Intramolecular quenching of excited singlet states by stable nitroxyl radicals

Topography of nicotinic acetylcholine receptor membrane-embedded domains

The topography of nicotinic acetylcholine receptor (AChR) membrane-embedded domains and the relative affinity of lipids for these protein regions were studied using fluorescence methods. Intact Torpedo californica AChR protein and transmembrane peptides were derivatized with N-(1-pyrenyl)maleimide (PM), purified, and reconstituted into asolectin liposomes. Fluorescence mapped to proteolytic fragments consistent with PM labeling of cysteine residues in alphaM1, alphaM4, gammaM1, and gammaM4. The topography of the pyrene-labeled Cys residues with respect to the membrane and the apparent affinity for representative lipids were determined by differential fluorescence quenching with spin-labeled derivatives of fatty acids, phosphatidylcholine, and the steroids cholestane and androstane. Different spin label lipid analogs exhibit different selectivity for the whole AChR protein and its transmembrane domains. In all cases labeled residues were found to lie in a shallow position. For M4 segments, this is compatible with a linear alpha-helical structure, but not so for M1, for which "classical" models locate Cys residues at the center of the hydrophobic stretch. The transmembrane topography of M1 can be rationalized on the basis of the presence of a substantial amount of non-helical structure, and/or of kinks attributable to the occurrence of the evolutionarily conserved proline residues. The latter is a striking feature of M1 in the AChR and all members of the rapid ligand-gated ion channel superfamily.

A spectroscopic and molecular mechanics investigation on a series of AIB-based linear peptides and a peptide template, both containing tryptophan and a nitroxide derivative as probes

Linear Aib-based hexapeptides, of the general formula Ac-Toac-(Aib)(n) -Trp-(Aib)(r) -OtBu [T(Aib)(n) Trp], where n + r = 4, and Toac is a nitroxide spin-labeled C(alpha,alpha)-disubstituted glycine, were investigated by steady-state and time-resolved fluorescence measurements in different solvent media. A related peptide, i.e., cyclo-¿Orn-[(Aib)(2)-Trp-(Aib)(2)-Z]-Asp-[(Aib)(2)-Toac-(Aib)(2)-+ ++OtBu ]¿ [T-cyclo-Trp], was also studied by the same techniques. It is a L-Orn, L-Asp diketopiperazine template, to which two Aib-based chains are covalently attached, each one containing one chromophore only, i.e., Trp or Toac. Whatever the solvent, in the former series of peptides quenching of the excited Trp exhibits three lifetime components and proceeds on a time scale from subnanoseconds to a few nanoseconds, while in the case of the template the same process occurs entirely on the nanoscale time scale, exhibiting two lifetimes only. The ir absorption spectral patterns suggest that the backbone of the peptides examined is in the 3(10)-helical conformation, as earlier determined by x-ray diffraction for T(Aib)(3)Trp in the crystal state. In all cases, the fluorescence results are satisfactorily described by a dipole-dipole interaction mechanism, in which electronic energy transfer takes place from the excited Trp to Toac, provided the mutual orientation between the fluorophore and Toac is taken into account. This implies that interconversion among conformational substates is slow on the time scale of the transfer process, allowing us to estimate the dynamics of the process. Molecular mechanics calculations coupled with time decay data made it possible to build up the most probable structures of these peptides in solution.

5-Doxylstearate-induced displacement of phencyclidine from its low-affinity binding sites on the nicotinic acetylcholine receptor

Fatty acids as well as phencyclidine (PCP) inhibit the ion channel activity of the nicotinic acetylcholine receptor (AChR) by a noncompetitive mechanism. However, the exact localization of the fatty acid binding sites is unknown and, thus, the noncompetitive inhibitory mechanism for these endogenous modulators remains to be elucidated. In an attempt to determine the location of the fatty acid binding sites, we study the mutually exclusive action between 5-doxylstearate (5-SASL), a derivative of the endogenous noncompetitive antagonist (NCA) stearic acid, and other exogenous NCAs. For this purpose, both equilibrium and competitive binding assays using fluorescent and radiolabeled ligands were performed on desensitized AChRs. More specifically, we determined: (i) the effect of 5-SASL on the binding of the exogenous NCA [(3)H]PCP; (ii) the effect of 5-SASL on the binding of either quinacrine or ethidium, two fluorescent NCAs from exogenous origin; and (iii) the PCP-induced displacement of quinacrine and ethidium from their respective high-affinity binding sites. Our first target (i) is carried out by measuring the [(3)H]PCP binding in the absence or in the presence of increasing concentrations of 5-SASL. We found that 5-SASL displaces PCP from its low-affinity binding sites. The low-affinity PCP binding sites were pharmacologically characterized by an apparent dissociation constant (K(d)) of 6.1 +/- 5.0 microM and a stoichiometry of 3.7 +/- 1.5 sites per AChR. The fact that 5-SASL increased the apparent K(d) without changing the number of sites per AChR is indicative of a mutually exclusive action. From these results, an apparent inhibition constant (K(i)) of 75 +/- 31 microM for 5-SASL was calculated. In addition, 5-SASL affected neither the apparent K(d) (0.46 +/- 0.37 microM) nor the stoichiometry (1.07 +/- 0.57 sites per AChR) of the high-affinity PCP binding site. The second objective (ii) is achieved by titrating either quinacrine or ethidium into AChR native membranes in the absence or in the presence of increasing concentrations of 5-SASL. These experiments showed that 5-SASL efficiently increased the apparent K(d) of quinacrine without perturbing the interaction of ethidium with its high-affinity locus. Considering that (a) 5-SASL effectively quenched the AChR-bound quinacrine fluorescence (H. R. Arias, Biochim. Biophys. Acta 1347, 9-22, 1997) and (b) fluorescence-quenching is a short-range process, it is possible to suggest that 5-SASL displaces quinacrine from its high-affinity binding site by a steric mechanism. In this regard, a K(i) of 38 +/- 5 microM for 5-SASL was calculated. Concerning the last objective (iii), AChR-bound quinacrine or ethidium was back titrated with PCP. Two PCP K(i) values were obtained by fitting the displacement plots by nonlinear regression with two components. The lowest K(i) values obtained for either quinacrine (0.86 +/- 0.37 microM) or ethidium (0. 29 +/- 0.23 microM) displacement from their respective high-affinity binding sites coincide with the previously determined high-affinity [(3)H]PCP K(d). In addition, the highest K(i) values obtained for either NCA displacement are in the same concentration range as the observed low-affinity [(3)H]PCP K(d). Taking into account all experimental data, we reached the following conclusions: (i) fatty acid molecules, or at least 5-SASL, sterically interact with both the PCP low-affinity and the quinacrine high-affinity binding sites; (ii) the low-affinity PCP binding sites, as well as the high-affinity quinacrine locus, are located at the nonannular lipid domain of the AChR; and, finally, (iii) fatty acid molecules are not accessible to the lumen of the ion channel, indicating an allosteric mode of action for fatty acids to inhibit ion flux. Thus, the 5-SASL, the quinacrine high-affinity, and the PCP low-affinity binding sites are all located at overlapping nonannular loci on the muscle-type AChR.

Analysis of protein and peptide penetration into membranes by depth-dependent fluorescence quenching: theoretical considerations

Depth-dependent fluorescence quenching in membranes is playing an increasingly important role in the determination of the low resolution structure of membrane proteins. This paper presents a graphical way of visualizing membrane quenching caused by lipid-attached bromines or spin labels with the help of a depth-dependent fluorescence quenching profile. Two methods are presently available to extract information on membrane penetration from quenching: the parallax method (PM; ) and distribution analysis (DA; A. S. Biophys. J. 64:290a (Abstr.); A. S. Methods Enzymol. 278:462-473). Analysis of various experimental and simulated data by these two methods is presented. The effects of uncertainty in the local concentration of quenching lipids (due to protein shielding or nonideality in lipid mixing), the existence of multiple conformations of membrane-bound protein, incomplete binding, and uncertainty in the fluorescence in nonquenching lipid are described. Regardless of the analytical form of the quenching profile (Gaussian function for DA or truncated parabola for PM), it has three primary characteristics: position on the depth scale, area, and width. The most important result, not surprisingly, is that one needs three fitting parameters to describe the quenching. This will keep the measures of the quenching profile independent of each other resulting in the reduction of systematic errors in depth determination. This can be achieved by using either DA or a suggested modification of the PM that introduces a third parameter related to quenching efficiency. Because DA utilizes a smooth fitting function, it offers an advantage for the analysis of deeply penetrating probes, where the effects of transleaflet quenching should be considered.

Binding sites for exogenous and endogenous non-competitive inhibitors of the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) is the paradigm of the neurotransmitter-gated ion channel superfamily. The pharmacological behavior of the AChR can be described as three basic processes that progress sequentially. First, the neurotransmitter acetylcholine (ACh) binds the receptor. Next, the intrinsically coupled ion channel opens upon ACh binding with subsequent ion flux activity. Finally, the AChR becomes desensitized, a process where the ion channel becomes closed in the prolonged presence of ACh. The existing equilibrium among these physiologically relevant processes can be perturbed by the pharmacological action of different drugs. In particular, non-competitive inhibitors (NCIs) inhibit the ion flux and enhance the desensitization rate of the AChR. The action of NCIs was studied using several drugs of exogenous origin. These include compounds such as chlorpromazine (CPZ), triphenylmethylphosphonium (TPMP+), the local anesthetics QX-222 and meproadifen, trifluoromethyl-iodophenyldiazirine (TID), phencyclidine (PCP), histrionicotoxin (HTX), quinacrine, and ethidium. In order to understand the mechanism by which NCIs exert their pharmacological properties several laboratories have studied the structural characteristics of their binding sites, including their respective locations on the receptor. One of the main objectives of this review is to discuss all available experimental evidence regarding the specific localization of the binding sites for exogenous NCIs. For example, it is known that the so-called luminal NCIs bind to a series of ring-forming amino acids in the ion channel. Particularly CPZ, TPMP+, QX-222, cembranoids, and PCP bind to the serine, the threonine, and the leucine ring, whereas TID and meproadifen bind to the valine and extracellular rings, respectively. On the other hand, quinacrine and ethidium, termed non-luminal NCIs, bind to sites outside the channel lumen. Specifically, quinacrine binds to a non-annular lipid domain located approximately 7 A from the lipid-water interface and ethidium binds to the vestibule of the AChR in a site located approximately 46 A away from the membrane surface and equidistant from both ACh binding sites. The non-annular lipid domain has been suggested to be located at the intermolecular interfaces of the five AChR subunits and/or at the interstices of the four (M1-M4) transmembrane domains. One of the most important concepts in neurochemistry is that receptor proteins can be modulated by endogenous substances other than their specific agonists. Among membrane-embedded receptors, the AChR is one of the best examples of this behavior. In this regard, the AChR is non-competitively modulated by diverse molecules such as lipids (fatty acids and steroids), the neuropeptide substance P, and the neurotransmitter 5-hydroxytryptamine (5-HT). It is important to take into account that the above mentioned modulation is produced through a direct binding of these endogenous molecules to the AChR. Since this is a physiologically relevant issue, it is useful to elucidate the structural components of the binding site for each endogenous NCI. In this regard, another important aim of this work is to review all available information related to the specific localization of the binding sites for endogenous NCIs. For example, it is known that both neurotransmitters substance P and 5-HT bind to the lumen of the ion channel. Particularly, the locus for substance P is found in the deltaM2 domain, whereas the binding site for 5-HT and related compounds is putatively located on both the serine and the threonine ring. Instead, fatty acid and steroid molecules bind to non-luminal sites. More specifically, fatty acids may bind to the belt surrounding the intramembranous perimeter of the AChR, namely the annular lipid domain, and/or to the high-affinity quinacrine site which is located at a non-annular lipid domain. Additionally, steroids may bind to a site located on the extracellular hydrophi

Location of diphenylhexatriene (DPH) and its derivatives within membranes: comparison of different fluorescence quenching analyses of membrane depth

The average membrane location of a series of diphenylhexatriene (DPH)-derived membrane probes was analyzed by measuring the quenching of DPH fluorescence with a series of nitroxide-labeled lipids in which the depth of the nitroxide group is varied. All DPH derivatives were located deeply within the bilayer. Some derivatives were anchored at a shallower depth than free DPH by attachment to cationic or anionic groups. However, the absolute change in DPH depth upon attachment to such groups was relatively modest (<4 A). In fact, protonated DPH fatty acid and a DPH fatty acyl group attached to a phosphatidylcholine were found to locate slightly more deeply than free DPH. The location of DPH derivatives can be explained by the length of the DPH group and its tendency to orient predominantly parallel to the fatty acyl chains of the bilayer. These factors allow a charged group attached to one end of a DPH molecule to be accommodated at the polar surface while maintaining a deep DPH location. Basically, it appears that most DPH derivatives probe the same region in the bilayer. We conclude previously reported differences in fluorescence polarization of free and anchored forms of DPH may reflect a direct effect of anchoring on motion rather than an effect on average DPH location. Other experiments showed the localization of DPH probes was found to be similar in the presence and absence of cholesterol. This implies that previously observed cholesterol-induced effects on DPH fluorescence polarization also largely reflect differences in DPH motion, not DPH location. From the quenching results it was also possible to define rules governing the location of a variety of chemical groups in membranes by comparison of the results obtained with DPH derivatives to those of similar derivatives of other fluorescent groups. Finally, an important goal of this study was to compare different methods of analysis of quenching data: parallax analysis, distribution (Gaussian) analysis (using a single Gaussian), and a second-order polynomial analysis. To evaluate the accuracy of these methods, the apparent depths of a series of fluorescence probes previously analyzed by parallax analysis was reanalyzed with all three methods. There was good agreement unless the fluorescent molecule was very shallow or very deep. In such cases, only parallax analysis gave physically reasonable results. This is likely to be due to the lack of a sufficient number of quenchers spanning a wide enough range for other analyses to compensate for deviations from ideal curves. Parallax analysis was also compared to distribution (Gaussian) analysis using a double Gaussian fit to account for quenching from the trans leaflet (Ladokhin, A. (1997) Methods Enzymol. 278, 462-473). Again more physically reasonable results were obtained from parallax analysis, likely due to non-Gaussian behavior of the depth dependence of quenching. Notwithstanding these observations, the significant number of cases where Gaussian curve fitting methods for quenching analysis are most powerful are discussed.

Topology of ligand binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of both alpha subunits there exist the binding sites for agonists such as the neurotransmitter acetylcholine (ACh) and for competitive antagonists such as d-tubocurarine. Agonists trigger the channel opening upon binding while competitive antagonists compete for the former ones and inhibit its pharmacological action. Identification of all residues involved in recognition and binding of agonist and competitive antagonists is a primary objective in order to understand which structural components are related to the physiological function of the AChR. The picture for the localisation of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are mainly located on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are sequentially identical, the observed high and low affinity for agonists on the receptor is conditioned by the interaction of the alpha subunit with the delta or the gamma chain, respectively. This relationship is opposite for curare-related drugs. This molecular interaction takes place probably at the interface formed by the different subunits. The principal component for the agonist/competitive antagonist binding sites involves several aromatic residues, in addition to the cysteine pair at 192-193, in three loops-forming binding domains (loops A-C). Other residues such as the negatively changed aspartates and glutamates (loop D), Thr or Tyr (loop E), and Trp (loop F) from non-alpha subunits were also found to form the complementary component of the agonist/competitive antagonist binding sites. Neurotoxins such as alpha-, kappa-bungarotoxin and several alpha-conotoxins seem to partially overlap with the agonist/competitive antagonist binding sites at multiple point of contacts. The alpha subunits also carry the binding site for certain acetylcholinesterase inhibitors such as eserine and for the neurotransmitter 5-hydroxytryptamine which activate the receptor without interacting with the classical agonist binding sites. The link between specific subunits by means of the binding of ACh molecules might play a pivotal role in the relative shift among receptor subunits. This conformational change would allow for the opening of the intrinsic receptor cation channel transducting the external chemical signal elicited by the agonist into membrane depolarisation. The ion flux activity can be inhibited by non-competitive inhibitors (NCIs). For this kind of drugs, a population of low-affinity binding sites has been found at the lipid-protein interface of the AChR. In addition, several high-affinity binding sites have been found to be located at different rings on the M2 transmembrane domain, namely luminal binding sites. In this regard, the serine ring is the locus for exogenous NCIs such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222, phencyclidine, and trifluoromethyliodophenyldiazirine. Trifluoromethyliodophenyldiazirine also binds to the valine ring, which is the postulated site for cembranoids. Additionally, the local anaesthetic meproadifen binding site seems to be located at the outer or extracellular ring. Interestingly, the M2 domain is also the locus for endogenous NCIs such as the neuropeptide substance P and the neurotransmitter 5-hydroxytryptamine. In contrast with this fact, experimental evidence supports the hypothesis for the existence of other NCI high-affinity binding sites located not at the channel lumen but at non-luminal binding domains. (ABSTRACT TRUNCATED)

The high-affinity quinacrine binding site is located at a non-annular lipid domain of the nicotinic acetylcholine receptor

This work deals with the localization of the high-affinity non-competitive quinacrine binding site on the muscle-type nicotinic acetylcholine receptor (AChR). Specifically, quantitative steady-state fluorescence spectroscopy is used to determine whether quinacrine binds to a site located at either the annular or the non-annular lipid domain. For this purpose, we measure the ability of spin-labelled phosphatidylcholine (SL-PC) to quench AChR-bound quinacrine, AChR-bound ethidium and membrane-partitioned 7-(9-anthroyloxy)stearate (7-AS) fluorescence. Additionally, we compare the accessibility of SL-PC which is considered to bind only to the annular lipid domain of the AChR with the accessibility of two non-annular domain-sensing lipids such as 5-doxylstearate (5-SAL) and spin-labelled androstane (ASL). Initial experiments using 7-AS established the experimental conditions for maximum SL-PC membrane partitioning. The non-specific quenching elicited by increasing turbidity of the sample after addition of SL-PC is corrected by means of parallel experiments with unlabelled egg yolk phosphatidylcholine. After correction, the SL-PC quenching experiments show the following order in quenching efficiency: 7-AS > quinacrine >> ethidium. The relative intrinsic sensitivity of quinacrine to TEMPO paramagnetic quenching in acetonitrile is considered to be approximately two times higher than that for 7-AS. Thus, SL-PC was found to be more accessible (about 5-fold) to the membrane-partitioned 7-AS than to the quinacrine locus. In addition, SL-PC was virtually not accessible to the high-affinity non-luminal binding site for ethidium. The relative capacity of SL-PC, 5-SAL, and ASL to quench AChR-bound quinacrine fluorescence indicated that the spin-labelled lipid accessibility to the quinacrine binding site follows the order: 5-SAL > ASL >> SL-PC. Examination of the effect of high concentrations of 5-SAL, of its unlabelled parent stearate, and of SL-PC on ethidium and quinacrine binding showed that: (a) both fatty acids displace quinacrine, but not ethidium, from its high-affinity binding site, however (b) 5-SAL was found to be more effective than stearate to displace quinacrine from its locus, whereas (c) SL-PC competes neither for the ethidium locus nor for the quinacrine binding site. The results suggest that the high-affinity binding site for quinacrine is located at a non-annular lipid domain of the AChR. This particular area has been considered to be located at the intramolecular interfaces of the five AChR subunits and/or at the interstices of the transmembrane domains.

Distance-Dependent Fluorescence Quenching of p-Bis[2-(5-phenyloxazolyl)]benzene by Various Quenchers

We report results of frequency-domain and steady-state measurements of the fluorescence quenching of p-bis-[2-(5-phenyloxazolyl)]benzene (POPOP) when quenched by bromoform (CHBr3), methyl iodide (CH3I), potassium iodide (KI), 1,2,4-trimethoxybenzene (TMB), or N,N-diethylaniline (DEA). The quenching efficiency of these compounds decreased in the order DEA, TMB, KI, CH3I, CHBr3. In the case of DEA and TMB the measurements clearly confirm the applicability of the exponential distance-dependent quenching (DDQ) model, in which the bimolecular quenching rate k(r) depends exponentially on the fluorophore–quencher separation r, k(r) = ka exp[−(r − a)/re], where a is the distance of closest approach. Simultaneous analysis of the frequency-domain and steady-state data significantly improved resolution of the recovered molecular parameters ka and re. The data for DEA and TMB cannot be satisfactorily fit using either the Smoluchowski or Collins–Kimball radiation boundary condition (RBC) model. The quenching behavior of the less efficient quenchers KI, CH3I, and CHBr3 can be adequately described with both the DDQ and RBC models, but this may be a simple consequence of less efficient quenching. The efficiency of quenching is discussed on the basis of the mechanisms of interaction between the fluorophore and quencher molecules, which involves electron transfer and/or heavy atom effects.

Quinacrine noncompetitive inhibitor binding site localized on the Torpedo acetylcholine receptor in the open state

Open-channel blockers of the nicotinic acetylcholine receptor (nAcChR) are widely thought to act sterically by entering and "plugging" the open channel of the nAcChR. However, quinacrine, a fluorescent open-channel blocker, has been recently shown to bind to the nAcChR at a site near the lipid bilayer while the receptor is in a closed, desensitized state, suggesting that at least one open-channel blocker might act allosterically outside the channel [Valenzuela et al. (1992) J. Biol. Chem. 267, 8238]. To determine whether or not quinacrine also binds near the lipid bilayer when the receptor is in an open state, a short-range lipophilic quencher (5-doxylstearate, 5-SA) was used to assess the proximity of the nAcChR-bound quinacrine to the lipid bilayer while the receptor was transiently open by an agonist. Initial experiments using a stopped-flow instrument established the conditions required to monitor a portion of the changes in quinacrine fluorescence associated with its binding to the receptor in the open state. 5-SA (80 microM) reduced the amplitude of the rapid agonist-induced change in quinacrine emission to 44% +/- 12% of the control value, indicating that the quinacrine was binding to a site proximal to the membrane-partitioned 5-SA. Control experiments established that 5-SA had no effect on the ability of the receptor to undergo agonist-induced conformational changes, suggesting that little, if any, 5-SA distributed into the channel lumen and perturbed the functional activity of the receptor. Together, the results indicate that quinacrine binds to a site on the open receptor that is in contact with the lipid bilayer and not in the channel lumen.

The transverse location of the fluorescent probe trans-parinaric acid in lipid bilayers

The transverse location of trans-parinaric acid in spherical vesicles made up from dipalmitoylphosphatidylcholine has been investigated by the differential quenching of the probe fluorescence by 5- and 16-doxylstearic acid derivatives. The quenching data are interpreted in terms of a local fluorophore concentration factor. In this way it was found that the polyene of t-PnA is located within the inner part of the bilayer (presumably aligned with the bilayer lipids), both in the gel and in the liquid crystalline phases.

Combined liquid chromatography/mass spectrometry of the radical adducts of a fluorescamine-derivatized nitroxide

Dynamic liquid secondary ion mass spectrometry (dyn-LSIMS) was employed to acquire continuous, on-line mass spectral data from the effluent of a reversed-phase high-performance liquid chromatograph (HPLC) used to separate a broad suite of carbon-centered radicals trapped as the O-alkylhydroxylamine adducts of an amino nitroxide that was subsequently derivatized with fluorescamine. Data obtained by the use of these combined techniques (LC/MS) can be employed to elucidate radical adduct structures; elemental compositions of the adducts can be confirmed by acquiring mass spectra at high resolution. At low resolution, introduction into the source of < 1 pmol of adduct yielded usable spectra. The first application of this technique to the identification of photochemically generated radicals in natural water samples is presented.

Agonist-induced displacement of quinacrine from its binding site on the nicotinic acetylcholine receptor: plausible agonist membrane partitioning mechanism

It was previously demonstrated that high concentrations of cholinergic agonists such as acetylcholine (ACh), carbamylcholine (CCh), suberyldicholine (SubCh) and spin-labelled acetylcholine (SL-ACh) displaced quinacrine from its high-affinity binding site located at the lipid-protein interface of the nicotinic acetylcholine receptor (AChR) (Anas, H. R. and Johnson, D. A. (1995) Biochemistry, 34, 1589-1595). In order to account for the agonist self-inhibitory binding site which overlaps, at least partially, with the quinacrine binding site, we determined the partition coefficient (Kp) of these agonists relative to the local anaesthetic tetracaine in AChR native membranes from Torpedo californica electric organ by examining (1) the ability of tetracaine and SL-ACh to quench membrane-partitioned 1-pyrenedecanoic acid (C10-Py) monomer fluorescence, and (2) the ability of ACh, CCh and SubCh to induce an increase in the excimer/monomer ratio of C10-Py-labelled AChR membrane fluorescence. To further assess the differences in agonist accessibility to the quinacrine binding site, we calculated the agonist concentration in the lipid membrane (CM) at an external agonist concentration high enough to inhibit 50% of quinacrine binding (IC50), which in turn was obtained by agonist back titration of AChR-bound quinacrine. Initial experiments established that high agonist concentrations do not affect either transmembrane proton concentration equilibria (pH) of AChR membrane suspension or AChR-bound quinacrine fluorescence spectra. The agonist membrane partitioning experiments indicated relatively small (< or = 20) Kp values relative to tetracaine. These values follow the order: SL-ACh>SubCh>>CCh-ACh. A direct correlation was observed between Kp and the apparent inhibition constant (Ki) for agonists to displace AChR-bound quinacrine. Particularly, agonist with high KpS such as SL-ACh and SubCh showed low Ki values, and this relationship was opposite for CCh and ACh. The calculated CM values indicated significant (between 7 and 54 mM) agonist accessibility to lipid membrane. By themselves, these results support the conjecture that agonist self-inhibition seems to be mediated by the quinacrine binding site via a membrane approach mechanism. The existence of an agonist self-inhibitory binding site, not located in the channel lumen would indicate an allosteric mechanism of ion channel inhibition; however, we can not discard that the process of agonist self-inhibition can also be mediated by a steric blockage of the ion channel.

Luminescence quenching by long range electron transfer: a probe of protein clustering and conformation at the cell surface

Quenching of luminescence from fluorescent and phosphorescent probes by nitroxide spin labels with a long range electron transfer (LRET) mechanism (44,45) has been tested as a tool to monitor association/clustering and conformational changes of cell surface proteins. The membrane proteins were labeled with monoclonal antibodies or Fab fragments conjugated with luminescent probes or water-soluble nitroxide spin labels. The method was tested as a probe of 3 different aspects of protein-protein association involving class I MHC molecules: (1) interaction between the heavy and light chains of the MHC molecules, (2) clustering, self-association of MHC molecules, (3) proximity of MHC molecules to transferrin receptors of fibroblasts or surface immunoglobulin molecules of B lymphoblasts. The extent of quenching upon increasing the fractional density of the quencher was sensitive for protein association in accordance with earlier immunoprecipitation and flow cytometric Förster-type energy transfer (FCET) data obtained on the same cells. These data suggest that the LRET quenching can be used as intra- or intermolecular ruler in a 0.5-2.5 nm distance range. This approach is simpler (measurements only on donor side) and faster than many other experimental techniques in screening physical association or conformational changes of membrane proteins by means of spectrofluorimetry, flow cytometry, or microscope based imaging.

Fluorescence study of a derivatized diacylglycerol incorporated in model membranes

A fluorescence study of a diacylglycerol derivatized with the n-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) chromophore (NBD) was carried out. Fluorescence self-quenching was observed for this probe in lecithin model membranes due to collisional interaction rather than to an aggregational behaviour of the probe. The efficient energy migration (Ro = 28 A) of the NBD fluorophore was studied via the fluorescence depolarization upon increase of probe concentration in membranes, and the results are compared with a model where a random distribution of the probes is assumed. A surface location of the chromophore was concluded for the NBD derivative of diacylglycerol, both from the fluorescence parameters and from the study of its fluorescence quenching by spin label probes. Very high lateral diffusion coefficients were obtained for these probes, both from the self-quenching (D = 2-6 x 10(-6) cm2 s-1) and from the spin probe quenching (D = 3.5 x 10(-6) cm2 s-1) studies. A concomitant fluorescence study of the related probe NBD-phosphatidylcholine revealed that its photophysical behaviour is similar to the derivatized diacylglycerol.

Calibration of the parallax fluorescence quenching method for determination of membrane penetration depth: refinement and comparison of quenching by spin-labeled and brominated lipids

We previously introduced the "parallax" method, which uses fluorescence quenching by spin-labeled lipids in order to measure the depth of molecules within a membrane [Chattopadhyay, A., & London, E. (1987) Biochemistry 26, 39-45]. In this report the accuracy of this method is established by comparison of spin-label quenching to that obtained using brominated lipids. To accomplish this, the fluorescent molecules used were a fatty acid labeled with a carbazole buried deeply within the acyl chain region of the membrane, an acyl-Trp with the Trp residue residing near the polar membrane region, and cytochrome b5, which has Trp residues in its membrane-inserted region. The depths calculated from the amount of bromine quenching agreed with those determined using parallax analysis. This indicates that the depth reported by parallax analysis is accurate and that the spin labels residue very close to their predicted locations in the membrane. Furthermore, there was good agreement when parallax analysis was applied both to quenching by brominated and spin-labeled molecules, suggesting that the analysis is valid in both cases. The effect that different distributions and motions of fluorophores and quenchers would have on parallax analysis was also examined. For uniform distributions of quenchers or fluorophores over a range of depths, it was found that the analysis reports the average fluorophore depth. In addition, experimental data suggest that motional effects do not significantly alter the measured depths. This is consistent with the motions during the short excited state lifetime of the fluorophores being relatively small and/or relatively isotropic.