Coupling of proton source and sink via H+-migration along the membrane surface as revealed by double patch-clamp experiments

Long-range proton transfer along the surface of black lipid bilayers was observed between two integral membrane channels (gramicidins), one operating as a proton source, the other as a sink, by patch-clamp technique. In contrast, potassium ions were shown to equilibrate with the aqueous bulk phase before being consumed. Both channels opened and closed simultaneously only if the charge between them was carried by protons. In this case an anomalous high conductance between two patched membrane fragments was measured, each of them containing one single gramicidin channel. The coupled state disappeared when the distance between these two channels was increased above the critical value. The latter was shown to increase with the channel lifetime. Our results support the idea of the 'localized' proton coupling, in which protons that have been pumped across membranes migrate along the membrane surface to reach another membrane protein that utilizes the established pH gradient.

Scanning electron microscopy and gramicidin patch clamp recordings of microvillous receptor neurons dissociated from the rat vomeronasal organ

Vomeronasal organs from female rats were dissociated and isolated microvillous receptor neurons were studied. The isolated receptor neurons kept the typical bipolar shape which they have in situ as observed by scanning electron microscopy. We applied the perforated patch-clamp technique using the cation-selective ionophore gramicidin on freshly isolated and well differentiated receptor neurons. The mean resting potential was -58+/-14 mV (n=39). The contribution of the sodium pump current to the resting potential was demonstrated by lowering the K+ concentration in the bath or by application of 100 microM dihydro-ouabain. The input resistance was in the range of 1-6 GOmega and depolarizing current pulses of a few pA were sufficient to trigger overshooting action potentials. In voltage clamp conditions a fast transient sodium inward current and a sustained outward potassium current were activated by membrane depolarization. These observations indicate that freshly isolated vomeronasal receptor neurons of rats can be recorded, using gramicidin, with little modification of the intracellular content. Their electrophysiological properties are very similar to those observed in situ. Four out of eight female vomeronasal receptor cells were depolarized by diluted rat male urine.

Functional GABA(A) receptors on human glioma cells

Glioma cells in acute slices and in primary culture, and glioma-derived human cell lines were screened for the presence of functional GABA(A) receptors. Currents were measured in whole-cell voltage clamp in response to gamma-aminobutyric acid (GABA). While cells from the most malignant glioma, the glioblastoma multiforme, did not respond to GABA, an inward current (under our experimental conditions with high Cl- concentration in the pipette) was induced in gliomas of lower grades, namely in 71% of oligodendroglioma cells and in 62% of the astrocytoma cells. Glioma cell lines did not express functional GABA(A) receptors, irrespective of the malignancy of the tumour they originate from. The currents elicited by application of GABA were due to activation of GABA(A) receptors; the specific agonist muscimol mimicked the response, the antagonists bicuculline and picrotoxin blocked the GABA-activated current and the benzodiazepine receptor agonist flunitrazepam augmented the GABA-induced current and the benzodiazepine inverse agonist DMCM decreased the GABA current. Cells were heterogeneous with respect to the direction of the current flow as tested in gramicidin perforated patches: in some cells GABA hyperpolarized the membrane, while in the majority it triggered a depolarization. Moreover, GABA triggered an increase in [Ca2+]i in the majority of the tumour cells due to the activation of Ca2+ channels. Our results suggest a link between the expression of GABA receptors and the growth of glioma cells as the disappearance of functional GABA(A) receptors parallels unlimited growth typical for malignant tumours and immortal cell lines.

Effect of PMA on the integrity of the membrane skeleton and morphology of epithelial MDCK cells is dependent on the activity of amiloride-sensitive ion transporters and membrane potential

The signaling pathways from an activation of protein kinase C (PKC) by phorbol myristate acetate (PMA) to the rearrangement of actin-based cytoskeleton and membrane skeleton of epithelial MDCK cells were studied by visualizing the cytoskeletal organization with immunofluorescence microscopy and by measuring intracellular pH, sodium ion concentration and membrane potential with the aid of fluorescent intracellular indicators. Upon PMA treatment the MDCK cells lost their cubic shape and acquired a spindle-like morphology. The stress fibers were depolymerized, and fodrin, the main component of the membrane skeleton, was released from the lateral walls to the cytosol. These changes were accompanied by depolarization of the cells, decrease in the intracellular pH and sodium ion concentration. In order to test the mutual correlation between the PMA-induced alterations we treated the cells with PMA in the presence of channel inhibitors or ionophores and in defined media. The effects of PMA on the membrane skeleton and morphology could be reversed in media lacking Na+ or K+ ions or by hyperpolarizing agents, dimethylamiloride and valinomycin, suggesting that the effects of PMA on the cytoskeleton were dependent on the ion gradients and membrane potential across the cell membrane. Moreover, the morphological changes and instabilization of the membrane skeleton of MDCK cells took place spontaneously without PMA in depolarizing conditions, in potassium gluconate buffer. We suggest that the membrane potential across the cell membrane of MDCK cells together with the activity of amiloride-sensitive cation transporters transmits signals in the protein kinase C (PKC) pathway leading from activation of PKC to fibroblast-like morphology and cytoplasmic localization of membrane skeleton components, features characteristic for cancer cells.

Effect of n-alkanols on lipid bilayer hydration

The effect of a homologous series of aliphatic n-alkanols on the presence of water within the head group and acyl chain region of lipid bilayers was investigated using time-resolved fluorescence spectroscopy according to a previously published approach [Ho, C., Slater, S. J., & Stubbs, C. D. (1995) Biochemistry 34, 6188-6195]. Upon addition of n-alkanols to phosphatidylcholine bilayers the fluorescence lifetime of N-[5-(dimethylamino)naphthalene-1-sulfonyl]dipalmitoylphosphatidyleth anolamine (dansyl-PE) decreased, indicative of an increased water content within the head group region, the effect being a linear function of n-alkanol chain length (C1-C8), based on the total n-alkanol concentration. The fluorescence lifetimes of 1-palmitoyl-2-[[2-[4-(6-phenyl-trans-1,3, 5-hexatrienyl)phenyl]ethyl]carbonyl]-3-sn-phosphatidylcholine (DPH-PC) and N-[[4-(6-phenyl-1,3, 5-hexatrienyl)phenyl]propyl]trimethylammonium p-toluenesulfonate (TMAP-DPH), and the fluorescence intensity ratio of the latter in D2O compared to that in H2O, were used to probe the level of water in the acyl chain region. There was a decrease in the lifetime and an increase in the D2O/H2O fluorescence intensity ratio upon addition of short-chain n-alkanols (C1-C3), suggestive of increased water content. By contrast, long-chain n-alkanols (C4-C8) increased the lifetime and decreased the ratio, suggestive of decreased water content. Acyl chain order, determined from DPH-PC fluorescence anisotropy, was decreased by all n-alkanols, indicating that the effects were not probe-dependent. The effects of short- and long-chain n-alkanols on the fluorescence lifetime of the tryptophans of gramicidin, incorporated into phosphatidylcholine bilayers as a model membrane protein, were similar to those obtained with TMAP-DPH and DPH-PC; ethanol decreased and hexanol increased the lifetime. Thus the effect of n-alkanols and general anesthetics on changes in the amount of water that may be accommodated within the acyl chain region of the bilayer is not predictable on the basis of the magnitude of effects on head group region or acyl chain order/fluidity.

A biosensor that uses ion-channel switches

Biosensors are molecular sensors that combine a biological recognition mechanism with a physical transduction technique. They provide a new class of inexpensive, portable instrument that permit sophisticated analytical measurements to be undertaken rapidly at decentralized locations. However, the adoption of biosensors for practical applications other than the measurement of blood glucose is currently limited by the expense, insensitivity and inflexibility of the available transduction methods. Here we describe the development of a biosensing technique in which the conductance of a population of molecular ion channels is switched by the recognition event. The approach mimics biological sensory functions and can be used with most types of receptor, including antibodies and nucleotides. The technique is very flexible and even in its simplest form it is sensitive to picomolar concentrations of proteins. The sensor is essentially an impedance element whose dimensions can readily be reduced to become an integral component of a microelectronic circuit. It may be used in a wide range of applications and in complex media, including blood. These uses might include cell typing, the detection of large proteins, viruses, antibodies, DNA, electrolytes, drugs, pesticides and other low-molecular-weight compounds.

THE BIOSYNTHESIS OF GRAMICIDIN S IN A CELL-FREE SYSTEM

1. A cell-free system prepared from Bacillus brevis cells, harvested in the late phase of growth and consisting of the 11000g supernatant, has been shown to incorporate into gramicidin S the five constituent amino acids added in labelled form. The results are consistent with complete synthesis and not merely a completion of pre-existing intermediate peptides. 2. The incorporation of (14)C-labelled amino acids by the 11000g supernatant into gramicidin S requires an energy source. Omission of phosphoenolpyruvate and pyruvate kinase from the incubation mixture prevents incorporation into gramicidin S. The cell-free system incorporates [(14)C]-leucine, -proline and -phenylalanine over a period of 4hr. With [(14)C]leucine, incorporation into gramicidin S takes place in the range pH6-9 with maximum incorporation at pH7.0. High concentrations of chloramphenicol or puromycin decreased the incorporation into gramicidin S by only about 20%. 3. The 50000g supernatant exhibited no decrease in ability of incorporating [(14)C]valine into gramicidin S as compared with the 11000g supernatant. About 40% of the incorporating ability remained in the 105000g supernatant after 3hr. centrifugation. When recombining the 105000g sediment with the 105000g supernatant, some increase in incorporation over that obtained with the supernatant alone was obtained. The findings tend to support the view that gramicidin S is synthesized in a different manner from that of proteins.

Two mechanisms of H+/OH- transport across phospholipid vesicular membrane facilitated by gramicidin A

Two rate-limiting mechanisms have been proposed to explain the gramicidin channel facilitated decay of the pH difference across vesicular membrane (delta pH) in the pH region 6-8 and salt (MCI, M+ = K+, Na+) concentration range 50-300 mM. 1) At low pH conditions (approximately 6), H+ transport through the gramicidin channel predominantly limits the delta pH decay rate. 2) At higher pH conditions (approximately 7.5), transport of a deprotonated species (but not through the channel) predominantly limits the rate. The second mechanism has been suggested to be the hydroxyl ion propogation through water chains across the bilayer by hydrogen bond exchange. In both mechanisms alkali metal ion transport providing the compensating flux takes place through the gramicidin channels. Such an identification has been made from a detailed study of the delta pH decay rate as a function of 1) gramicidin concentration, 2) alkali metal ion concentration, 3) pH, 4) temperature, and 5) changes in the membrane order (by adding small amounts of chloroform to vesicle solutions). The apparent activation energy associated with the second mechanism (approximately 3.2 kcal/mol) is smaller than that associated with the first mechanism (approximately 12 kcal/mol). In these experiments, delta pH was created by temperature jump, and vesicles were prepared using soybean phospholipid or a mixture of 94% egg phosphatidylcholine and 6% phosphatidic acid.

GRAMICIDIN AND ION TRANSPORT IN ISOLATED LIVER MITOCHONDRIA

1. Gramicidin caused H(+) production by liver mitochondria; this process was dependent on the presence of Na(+), Li(+), Rb(+) or Cs(+). In the presence of one of these alkali-metal ions and phosphate, gramicidin caused mitochondrial swelling and increased oxygen consumption. 2. Uncoupling agents, anaerobic conditions or respiratory inhibitors both inhibited and reversed H(+) production and swelling. Both these processes could be supported by coupled electron transport through even a restricted portion of the respiratory chain. 3. NH(4) (+) also caused stimulation of respiration in the presence of gramicidin. In this case phosphate was not required. NH(4) (+), in the presence of gramicidin, caused contraction of the mitochondria and a reversal of K(+)-induced swelling and H(+) production. 4. Uncoupling agents or NH(4) (+) together with gramicidin caused the release of Sr(2+) that had been accumulated by mitochondria in the presence of phosphate. 5. These results are discussed in relation to a postulated respiration-dependent H(+) pump located in the mitochondrial membrane. It is suggested that gramicidin allows alkali-metal ions to pass through the mitochondrial membrane.

Valence selectivity of the gramicidin channel: a molecular dynamics free energy perturbation study

The valence selectivity of the gramicidin channel is examined using computer simulations based on atomic models. The channel interior is modeled using a gramicidin-like periodic poly (L,D)-alanine beta-helix. Free energy perturbation calculations are performed to obtain the relative affinity of K+ and Cl- for the channel. It is observed that the interior of the gramicidin channel provides an energetically favorable interaction site for a cation but not for an anion. Relative to solvation in bulk water, the carbonyl CO oxygens can provide a favorable interaction to stabilize K+, whereas the amide NH hydrogens are much less effective in stabilizing Cl-. The results of the calculations demonstrate that, as a consequence of the structural asymmetry of the backbone charge distribution, a K+ cation can partition spontaneously from bulk water to the interior of the gramicidin channel, whereas a Cl- anion cannot.

Excitatory actions of GABA in developing rat hypothalamic neurones

1. Gramicidin-perforated patch clamp recording was employed to study GABA-mediated responses in rat hypothalamic neurones (n = 102) with an intracellular Cl- concentration unaltered by the pipette solution. 2. In young cultures after 1-7 days in vitro (DIV), GABA induced depolarizing membrane potentials (+16.5 +/- 1.3 mV) that often surpassed the threshold for the firing of action potentials (-42 +/- 1 mV) and resulted in an increase in neuronal activity. The depolarizing responses to GABA in young cultures were dose dependent. The concentration of GABA necessary to evoke the half-maximal depolarization (EC50) was 2.8 microM. In contrast, GABA induced hyperpolarizing membrane potentials (-12.0 +/- 1.4 mV) and a decrease in neuronal activity in older neurones (20-33 DIV). Both the depolarization and the hyperpolarization induced by GABA were blocked by bicuculline, indicating a mediation by GABAA receptors. 3. The reversal potentials of the GABA-evoked currents were between -40 to -50 mV during the first week of culture, and shifted to below -70 mV after 3 weeks of culture. In parallel, neurones that were dissociated from older animals (postnatal day 5) had a more negative reversal potential for the GABA-evoked currents than cells from younger animals (embryonic day 15), suggesting that the negative shift of the reversal potential occurs both in vitro and in vivo. Our data suggest that the mechanism for GABA-induced depolarization is the depolarized Cl- reversal potential found in young but not older neurones. 4. Consistent with the depolarizing response to exogenous application of GABA, some spontaneous depolarizing postsynaptic potentials in young cultures were insensitive to AP5-CNQX, but were eliminated by bicuculline, indicating that synaptically released GABA mediated excitatory synaptic transmission in early development. 5. By combining a rapid computer-controlled delivery of GABA with subthreshold positive current injections into recorded neurones, we found in young cultures that the GABA-evoked depolarization could directly trigger action potentials, facilitate some depolarizing input to fire action potentials, and shunt other depolarizing input. Whether the GABA-induced depolarization is excitatory or inhibitory would be determined by the reversal potential of the GABA-evoked current, and the temporal relationship between GABA-evoked depolarizations and other excitatory events. 6. We conclude that the reversal potential of the GABA-evoked current shifts negatively from depolarizing to hyperpolarizing in developing hypothalamus. Consequently, GABA neurotransmission may serve both excitatory and inhibitory roles during early development.

Formation of membrane domains created during the budding of vesicular stomatitis virus. A model for selective lipid and protein sorting in biological membranes

Vesicular stomatitis virus buds from domains of the plasma membrane that have a unique protein and lipid composition. Fluorescence digital imaging microscopy and resonance energy transfer were used to determine how the two viral envelope-associated proteins, the G and the M proteins, could alter the lateral distribution of lipids in large unilamellar vesicles and form domains. The G protein formed large domains in vesicles containing phosphatidic acid but not with phosphatidylserine, while the M protein formed domains enriched in both acidic phospholipids. Domains enriched in sphingomyelin were observed only when both the G protein and the M protein were present in vesicles containing phosphatidic acid. Phosphatidylcholine and gramicidin (chosen to represent a host membrane protein) were excluded from the domains. Cholesterol was induced to partition into the domains only in vesicles containing phosphatidic acid and sphingomyelin along with both of the proteins. Phosphatidylethanolamine was not enriched or depleted in the domains. Domains of similar composition were formed using vesicles made from dioleoylphospholipids and the lipids extracted from BHK-21 plasma membranes, indicating that the fatty acid composition was not as important as the polar head groups of the phospholipids. The phospholipid and cholesterol compositions of the domains formed by the G and the M proteins in vesicles were very similar to the composition of the viral envelope, suggesting that the domains represent the areas in the plasma membrane where the virus buds. This study provides a model for selective lipid and protein sorting that occurs in biological membranes.

Potassium flux through gramicidin ion channels is augmented in vesicles comprised of plasmenylcholine: correlations between gramicidin conformation and function in chemically distinct host bilayer matrices

The functional role of distinct phospholipid subclasses and molecular species in modulating gramicidin-mediated K+ flux was characterized through quantification of changes in the fluorescence intensity of ion specific fluorescent probes encapsulated inside vesicles comprised of individual molecular species of plasmenylcholine and phosphatidylcholine. The rate constant of gramicidin-mediated K+ ion flux across bilayers comprised of 1-O-(Z)-hexadec-1'-enyl-2-octadec-9'-enoyl-sn-glycero-3-p hos phocholine (plasmenylcholine) was 18.9 +/- 1.7 s-1, while that present across bilayers comprised of 1-hexadecanoyl-2-octadec-9'-enoyl-sn-glycero-3-phosphocholine (phosphatidylcholine) was 12.3 +/- 1.5 s-1. The observed changes were not due to alterations in the nature of the sn-2 aliphatic chain or the net surface charge present at the membrane interface and were unaltered by the addition of several amphiphilic agents (including charged amphiphiles), suggesting that the observed alterations specifically reflect changes in channel function which result from the covalent alteration of host phospholipid in the proximal portion of the sn-1 aliphatic chain (i.e., phospholipid subclass-specific alterations). Addition of cholesterol to bilayer matrices comprised of plasmenylcholine resulted in dose-dependent attenuation of the rate of gramicidin-mediated K+ flux, but did not alter the rate of gramicidin-mediated K+ flux in membranes comprised of phosphatidylcholine. Gramicidin ion channels experience distinct environments in membranes comprised of phosphatidylcholine and plasmenylcholine host lipids demonstrated by both the different fluorescence anisotropies of endogenous tryptophan residues and the different C=O stretching frequencies of intramonomer carbonyls in gramicidin incorporated into these two choline glycerophospholipid subclasses.(ABSTRACT TRUNCATED AT 250 WORDS)

Gramicidin tryptophans mediate formamidinium-induced channel stabilization

Compared with alkali metal cations, formamidinium ions stabilize the gramicidin A channel molecule in monoolein bilayers (Seoh and Busath, 1993a). A similar effect is observed with N-acetyl gramicidin channel molecules in spite of the modified forces at the dimeric junction (Seoh and Busath, 1993b). Here we use electrophysiological measurements with tryptophan-to-phenylalanine-substituted gramicidin analogs to show that the formamidinium-induced channel molecule stabilization is eliminated when the four gramicidin tryptophans are replaced with phenylalanines in gramicidin M-. This suggests that the stabilization is mediated by the tryptophan side chains. Tryptophan residues 9, 13, and 15 must cooperate to produce the effect because replacement of any one of the three with phenylalanine significantly reduces stabilization; replacement of Trp-11 with phenylalanine causes negligible decrease in stabilization. In addition, formamidinium-related current-voltage supralinearity and open-channel noise are absent with gramicidin M-. When the lipid bilayer was formed with monoolein ether rather than monoolein ester, the channel lifetimes were reduced markedly and, at low voltage and relative to those in KCl solution, were decreased by a factor of 2, whereas the open-channel noise was unaffected and the current-voltage relation was only modestly affected. These results suggest that formamidinium modifies the state of the tryptophan side chains, which, in turn, affects channel lifetime, current-voltage supralinearity, and open-channel noise through interactions with water or lipid headgroup atoms including the lipid ester carbonyl.

Influence of obstacles on lipid lateral diffusion: computer simulation of FRAP experiments and application to proteoliposomes and biomembranes

Fluorescence Recovery After Photobleaching experiments were simulated using a computer approach in which a membrane lipid leaflet was mimicked using a triangular lattice obstructed with randomly distributed immobile and non-overlapping circular obstacles. Influence of the radius r and area fraction c of these obstacles and of the radius R of the observation area on the relative diffusion coefficient D* (Eq. (1)) and mobile fraction M was analyzed. A phenomenological equation relating D* to r and c was established. Fitting this equation to the FRAP data we obtained with the probe NBD-PC embedded in bacteriorhodopsin/egg-PC multilayers suggests that this transmembrane protein rigidifies the surrounding lipid phase over a distance of about 18 A (approximately equal to two lipid layers) from the protein surface. In contrast, analysis of published diffusion constants obtained for lipids in the presence of gramicidin suggests that in terms of lateral diffusion, this relatively small polypeptide does not significantly affect the surrounding lipid phase. With respect to the mobile fraction M, and for point obstacles above the percolation threshold, an increase in R led to a decrease in M which can be associated with the existence of closed domains whose average size and diffusion properties can be determined. Adaptation of this model to the re-interpretation of the FRAP data obtained by Yechiel and Edidin (J Cell Biol (1987) 115:755-760) for the plasma membrane of human fibroblasts consistently leads to the suggestion that the lateral organization of this membrane would be of the confined type, with closed lipid domains of approximately equal to 0.5 microns 2 in area.

The "independence principle" in the processes of water transport

The processes of membrane transport exhibiting permeability coefficients depending on the species activities do not obey the "independence principle" and are assumed to take place by a mechanism of discrete nature, analyzable by a kinetic formalism. In this article, we study the dependence of the osmotic permeability coefficient on the water activities, from the steady-state analysis of a kinetic model of single-file water transport that simultaneously incorporates the vacancy-mediated and "knock-on" mechanisms into the state diagram. In particular, we study the relation between the near-equilibrium osmotic permeability (Pe) and the equilibrium water activity of the compartments (w). The analysis and numerical calculations performed for a simple case of the model show that, for values of the parameters consistent with experimental data, Pe exhibits only a small variation with w within the physiological range in the majority of the situations considered here. It is not possible to predict, from the study of these simple models, whether more complicated kinetic diagrams of water transport may be characterized by permeability coefficients with a more evident dependence on the water activities. Nevertheless, the results obtained here suggest that, for the case of physiological water pores, the analysis of the kinetic dependence of the permeability coefficients on the water activities may not yield evidence pointing to a discrete nature for the transport process.

Combination of the electrogenic ionophores, valinomycin and CCCP, can lead to non-electrogenic K+/H+ exchange on bilayer lipid membranes

The method of pH shift measuring by means of a pH microelectrode was applied to measure hydrogen ion fluxes across a planar bilayer lipid membrane (BLM) in the presence of the potassium ion ionophore, valinomycin, and a protonophore, carbonylcyanide m-chlorophenylhydrazone (CCCP), under conditions of the voltage clamp. The voltage dependence of the flux was determined to be in the range of +/- 150 mV under the conditions of both symmetrical KCl as well as a KCl gradient across the BLM. Surprisingly, at a clamped zero voltage on BLM a significant hydrogen ion flux was observed in the presence of a KCL gradient and both valinomycin and CCCP. This finding was interpreted as a result of induction of non-electrogenic K+/H+ exchange in the presence of valinomycin and CCCP, presumably through the formation of electrically neutral complexes of these two ionophores and K+ (H+) ions: valinomycin-K(+)-CCCP- and/or possibly valinomycin-CCCP(-)-H+.

Gravity effects on membrane processes

Application of the Gouy-Chapman-Debye-Hückel (GCDH) theory to a model membrane in contact with electrolytes of various concentrations and composition predict density variations within an interfacial layer. Assuming that on cellular dimensions hydrodynamics can be applied (the objections are briefly discussed) two types of gravity effects can be defined, 1. convection along the surface of vertically oriented membranes and 2. surface potential variations by layer deformations at horizontally oriented membranes. Both effects should affect transport across the layer to the membrane surface and across the membrane. According to the theoretical predictions first experiments with gramicidin channels incorporated into artificial phosphatidylserine bilayer membranes show a significant difference in single channel currents in vertical and horizontal membranes. The complexity of biological membrane functions requires investigation of isolated membrane surface reactions and transport systems to study the gravisensitivity for each process separately.

Partitioning of gramicidin A' between coexisting fluid and gel phospholipid phases

The partitioning behavior of gramicidin A' was investigated in four binary phospholipid mixtures with coexisting fluid and gel phases. The ratio of the equilibrium peptide concentration in the fluid phase to that in the gel phase (i.e., the partition coefficient, Kp) was determined by analysis of the quenching of gramicidin A' tryptophanyl fluorescence by a spin-labeled phosphatidylcholine. The partition coefficient was used as a measure of the relative solubility of gramicidin A' in the four types of gel phases analyzed. The composition of the gel phase was entirely Ca(dioleoylphosphatidylserine)2 (Ca(di18:1-PS)2), or was rich in either distearoylphosphatidylcholine (di18:0-PC), dipalmitoylphosphatidylcholine (di16:0-PC), or dimyristoylphosphatidylcholine (di14:0-PC). Except in the last case, the gel phase was depleted of gramicidin A': Kp approximately 30 when the gel phase was Ca(di18:1-PS)2 or di18:0-PC-rich, Kp approximately 10 when the gel phase was di16:0-PC-rich, and Kp approximately 1 when the gel phase was di14:0-PC-rich. The hydrophobic mismatch between the length of gramicidin A' and the length of the phospholipid acyl chains in the bulk gel phase is greatest with di18:1-PS and di18:0-PC, intermediate with di16:0-PC, and least with di14:0-PC. The Kp measurements presented here are consistent with increasing solubility of gramicidin A' in the gel phase with decreasing hydrophobic mismatch.