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K H Cheng

Publications and source records attributed to K H Cheng.

At least 37 records · Page 2Linked to original sources
PubMed1994

Detection and characterization of the onset of bilayer packing defects by nanosecond-resolved intramolecular excimer fluorescence spectroscopy.

Bilayer packing defects in binary dilinoleoylphosphatidylethanolamine and 1-palmitoyl-2-oleoylphosphatidylcholine (DLPE/POPC) lipid mixtures have been studied by the use of nanosecond-resolved intramolecular excimer fluorescence spectroscopy. Frequency-domain fluorescence intensity decays of dual-chain labelled dipyrenyl lipids of different chain lengths in DLPE/POPC mixtures were acquired at both the monomer (392 nm) and excimer (475 nm) emission channels and at 20 degrees C. On the basis of a new intramolecular excimer formation kinetic model, the extent of aggregation and the rotational mobility, in terms of the equilibrium constant of the monomer to aggregated state and the excimer association rate constant, respectively, of the intralipid pyrenes were calculated from the frequency-domain data. Within the range of 60-100% DLPE where bilayer defects are known to coexist with bilayer and non-bilayer states, a prominent peak in the equilibrium constant and a concomitant dip in the excimer association constant at approximately 80% DLPE were observed. Our nanosecond-resolved fluorescence results suggest that the intramolecular excimer kinetic parameters of dipyrenyl lipids are very sensitive to the onset of bilayer packing defects in lipid membranes. Moreover, the onset of bilayer defect state is characterized by the greater extent of aggregation and more hindered rotational mobility of the acyl chains as compared with the bilayer (0% DLPE) and non-bilayer inverted hexagonal (100% DLPE) states of the lipid membranes.

Kinetics↗
PubMed1994

Infrared study of the bilayer stability behavior of binary and ternary phospholipid mixtures containing unsaturated phosphatidylethanolamine.

The bilayer stability of liposomes containing unsaturated phosphatidylethanolamine (PE) has been investigated by measuring the C = O and CH2 stretching frequencies of the lipids at different lipid compositions and temperatures. Binary mixtures of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and dilinoleoyl-PE (DLPE) are known to exhibit lamellar liquid crystalline (L alpha), inverted hexagonal (HII) and metastable intermediate (I) phases. Abrupt increases in the C = O and CH2 stretching frequencies at 65-75 and 90-95% PE, respectively, were found as the PE content of the DLPE/POPC mixtures was increased from 0 to 100%. These transitions were associated with the L alpha -I and I-HII phase transitions of the DLPE/POPC mixtures, accordingly. The effects of three lipid perturbants, butylated hydroxytoluene (BHT), diacylglycerol (DG) and cholesterol (CL), on the above L alpha -I and I-HII transitions were also examined. All perturbants were found to be effective in shifting the L alpha -I transition of the DLPC/POPC mixtures to a lower PE% as detected by the C = O stretching frequency measurements. On the other hand, the perturbants appeared to eliminate the I-HII transition of the DLPE/POPC mixtures as detected by the CH2 stretching frequency measurements. The effectiveness of the perturbants in promoting the L alpha -I phase transition of the DLPE/POPC mixtures followed the order of DG > BHT > CL.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipid Bilayers↗
PubMed1994

In vivo tissue characterization of human brain by chisquares parameter maps: multiparameter proton T2-relaxation analysis.

The heterogeneous proton MR relaxation decay process in human brain has been investigated by performing region-of-interest and pixel-by-pixel calculations on the multiecho MR images with different repetition times (TR) of human brains using a clinical 1.5-T whole-body superconducting MR scanner. Based on the monoexponential, biexponential, and continuous gaussian distribution relaxation models, first-order proton relaxation parameters (proton density, T1 and T2) and higher-order transverse proton relaxation parameters (T2-long, T2-short, T2-long fraction, T2-average, and T2-distribution width) were calculated. On the basis of an F test (p < .01), the statistical significance of the higher-order (biexponential and distribution) fits over the monoexponential fit was evaluated. Here, a significant improvement in the biexponential fit was found for some of the regions containing the ventricular cerebrospinal fluid (CSF) (T2-long = 2780 +/- 570 ms; T2-short = 159 +/- 42 ms; T2-long fraction = 0.51 +/- 0.08 ms) due to the partial volume effect but not for most of the white matter (WM). On the other hand, an improvement of fit to WM was obtained when distribution (T2-average = 80 +/- 8 ms; T2-distribution half-width = 21 +/- 4 ms) as opposed to monoexponential (T2 = 89 +/- 10 ms) fit was used. As internal controls, tubes of CuSO4 solution (T2 = 1293 +/- 128 ms) and agarose gel (T2 = 111 +/- 10 ms) which have similar T2 values as the CSF and WM of the brain, respectively, were attached to the human head and imaged concomitantly. No significance improvements in either the biexponential or distribution fits over the monoexponential fit were found for all the controls. In addition to the first-order and higher-order relaxation parameter maps, the monoexponential chisquares, as well as the chisquares ratio (chisquares of the monoexponential fit divided by that of the higher-order fit), maps were also generated. Unlike the higher-order T2-relaxation parameter maps, the chisquares parameter maps required no selection of any predetermined statistical confidence level. Therefore, these chisquares parameter maps provided a somewhat nonsubjective spatial profile of the heterogeneous transverse relaxation process in the brain. Our results led us to propose that the use of chisquares parameter maps, together with the first-and higher-order relaxation parameter maps, may further improve the in vivo tissue characterization capability of MRI in future clinical diagnosis and staging of intracranial diseases.

Adult↗
PubMed1994

Intramolecular excimer kinetics of fluorescent dipyrenyl lipids: 1. DMPC/cholesterol membranes.

The intramolecular dynamics of the excimer forming dipyrenyl lipids (DipynPC) of different chain lengths (n) in ethanol and in dimyristoylphosphatidycholine (DMPC) membranes was investigated by the use of frequency-domain fluorescence intensity decay technique. Based on a 3-state model, the extent of aggregation and rotational rate of the two intralipid pyrene moieties in the dipyrenyl lipids were estimated from the frequency-domain data. In ethanol (20 degrees C), the rotational rate for DipynPC increased progressively as n was varied from 4 to 12. At the gel (L beta)-to-liquid crystalline (L alpha) phase transition of DMPC (approximately 23 degrees C), the rotational rate increased and aggregation decreased significantly for Dipy10PC, whereas only the rotational rate was changed for Dipy4PC. In the presence of 30 mol% cholesterol, significant increases in both the rotational rate and aggregation were observed for Dipy10PC in both L beta and L alpha phases. However, for the case of Dipy4PC, an increase in the rotational rate but a decrease in the aggregation were noticed only in the L beta phase, and no similar changes were detected in the L alpha phase. Our results indicate differential effects of cholesterol on the conformational dynamics of acyl chains at different depths of the membranes.

Cholesterol↗
PubMed1994

Intramolecular excimer kinetics of fluorescent dipyrenyl lipids: 2. DOPE/DOPC membranes.

The intramolecular dynamics of the excimer-forming dipyrenyl lipids (DipynPE) of different chain lengths (n) in fully hydrated dioleoylphosphatidylethanolamine (DOPE) and dioleoylphosphatidylcholine (DOPC) binary mixtures was investigated by the use of frequency-domain fluorescence intensity dcay technique. Using a 3-state model (see companion paper), the extent of aggregation and rotational rate of the two covalently attached pyrene moieties in DipynPE were estimated from the frequency-domain data. At 1 degrees C, the rotational rate and aggregation for Dipy4PE and Dipy10PE were insensitive to DOPE% of the lipid bilayer. At 27 degrees C, the rotational rate decreased, whereas the aggregation increased steadily for Dipy10PE as the DOPE% of the bilayer increased from 0 to 80. However, an abrupt increase in the rotational rate and a decrease in the aggregation for Dipy10PE were detected as the DOPE% reached 100, at which point the membranes are in the inverted hexagonal (HII) phase. No similar changes were found for Dipy4PE. These results indicate that the presence of PE with large intrinsic-curvature increases the lateral stress at the region near the center of the bilayer, and that this stress can be relieved as the membranes enter the highly curved HII phase.

Kinetics↗
PubMed1993

Quantitation of non-Einstein diffusion behavior of water in biological tissues by proton MR diffusion imaging: synthetic image calculations.

The non-Einstein diffusion behavior of water in a model biological tissue system, intact duck embryos, has been investigated by the use of an in vivo proton pulsed-gradient spin-echo (PGSE) MR imaging technique. Multiple-frame MR images of the intact duck embryos and control solution (0.5 mM CuSO4 doped water) were acquired systematically at different diffusion times and strengths of the diffusion-sensitizing magnetic field gradients of the PGSE sequence. These raw images were then used to generate various dynamic (self-diffusion coefficient) and structural (fractal, residual attenuation, and compartment fraction) diffusion parameter maps of water in the imaging objects on the basis of different Einstein and higher order (non-Brownian, Residual, and 2-compartment) diffusion models. The self-diffusion coefficients of the body tissues of the embryos obtained from all diffusion models were significantly lower than those of the surrounding embryonic fluid. The structural diffusion parameter maps obtained from the higher order diffusion models revealed that water molecules exhibited either non-Brownian, restricted, or compartmentalized diffusion behavior in the embryonic tissues, but Einstein or Brownian diffusion behavior in the embryonic fluid and control solution. The diffusion parameter maps, both dynamic and structural, were found to provide much better contrasts than the conventional relaxation time (T1, T2, and biexponential T2) maps in separating the tissues from the surrounding embryonic fluid in the duck embryos. The mathematical models and procedures for generating the dynamic and structural diffusion parameter maps are also presented in this paper.

Animals↗
PubMed1993

Transjugular intrahepatic portosystemic shunt. Requiem for the surgical portosystemic shunt?

Is transjugular intrahepatic portosystemic shunt (TIPS) preferable to a surgical shunting procedure in patients who are expected to benefit from a portal-systemic shunt? Since randomized trials comparing these procedures have not yet been reported, we attempted to define the present best therapeutic strategy by reviewing both the recent literature on TIPS and surgical shunting and our first experience with TIPS. The results suggest that TIPS is just as effective as surgical shunting but is associated with a lower morbidity and mortality. Procedure related deaths seem rare. In our series of 16 patients there was one death within 30 days. Seven early complications including stent dislodgement, early occlusion, encephalopathy and haemolysis were noted. The incidence of long-term complications, especially encephalopathy and shunt occlusion, seems comparable for both shunting procedures. Major advantages of TIPS are its therapeutic efficacy in patients with ascites and the fact that the technical difficulties of performing liver transplantation are not increased. We conclude that TIPS, performed by an experienced team, is at present the procedure of choice in patients who are candidates for a portal-systemic shunt, especially in patients in whom liver transplantation is a future option. Surgical shunts can be reserved for patients in whom TIPS is not feasible or has failed.

Ascites↗
PubMed1992

Magnetic resonance diffusion imaging detects structural damage in biological tissues upon hyperthermia.

The use of quantitative nuclear magnetic resonance (MR) imaging to investigate the extent and mechanism of hyperthermic damage in biological tissues has been studied. By using the multiple delay-multiple echo and pulsed-gradient spin echo MR imaging sequences, multiple frame MR images of freshly harvested rabbit tissues (brain, kidney, and muscle) and intact duck embryos in shells were obtained before and after heat treatment (45 degrees C for 30 min) using a clinical 1.5-Tesla whole-body superconducting MR scanner. Based on the relaxation and diffusion models, maps of the proton spin density, relaxation times, and various self-diffusion parameters of tissue water were generated from these multiple frame MR images. Our results indicated that the values of the diffusion barrier size and fractal parameter of the tissues and the self-diffusion coefficient of tissue water increased significantly, i.e., approached that of free water, after the heat treatment. In comparison, only slight changes in the spin density and relaxation times of the tissue water were found after the identical heat treatment. We concluded that the significant changes in the self-diffusive behavior of the tissue water are due to the denaturation of macromolecules (e.g., protein and fiber) within the tissues at elevated temperatures. We further suggested that MR diffusion imaging represents a powerful tool to investigate the extent and mechanism of heat damage of biological tissues in vivo and therefore bears important potential in the clinical assessment of the therapeutic efficacy of hyperthermia in cancer therapy.

Animals↗
PubMed1992

Inactivation of calcium uptake by EGTA is due to an irreversible thermotropic conformational change in the calcium binding domain of the Ca(2+)-ATPase.

Calcium uptake by rabbit skeletal sarcoplasmic reticulum (SR) is inhibited with an effective inactivation temperature (TI) of 37 degrees C in EGTA with no effect on ATPase activity. Since the Ca-ATPase denatures at a much higher temperature (49 degrees C) in EGTA, this suggests that a small or localized conformational change of the Ca-ATPase at 37 degrees C results in inability to accumulate calcium by the SR. Using a fluorescent analogue of dicyclohexylcarbodiimide, N-cyclohexyl-N'-[4-(dimethylamino)-alpha-naphthyl]-carbodiimide (NCD-4), the region of the calcium binding sites of the SR Ca-ATPase was labeled. Steady-state and frequency-resolved fluorescence measurements were subsequently performed on the NCD-4-labeled Ca-ATPase. Site-specific information pertaining to the hydrophobicity and segmental flexibility of the region of the calcium binding sites was derived from the steady-state fluorescence intensity, lifetime, and rotational rate of the covalently bound NCD-4 label as a function of temperature (0-50 degrees C). A reversible transition at approximately 15 degrees C and an irreversible transition at approximately 35 degrees C were deduced from the measured fluorescence parameters. The low-temperature transition agrees with the previously observed break in the Arrhenius plot of ATPase activity of the native Ca-ATPase at 15-20 degrees C. The high-temperature transition conforms well with the conformational transition, resulting in uncoupling of Ca translocation from ATP hydrolysis as predicted from the irreversible inactivation of Ca uptake at 31-37 degrees C in 1 mM EGTA.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Quantitation of lateral stress in lipid layer containing nonbilayer phase preferring lipids by frequency-domain fluorescence spectroscopy.

Frequency-resolved fluorescence measurements have been performed to quantitate the lateral stress of the lipid layer containing nonbilayer phase preferring dioleoylphosphatidylethanolamine (DOPE). On the basis of a new rotational diffusion model, the wobbling diffusion constant (Dw), the curvature-related hopping diffusion constant (DH), and the two local orientational order parameters ([P2] and [P4]) of 1-palmitoyl-2-[[2-[4-(6-phenyl-trans-1,3,5-hexatrienyl)phenyl]ethyl] carbonyl]-3-sn-phosphatidylcholine (DPH-PC) in fully hydrated DOPE and DOPE/dioleoylphosphatidylcholine (DOPC) mixtures were calculated from the frequency-domain anisotropy data. The values of [P2], [P4], and DH for DOPE were found to increase significantly at approximately 12 degrees C, the known lamellar liquid crystalline (L alpha) to inverted hexagonal (HII) phase transition temperature of DOPE. Similar features as well as a decline of Dw were detected in the DOPE/DOPC mixtures as the DOPE content was increased from 85% to 90% at 23 degrees C, corresponding to the known lyotropic phase transition of the DOPE/DOPC. In contrast, for DOPC (0-40 degrees C) and DOPE/DOPC (0-100% DOPE at 3 degrees C), which remained in the L alpha phase, these changes were not detected. The most probable local orientation of DPH-PC in the DOPE/DOPC mixtures shifted progressively toward the normal of the lipid/water interface as the content of DOPE increased. We concluded that the curvature-related lateral stress in the lipid layer increases with the content of the nonbilayer phase preferring lipids.

Diffusion↗
PubMed1992

Activation energy and entropy for intramolecular excimer formation in a dipyrenylphosphatidylcholine probe in lamellar and hexagonal lipid phases.

Intramolecular excimer formation in pyrene-labeled phosphatidylcholine was used as a tool to determine thermodynamic characteristics of the lamellar to hexagonal phase transitions in a binary lipid system dilinoleoylphosphatidylethanolamine (DLPE)/palmitoyloleoylphosphatidylcholine (POPC). Upon an L alpha/HII phase transition, the activation energy Ea for excimer formation increased from 5.6 +/- 0.2 kcal/mol to 6.3 +/- 0.2 kcal/mol, while the activation entropy delta S decreased from -40.0 +/- 0.8 cal/K.mol to -38.4 +/- 0.8 cal/K.mol. The results are consistent with the idea of molecular splaying of the acyl chains in the hexagonal phase. It is estimated that the molecular area at the terminal carbon of the lipid acyl chains increases by a factor of 2.2 upon the L alpha HII transition in DLPE/POPC.

Lipids↗
PubMed1991

Improvement of visual acuity and corneal physiology in keratoconus by fitting aspherical, high oxygen-permeable contact lenses.

Keratoconus is a condition in which the cornea assumes a complex irregular curvature caused by central corneal thinning. The abnormal topography of the cornea in combination with central corneal scarring results in an impaired visual acuity. Even in mild cases spectacles do not correct vision adequately. The use of hard contact lenses with a spherical geometry in the past has already given a marked therapeutic improvement. The use of these lenses however, is complicated by hypoxia and mechanical trauma of the cornea. These complications could theoretically be avoided by fitting elliptical lenses with a high oxygen transmission. To investigate this hypothesis we compared low oxygen-permeable spherical lenses with high oxygen-permeable elliptical contact lenses in a group of twenty patients with mild keratoconus. Our results showed both a marked subjective and objective visual improvement after fitting elliptical lenses with a low incidence of complications.

Adult↗
PubMed1991

Intramolecular excimer formation of pyrene-labeled lipids in lamellar and inverted hexagonal phases of lipid mixtures containing unsaturated phosphatidylethanolamine.

The rates of intramolecular excimer formation of di(1'-pyrenemyristoyl)phosphatidylcholine (dipyPC) in dioleoylphosphatidyl-ethanolamine (DOPE), egg PE/diolein (DG) and dilinoleoyl-PE (DLPE)/1-palmitoyl-2-oleoyl-PC (POPC) were studied at different temperatures and lipid compositions. Both the excimer-to-monomer intensity ratio and the excimer association rate constant were employed to quantify the rate of excimer formation. The latter was calculated from the measured monomer fluorescence lifetime of dipyPC. We observed that the rate of excimer formation was sensitive to either the temperature-induced or lipid composition-induced lamellar-to-inverted hexagonal phase transition of the above lipid systems. As the lipids entered the inverted hexagonal phase, the rate of excimer formation increased at the temperature-induced phase transition for DOPE, but decreased at the composition-induced phase transition for both TPE/DG and DLPE/POPC systems by increasing the DG% and decreasing the PC%, respectively. We conclude that the rate of intramolecular excimer formation of dipyPC in the non-lamellar phase is influenced both by the intra-lipid free volume of the hydrocarbon region and the intra-rotational dynamics of the two lipid acyl chains.

Chemical Phenomena↗
PubMed1990

Thermal denaturation of the Ca2(+)-ATPase of sarcoplasmic reticulum reveals two thermodynamically independent domains.

Inactivation of Ca2+ uptake and ATPase activity of the Ca2(+)-ATPase of rabbit sarcoplasmic reticulum was measured and compared to the thermal denaturation of the enzyme as measured by differential scanning calorimetry (DSC) and fluorescence spectroscopy. Two fluorophores were monitored: intrinsic tryptophan (localized in the transmembrane region) and fluorescein isothiocyanate (FITC)-labeled Lys-515 (located in the nucleotide binding domain). Inactivation, defined as loss of activity, and denaturation, defined as conformational unfolding, were irreversible under the conditions used. Activation energies (EA) and frequency factors (A) for inactivation were obtained for the enzyme in 1 mM EGTA and 1 mM Ca2+. These were transformed to a transition temperature for inactivation, Tm (defined as the temperature of half-inactivation when temperature is scanned upward at 1 degree C/min). All denaturation profiles were fit with an irreversible model to obtain EA and Tm for each transition, and the values of these parameters for denaturation were compared to the values for inactivation. In EGTA, denaturation obeys a single-step model (Tm = 49 degrees C), but a two-step model is required to fit the DSC provile of the enzyme in 1 mM Ca2+. The specific locations of tryptophan and the fluorescein label were used to demonstrate that denaturation in Ca2+ occurs through two distinct thermodynamic domains. Domain I (Tm = 50 degrees C) consists of the nucleotide binding region and most likely the phosphorylation and transduction regions [MacLennan, D. H., Brandl, C. J., Korczak, B., & Green, N. M. (1985) Nature 316, 696-700].(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Lateral diffusion study of excimer-forming lipids in lamellar to inverted hexagonal phase transition of unsaturated phosphatidylethanolamine.

Using multi-frequency cross-correlation fluorometry, the monomer fluorescence lifetime of 1-palmitoyl-2-[10-(1-pyrenyl)decanoyl)phosphatidylcholine (Py-PC) was employed to determine the lateral diffusion constant (DT) of dioleoylphosphatidylethanolamine (DOPE) in both the lamellar (L alpha) and the inverted hexagonal (HII) phases. The values of DT increased with temperature in both phases. However, the rate of increase of DT declined abruptly at approximately 10-13 degrees C (L alpha -HII transition temperature), as indicated by the existence of an inflection point in the log (DT/T) vs. 1/T plot. This observation suggests that the translational motion of lipids in the HII phase is lower than that in the L alpha phase upon temperature extrapolation. Lipid perturbants, cholesterol and diacylglycerol, were found to destabilize the L alpha phase of DOPE. This was demonstrated by a down-shift of the inflection point in the log(DT/T) vs. 1/T plot in the presence of the perturbants. Both cholesterol and 1,2-dioleoyl-sn-glycerol (diolein) decreased the lateral diffusion constant in both phases. Diolein promoted the HII phase more effectively than did the cholesterol. This is explained by an intrinsic wedge-shape geometry of diolein which strongly favors the formation of inverted cylindrical packing of the lipids.

Chemical Phenomena↗
PubMed1990

Headgroup hydration and motional order of lipids in lamellar liquid crystalline and inverted hexagonal phases of unsaturated phosphatidylethanolamine--a time-resolved fluorescence study.

By the use of frequency domain cross-correlation fluorometry, the fluorescence lifetime of the water soluble probe 8,1-anilinonapthalene sulfonic acid (ANS) in aqueous dispersions of dioleoylphosphatidylethanolamine (DOPE) and phosphatidylethanolamine transphosphatidylated from egg phosphatidylcholine (TPE) was measured. The orientational order parameter and rotational diffusion constant of the lipophilic probe 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) were also determined in TPE dispersions. In agreement with a previous study on DOPE (Cheng (1989) Biophys. J. 55, 1025-1031), abrupt changes in both the order packing and rotational diffusion constant were found at the lamellar liquid crystalline (L alpha) to inverted hexagonal (HII) phase transition of TPE. Owing to the subnanosecond resolution capability of this frequency domain fluorometric technique, the heterogeneous fluorescence decay of ANS was resolved into three distinct components with different decay lifetimes (tau's). They were 0 less than tau less than 0.5 ns, 2 less than tau less than 9 ns and tau greater than 15 ns. These lifetime regions were attributed to the partitioning of ANS into the bulk aqueous medium, the lipid/water interface and the lipid hydrocarbon region, respectively. These classifications of lifetime regions were further supported by the sensitivity of those lifetime components with the solvent isotopic shift of D2O. Similar to the changes of orientational order and rotational diffusion of lipophilic probe, the lifetime and intensity fraction of ANS associated with the lipid/water interfacial region declined abruptly at the L alpha-HII transition of both DOPE and TPE. This observation suggested that a dehydration of the lipid headgroup surface occurs at the L alpha-HII transition. This study provided evidence that both the lipid headgroup surface hydration and the lipid dynamics change drastically as a result of the macroscopic rearrangement of lipids at the L alpha-HII transition.

Anilino Naphthalenesulfonates↗
PubMed1990

Infrared and time-resolved fluorescence spectroscopic studies of the polymorphic phase behavior of phosphatidylethanolamine/diacylglycerol lipid mixtures.

Fourier transform infrared (FTIR) and time-resolved fluorescence spectroscopy have been employed to examine the structural dynamics of lipid fatty acyl chains and lipid/water interfacial region of a binary lipid mixture containing unsaturated phosphatidylethanolamine (PE) and diacylglycerol (DG). Infrared vibrational frequencies of the CH2 symmetric stretching and the C = O stretching bands of the lipids were measured at different lipid compositions and temperatures. For 0% DG, the lamellar gel to lamellar liquid crystalline (L beta-L alpha) and the L alpha to inverted hexagonal (L alpha-HII) phase transitions were observed at approximately 15 degrees and 55 degrees C, respectively. As the DG content increased gradually from 0% to 15%, the L alpha-HII phase transition temperature decreased drastically while the L beta-L alpha phase transition temperature decreased only slightly. At 10% DG, a merge of these two phase transitions was noticed at approximately 10 degrees C. For the composition study at 23 degrees C, the L alpha-HII transition occurred at approximately 6-10% DG as indicated by abrupt increases in both the CH2 and C = O stretching frequencies at those DG contents. Using time-resolved fluorescence spectroscopy, abrupt decreases in both the normalized long time residual and the initial slope of the anisotropy decay function of lipid probes, 1-palmitoyl-2-[[2-[4-(6-phenyl-trans-1,3,5- hexatrienyl)phenyl]ethyl]carbonyl]-3-sn-phosphatidylcholine, in these PE/DG mixtures were observed at the L alpha-HII phase transition. These changes in the anisotropy decay parameters suggested that the rotational dynamics and orientational packing of the lipids were altered at the composition-induced L alpha-HII transition, and agreed with a previous temperature-induced L alpha-HII transition study on pure unsaturated PE (Cheng (1989) Biophys. J. 55, 1025-1031). The fluorescence lifetime of water soluble probes, 8,1-anilinonapthalenes sulfonate acid, in PE/DG mixtures increased abruptly at the L alpha-HII phase transition, suggesting that the conformation and hydration of the lipid/water interfacial region also undergo significant changes at the L alpha-HII transition.

Diglycerides↗