Fernandez and Fromherz (1977; J. Phys. Chem. 81, 1755-1761) let us infer that flunitrazepam is localized a region with D = 60. This D value is lower that Dwater = 78 and higher than D of hydrocarbon chains zone (D = 2-5) and would correspond to D of the region of polar groups. This result is compatible with the hypothesis.

Use of a novel method for determination of partition coefficients to compare the effect of local anesthetics on membrane structure

A new, simple procedure for the determination of partition coefficients (P) was developed based on spectral effects caused upon addition of solutes to spin labeled model lipid membranes, and on the knowledge of their water solubility. Values of P were determined for nine local anesthetics (LA), amino-esters and amino-amides. The results were in good agreement with those found by phase separation and by a more complex, previously reported, methodology (Lissi et al. (1990) Biochim. Biophys. Acta 1021, 46-50) applied to either EPR or fluorescence spectra of probes incorporated in the bilayers. Both the present and the previously reported procedures make use of effects on membrane structure evaluated by spectroscopic techniques and offer the advantage of not requiring phase separation. The spectral effects, indicative of a decrease in bilayer organization increased with LA concentration, reaching a maximum at the drug water solubility, indicating that partitioning in the membrane is limited by saturation of the aqueous phase. A thermodynamic analysis of the partition data according to Hill (Hill, M.W. (1974) Biochim. Biophys. Acta 356, 117-124) showed that the LAs did not display ideal behavior. Knowledge of the partition coefficients allowed a comparison between effects at the same drug concentration in the membrane. Within a given family (esters, acyclic amides, cyclic amides) no clear proportionality was observed between effect and LA hydrophobicity, as reflected in the partition coefficient. Rather, the membrane perturbing ability is a result of steric effects originating in the mismatch between anesthetic and phospholipid shapes.

Differential influence of anionic and cationic charge on the ability of amphiphilic drugs to interact with DPPC-liposomes

The compounds clofibric acid and chlorphentermine have identical aromatic ring systems, but when charged their side chains are anionic or cationic, respectively. The drugs were applied as tools to investigate whether the interaction of amphiphilic drugs with the zwitterionic dipalmitoyl-phosphatidylcholine (DPPC) depends on the charge of the polar side chain. In suspensions of DPPC-liposomes, the drug-effect on the phase-transition temperature (Tt) was evaluated by means of differential scanning calorimetry. The drug-binding was determined spectrophotometrically. The clofibric acid anion had a much weaker depressing effect on Tt than the chlorphentermine-cation and a considerably lower ability to bind to the DPPC-liposomes. Furthermore, a plot of the effect versus the binding suggested that the clofibric acid-anion had a lower intrinsic activity to reduce Tt compared with the chlorphentermine-cation. In contrast, when the dissociation-equilibrium was shifted towards the uncharged state both drugs were indistinguishable with respect to effect and binding, suggesting that the differences observed with the charged forms could indeed be attributed to the opposite charges. The findings are tentatively explained to result from a different ability of the anionic and the cationic form to reach the hydrophobic interior of the DPPC-bilayer.

Dibucaine interaction with phospholipid vesicles. A resonance energy-transfer study

Resonance energy transfer between a local anaesthetic, dibucaine (donor) and a set of functionalized probes [n-(9-anthroyloxy)stearic acids, n = 2, 3, 6, 7, 9 and 12 and 16-(9-anthroyloxy)palmitic acid] (acceptors) was found to be an efficient process with a critical radius of transfer Ro = 2.1 nm, this interaction being used to locate the drug in a model membrane system, small unilamellar vesicles of dipalmitoylglycerophosphocholine, both above and below the temperature of the gel-to-the-liquid-crystal transition of the phospholipid. From the sequence of relative quenching efficiencies of dibucaine fluorescence upon incorporation of the probes, it was concluded that the drug intercalates in the membrane near the glycerol backbone of the lipid. In addition, it was found that dibucaine location is not significantly affected upon crossing the phase-transition temperature of the phospholipid. Dibucaine photophysics was also studied and the short lifetime of the neutral form of the anaesthetic with respect to that of the monoprotonated species was attributed to an intramolecular charge-transfer interaction. From the study of its partition coefficient between the membrane and the aqueous phase, it was concluded that the only significant species present in the membrane is the charged one.

Differential affinity of charged local anesthetics to solid-gel and liquid-crystalline states of dimyristoylphosphatidic acid vesicle membranes

Cationic local anesthetics decreased the transition temperature of the anionic phospholipid (dimyristoylphosphatidic acid, DMPA) vesicles. The counterion concentration changes the electrical double layer effect, and affects the magnitude of temperature depression caused by anesthetics. From the counterion effect on the transition-temperature depression, the partition coefficients of cationic local anesthetics to liquid-crystalline and solid-gel DMPA membranes were separately estimated. The differences in the partition coefficients between solid-gel and liquid-crystalline membranes correlated to the nerve blocking potencies. There are at least two states in the nerve membranes: resting state at higher temperature and excited state at lower temperature. We speculate that the resting state corresponds to the liquid-crystalline state, and the excited state to the solid-gel state. The difference in the partition coefficients to the resting and excited states is the cause of local anesthesia.