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Under ultrasonic irradiation, 1,2-dihexadecyl-sn-glycero-3-phosphorylcholine (DHDPC) forms microvesicular liposomes in D20. These liposomes have been widely used as a model of a cell membrane.We have studied the interaction of some general anesthetics, i.e., chloroform, halothane, methoxyflurane, and enflurane, with synthetic DHDPC-D2O liposomes by Fourier transform proton magnetic resonance (PMR). Our previous work (1) showed that chloroform and halothane interacted with DHDPC-D20 liposomes (about 1.5-2% wt/wt) that had not been sonicated or that had been sonicated in an ice-water bath (sonication temperature, 70). The choline group was fluidized first, and the motional narrowing of the fatty acid (hexadecanoic acid) methyl and methylene groups was observed after addition of larger quantities of anesthetics at ambient temperature (28-30'). The result was compared with the temperature effect on the liposomes (2, 3). Chloroform and halothane acted on the liposomes and caused a phase transition (from crystalline to liquid-crystalline phase). The phase transition temperature occurred at 42°for the unsonicated liposomes and around 400 (in a range) for the sonicated liposomes (2, 3). The PMR intensity and linewidth below the phase transition temperature depend on the vesicle size.The DHDPC-D20 liposomes, whether or not they have been sonicated in an ice-water bath, show only the choline peak at 290 with the addition of methoxyflurane (>0.15 M). The linewidth of the unsonicated sample was about 30 Hz; that of the sonicated sample was about 15 Hz. The fatty acid methyl and methylene groups are only appreciably observable (line-

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A pH-indicator dye, bromothymol blue, was used to probe the hydrophilic surface of dimyristoyl-, dipalmitoyl-, and distearoylphosphatidylcholine bilayer vesicles. The apparent pK of the surface-adsorbed dye was larger than the bulk pK value. The contribution of the choline positive charge on the dissociation constant of the dye adsorbed on the vesicle surface was estimated by screening the charge interaction with 2 M KC1. The effective surface potentials interacting with the dye were thus estimated to be 33.2, 45.6, and 46.8 mV, respectively, for the dimyristoyl-, dipalmitoyl-, and distearoylphosphatidylcholine vesicles. From the differences between the obtained effective potentials and the calculated surface potentials of the charge-determining plane of the choline head, the distances between the prototropic part of the dye and the choline charge-determining plane were estimated to be 10.5, 8.0, and 7.8 A, respectively. These values were obtained at 250C; the dimyristoylphosphatidylcholine membrane was in the liquid-crystalline phase and the other two were in the solid gel phase. Addition of an inhalation anesthetic, enflurane, decreased the distance in the dimyristoylphosphatidylcholine vesicles and increased the distance in the dipalmitoyl-and distearoylphosphatidylcholine vesicles. The increase of precessional motion of choline head by the inhalation anesthetic is apparently responsible for the changes.Fluidizing and disordering effects of anesthetics upon phospholipid membranes are well documented (1-4). We have previously reported (5, 6), by a proton nuclear magnetic resonance study, that inhalation anesthetics at clinical tensions increased the signal strength of the choline methyl protons of the dipalmitoylphosphatidylcholine vesicles without noticeable effects upon the hydrcarbon-chain protons. It was concluded that the primary effect of inhalation anesthetics at clinical tensions on the phospholipid membrane is directed to the interfacial hydrophilic part of the membrane.In the present study, a pH-indicator dye, bromothymol blue, was used to measure conformational change of the hydrophilic head group of zwitterionic phosphatidylcholine vesicles. Bromothymol blue is a negatively charged weak electrolyte and binds to the phospholipid vesicles presumably between the hydrophilic head group and the hydrophobic tails (see, for instance, ref. 7) without penetrating into the strongly hydrophobic membrane core. The chromophore reports physical characteristics of the interfacial region by changing its color. The pH-indicator dyes have been used for the study of surface potential of micells, phospholipid vesicles, and proteins (7-9).The color of pH-indicator dyes adsorbed to interfaces is different from that in the bulk solution. This is partly due to the electrostatic effect of the surface charges on the local proton activity at the interfacial region (10). According to Boltzmann's law, the hydrogen ion activity, aH+, near the surface charge I'0, is related to the bulk hydrogen ion activity by aH+s = aH+...

Hydrogen bond strengths in terms of the proton chemical shifts of five potent inhalation anesthetics containing acidic hydrogen were measured in cyclohexane and in methanol using the proton magnetic resonance spectroscopic method. The purpose of this study is to quantitatively compare the relative polar character of potent anesthetics. The hydrogen bond shift (delta ppm) of each anesthetic is the difference in the chemical shifts of the infinitely diluted unassociated anesthetic in cyclohexane and that of the infinitely diluted hydrogen bonded anesthetic in methanol. It was found that the hydrogen bond shifts (in delta ppm) are as follows: methoxyflurane, 0.72; chloroform, 0.75; halothane, 1.06; isoflurane, 1.38; enflurane, 1.44. There is a good correlation between the hydrogen bond shifts and the clinical potencies (minimum alveolar concentration in man). The conclusion from this study is that the acidic halogenated inhalation anesthetics are more potent if they form weaker hydrogen bonds.