The Interfacial Properties of ApoA-I and an Amphipathic α-Helix Consensus Peptide of Exchangeable Apolipoproteins at the Triolein/Water Interface (2004)

Apolipoprotein A-I (apoA-I) is the major protein in high density lipoprotein (HDL). During lipid metabolism, apoA-I moves among HDL and triacylglycerol-rich lipoproteins. The main structure and the major lipid binding motif of apoA-I is the amphipathic α-helix. To understand how apoA-I behaves at hydrophobic lipoprotein interfaces, the interfacial properties of apoA-I and an amphipathic α-helical consensus sequence peptide (CSP) were studied at the triolein/water (TO/W) interface. CSP ((PLAEELRARLRAQLEELRERLG)2-NH2) contains two 22-residue tandem repeat sequences that form amphipathic α-helices modeling the central part of apoA-I. ApoA-I or CSP added into the aqueous phase surrounding a triolein drop lowered the interfacial tension (γ) of TO/W in a concentration- and time-dependent fashion. The γTO/W was lowered ∼16 millinewtons (mN)/m by apoA-I at 1.4 × 10–6 m and ∼15 mN/m by CSP at 2.6 × 10–6 m. At equilibrium γ, both apoA-I and CSP desorbed from the interface when compressed and readsorbed when expanded. The maximum surface pressure CSP could withstand without being ejected (ΠMAX) was 16 mN/m. The ΠMAX of apoA-I was only 14.8 mN/m, but re-adsorption kinetics suggested that only part of the apoA-I desorbed at Π between 14.8 and 19 mN/m. However, above ∼19 mN/m (ΠOFF) the entire apoA-I molecule desorbed into the water. ApoA-I was more flexible at the TO/W interface than CSP and showed more elasticity at oscillation periods 4–128 s even at high compression, whereas CSP was elastic only at faster periods (4 and 8 s) and moderate compression. Flexibility and surface pressure-mediated desorption and re-adsorption of apoA-I probably provides lipoprotein stability during metabolic-remodeling reactions in plasma. Apolipoprotein A-I (apoA-I) is the major protein in high density lipoprotein (HDL). During lipid metabolism, apoA-I moves among HDL and triacylglycerol-rich lipoproteins. The main structure and the major lipid binding motif of apoA-I is the amphipathic α-helix. To understand how apoA-I behaves at hydrophobic lipoprotein interfaces, the interfacial properties of apoA-I and an amphipathic α-helical consensus sequence peptide (CSP) were studied at the triolein/water (TO/W) interface. CSP ((PLAEELRARLRAQLEELRERLG)2-NH2) contains two 22-residue tandem repeat sequences that form amphipathic α-helices modeling the central part of apoA-I. ApoA-I or CSP added into the aqueous phase surrounding a triolein drop lowered the interfacial tension (γ) of TO/W in a concentration- and time-dependent fashion. The γTO/W was lowered ∼16 millinewtons (mN)/m by apoA-I at 1.4 × 10–6 m and ∼15 mN/m by CSP at 2.6 × 10–6 m. At equilibrium γ, both apoA-I and CSP desorbed from the interface when compressed and readsorbed when expanded. The maximum surface pressure CSP could withstand without being ejected (ΠMAX) was 16 mN/m. The ΠMAX of apoA-I was only 14.8 mN/m, but re-adsorption kinetics suggested that only part of the apoA-I desorbed at Π between 14.8 and 19 mN/m. However, above ∼19 mN/m (ΠOFF) the entire apoA-I molecule desorbed into the water. ApoA-I was more flexible at the TO/W interface than CSP and showed more elasticity at oscillation periods 4–128 s even at high compression, whereas CSP was elastic only at faster periods (4 and 8 s) and moderate compression. Flexibility and surface pressure-mediated desorption and re-adsorption of apoA-I probably provides lipoprotein stability during metabolic-remodeling reactions in plasma. Apolipoprotein A-I (apoA-I) 1The abbreviations used are: apoA-I, apolipoprotein A-I; HDL, high density lipoprotein; TAG, triacylglycerol; TO/W, triolein/water; VLDL, very low density lipoprotein; DMPC, dimyristoyl phosphatidylcholine; A/W, air/water; PC, phosphatidylcholine; DD/W, dodecane/water; apoA-IV/A-IV, apolipoprotein A-IV; apoE-3, apolipoprotein E-3; AβS, amphipathic β strands; CSP, consensus sequence peptide; mN, millinewton(s).1The abbreviations used are: apoA-I, apolipoprotein A-I; HDL, high density lipoprotein; TAG, triacylglycerol; TO/W, triolein/water; VLDL, very low density lipoprotein; DMPC, dimyristoyl phosphatidylcholine; A/W, air/water; PC, phosphatidylcholine; DD/W, dodecane/water; apoA-IV/A-IV, apolipoprotein A-IV; apoE-3, apolipoprotein E-3; AβS, amphipathic β strands; CSP, consensus sequence peptide; mN, millinewton(s). is the major protein of high density lipoprotein (HDL) (1Atkinson D. Small D.M. Annu. Rev. Biophys. Biophys. Chem. 1986; 15: 403-456Crossref PubMed Scopus (135) Google Scholar). It promotes cholesterol efflux from tissues to the liver for excretion and is a cofactor for lecithin:cholesterol acyltransferase (LCAT), which is responsible for the formation of most plasma cholesterol esters. ApoA-I exchanges off HDL onto other lipoproteins like very low density lipoprotein (VLDL) and moves back and forth among lipoprotein classes in the course of its metabolism. Thus the conformational adaptability of apoA-I to the polar environment in plasma and the apolar environment on lipoprotein particles is essential for their function. The predominant secondary structure feature of apoA-I is the 11/22-mer tandem repeat amino acid segments with high propensity to form amphipathic α-helices (1Atkinson D. Small D.M. Annu. Rev. Biophys. Biophys. Chem. 1986; 15: 403-456Crossref PubMed Scopus (135) Google Scholar, 2Segrest J.P. Jackson R.L. Morrisett J.D. Gotto Jr., A.M. FEBS Lett. 1974; 38: 247-253Crossref PubMed Scopus (485) Google Scholar, 3Segrest J.P. Jones M.K. DeLoof H. Brouilette C.G. Venkatachalapathi Y.V. Anantharamaiah G.M. J. Lipid Res. 1992; 33: 141-166Abstract Full Text PDF PubMed Google Scholar). Many of the amphipathic α-helices in apoA-I are class A amphipathic α-helices, which have a large (30–50%) apolar face with the positive residues distributed in the polar-nonpolar interface and are highly suited to lipid binding. This structure is thought to serve as the lipid association motif of apoA-I and to stabilize the lipoprotein surface. ApoA-I exists in at least three states during metabolism: lipid-poor or free in solution, bound to a discoidal nascent HDL assembled with phospholipids and free cholesterol in a bilayer, or at the surface of a spherical lipoprotein particle (HDL or VLDL) with an apolar core of cholesterol esters and triglycerides, and a surface of phospholipids and free cholesterol (4Small D.M. Kreisberg R.A. Segrest J.P. Current Issues in Endocrinology and Metabolism: Plasma Lipoproteins and Coronary Artery Disease. Blackwell Scientific Publications, Boston, MA1992: 57-91Google Scholar). It is believed that apoA-I interacts primarily with the phospholipid hydrocarbon chains on discoidal HDL, and it is possible that it also interacts with the hydrophobic core of HDL particles and with the hydrophobic core of VLDL. Intensive studies (1Atkinson D. Small D.M. Annu. Rev. Biophys. Biophys. Chem. 1986; 15: 403-456Crossref PubMed Scopus (135) Google Scholar, 5Tall A.R. Small D.M. Deckelbaum R.J. Shipley G.G. J. Biol. Chem. 1977; 252: 4701-4717Abstract Full Text PDF PubMed Google Scholar, 6Pownall H.J. Massey J.B. Kusserow S.K. Gotto Jr., A.M. Biochemistry. 1979; 181: 574-579Crossref Scopus (83) Google Scholar, 7Brouillette C.G. Anantharamaiah G.M. Biochim. Biophys. Acta. 1995; 1256: 103-129Crossref PubMed Scopus (165) Google Scholar, 8Brouillette C.G. Anantharamaiah G.M. Engler J.A. Borhani D.W. 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Biochemistry. 1979; 181: 574-579Crossref Scopus (83) Google Scholar, 7Brouillette C.G. Anantharamaiah G.M. Biochim. Biophys. Acta. 1995; 1256: 103-129Crossref PubMed Scopus (165) Google Scholar, 8Brouillette C.G. Anantharamaiah G.M. Engler J.A. Borhani D.W. Biochim. Biophys. Acta. 2001; 1531: 4-46Crossref PubMed Scopus (223) Google Scholar). Sonication or chaotropic agents are required to form quasi-spherical HDL-like particles with apoA-I, phospholipid, and cholesterol ester (9Scanu A.M. Biochim. Biophys. Acta. 1972; 265: 471-508Crossref Scopus (112) Google Scholar, 10Ritter M.C. Scanu A.M. J. Biol. Chem. 1977; 252: 1200-1216Abstract Full Text PDF Google Scholar, 11Pittman R.C. Glass C.A. Atkinson D.A. Small D.M. J. Biol. Chem. 1987; 262: 2435-2442Abstract Full Text PDF PubMed Google Scholar). Deletion mutations of apoA-I (12Fang Y. Gursky O. Atkinson D. Biochemistry. 2003; 42: 13260-13268Crossref PubMed Scopus (50) Google Scholar, 13Gorshkova I.N. Liu T. Zannis V.I. Atkinson D. Biochemistry. 2002; 41: 10529-10539Crossref PubMed Scopus (39) Google Scholar, 14Saito H.S. Dhanasekaran P. Nguyen D. Holvoet P. Lund-Katz S. Phillips M.C. J. Biol. Chem. 2003; 278: 23227-23232Abstract Full Text Full Text PDF PubMed Scopus (161) Google Scholar), synthetic peptides encompassing specific regions in apoA-I (15Palgunachari M.N. Mishra V.K. Lund-Katz S. Phillip M.C. Adeyeye S.O. Alluri S. Anantharamaiah G.M. Segrest J.P. Arterioscler. Thromb. Vasc. Biol. 1996; 16: 328-338Crossref PubMed Scopus (201) Google Scholar), and "idealized" designed synthesized model peptides of apoA-I (16Chao, Y. (2002) Conformational Studies of a Consensus Sequence Peptide (CSP) and a Real Sequence Peptide (RSP) of Apolipoproteins by Circular Dichroism and X-ray Crystallography. Ph.D. thesis, Department of Physiology and Biophysics, Boston University School of Medicine, Boston, MAGoogle Scholar) have been used in studies to estimate the lipid binding function of different regions of apoA-I. It is generally agreed that the C-terminal region of apoA-I (185–243 amino acids) has the highest affinity for lipid and plays a critical role in initiating the lipid binding. The N-terminal amphipathic region (residues 44–65) also has a higher affinity for lipids (17Zhu H. Atkinson D. Biochemistry. 2004; 43: 13156-13164Crossref PubMed Scopus (34) Google Scholar), whereas the middle region of the protein has a significant but lower affinity (15Palgunachari M.N. Mishra V.K. Lund-Katz S. Phillip M.C. Adeyeye S.O. Alluri S. Anantharamaiah G.M. Segrest J.P. Arterioscler. Thromb. Vasc. Biol. 1996; 16: 328-338Crossref PubMed Scopus (201) Google Scholar). To understand how apolipoproteins behave at the lipoprotein surface, extensive studies have been carried out on apolipoproteins interacting with phospholipid monolayers spread at the air/water (A/W) interface (15Palgunachari M.N. Mishra V.K. Lund-Katz S. Phillip M.C. Adeyeye S.O. Alluri S. Anantharamaiah G.M. Segrest J.P. Arterioscler. Thromb. Vasc. Biol. 1996; 16: 328-338Crossref PubMed Scopus (201) Google Scholar, 18Phillips M.C. Krebs K.E. Methods Enzymol. 1986; 128: 387-403Crossref PubMed Scopus (54) Google Scholar, 19Krebs K.E. Phillips M.C. Sparkes C.E. Biochim. Biophys. Acta. 1983; 751: 470-473Crossref PubMed Scopus (23) Google Scholar, 20Shen B.W. Scanu A.M. Biochemistry. 1980; 19: 3643-3650Crossref PubMed Scopus (25) Google Scholar, 21Krebs K.E. Ibdah J.A. Phillips M.C. Biochim. Biophys. Acta. 1988; 959: 229-237Crossref PubMed Scopus (47) Google Scholar, 22Ibdah J.A. Phillips M.C. Biochemistry. 1988; 27: 7155-7162Crossref PubMed Scopus (70) Google Scholar, 23Ibdah J.A. Lund-Katz S. Phillips M.D. Biochemistry. 1989; 28: 1126-1133Crossref PubMed Scopus (85) Google Scholar, 24Ibdah J.A. Krebs K.E. Phillips M.C. Biochim. Biophys. Acta. 1989; 1004: 300-308Crossref PubMed Scopus (54) Google Scholar, 25Weinberg R.B. Ibdah J.A. Phillips M.C. J. Biol. Chem. 1992; 267: 8977-8983Abstract Full Text PDF PubMed Google Scholar, 26Weinberg R.B. Cook V.R. DeLozier J.A. Shelness G.S. J. Lipid Res. 2000; 41: 1419-1427Abstract Full Text Full Text PDF PubMed Google Scholar). As one could expect, apolipoproteins (apoA-I, apoA-II, apoC-I, apoC-II, apoA-IV, and apoE) are all surface-active and lower the surface energy. Exclusion pressures (Πe) at which the apoproteins could no longer penetrate into the phospholipids monolayer have been measured. For instance, apoA-I has an Πe of ∼33 mN/m (18Phillips M.C. Krebs K.E. Methods Enzymol. 1986; 128: 387-403Crossref PubMed Scopus (54) Google Scholar) at the egg phosphatidylcholine (PC) A/W interface, whereas the Πe of apoA-II is 34 mN/m and that of apoA-IV is 29 mN/m at the egg PC A/W interface. The Πe of synthesized 8 tandem-repeating 22-mer domains of apoA-I at egg PC A/W interface has also been measured (15Palgunachari M.N. Mishra V.K. Lund-Katz S. Phillip M.C. Adeyeye S.O. Alluri S. Anantharamaiah G.M. Segrest J.P. Arterioscler. Thromb. Vasc. Biol. 1996; 16: 328-338Crossref PubMed Scopus (201) Google Scholar). Among the 22-mers, the N- and C-terminal peptides (44–65 amino acids and 220–241 amino acids) exhibited the highest Πe (28 mN/m and 30 mN/m), whereas other central peptides exhibited lower Πe values (<23 mN/m). A number of consensus sequences of amphipathic α-helices modeling different sequences of apolipoproteins (7Brouillette C.G. Anantharamaiah G.M. Biochim. Biophys. Acta. 1995; 1256: 103-129Crossref PubMed Scopus (165) Google Scholar, 8Brouillette C.G. Anantharamaiah G.M. Engler J.A. Borhani D.W. Biochim. Biophys. Acta. 2001; 1531: 4-46Crossref PubMed Scopus (223) Google Scholar) have been investigated using the Πe technique as well. Some of these peptides, as short as 18 amino acids, have high Πe in phospholipid monolayers, and others exhibit lower Πe depending upon the specific amino acid arrangement of the amphipathic helical segment. For instance, upon substituting phenylalanine (Phe) for other hydrophobic residues in an A-type 18-residue α-helix (18A), Πe rose from 30 mN/m for 18A-1F to 46 mN/m for 18A-6F (27Datta G. Chaddha M. Hama S. Navab M. Fogelman A.M. Garber D.W. Mishra V.K. Epand R.M. Epand R.F. Lund-Katz S. Phillips M.C. Segrest J.P. Anantharamaiah G.M. J. Lipid Res. 2001; 42: 1096-1104Abstract Full Text Full Text PDF PubMed Google Scholar). To date only a few studies (26Weinberg R.B. Cook V.R. DeLozier J.A. Shelness G.S. J. Lipid Res. 2000; 41: 1419-1427Abstract Full Text Full Text PDF PubMed Google Scholar, 28Wang L. Atkinson D. Small D.M. J. Biol. Chem. 2003; 278: 37480-37491Abstract Full Text Full Text PDF PubMed Scopus (32) Google Scholar) have concerned the surface behavior of apolipoproteins or their consensus peptides on oil/water interfaces. In a previous report (28Wang L. Atkinson D. Small D.M. J. Biol. Chem. 2003; 278: 37480-37491Abstract Full Text Full Text PDF PubMed Scopus (32) Google Scholar), we have compared the interfacial properties of a synthesized consensus sequence peptide (CSP) of exchangeable apolipoproteins at the dodecane/water (DD/W) and the A/W interface. CSP consists of two 22-residue tandem repeat sequences derived from apoA-I, apoA-IV, and apoE-3. It is expected to comprise two antiparallel 20-residue amphipathic α-helices linked by a 4-residue proline-containing turn and is an idealized fundamental structural motif of exchangeable apolipoproteins (16Chao, Y. (2002) Conformational Studies of a Consensus Sequence Peptide (CSP) and a Real Sequence Peptide (RSP) of Apolipoproteins by Circular Dichroism and X-ray Crystallography. Ph.D. thesis, Department of Physiology and Biophysics, Boston University School of Medicine, Boston, MAGoogle Scholar, 29Nolte R.T. Atkinson D. Biophys. J. 1992; 63: 1221-1239Abstract Full Text PDF PubMed Scopus (152) Google Scholar). Our results indicated that CSP binds strongly to the DD/W interface and the A/W interface to lower the interfacial tension (γ) and the free energy (28Wang L. Atkinson D. Small D.M. J. Biol. Chem. 2003; 278: 37480-37491Abstract Full Text Full Text PDF PubMed Scopus (32) Google Scholar). CSP binds more tightly to the DD/W interface than the A/W interface, but it lies flat on both interfaces at saturation with an average area per residue of 14–16 Å2. It can be compressed 6–12% while remaining on the surface, but it is ejected from the surface above a critical surface pressure (ΠMAX). ΠMAX on the A/W interface is 20.3 mN/m, but more than 10 mN/m higher on DD/W (31.7 mN/m). We suggest that surface pressure-mediated desorption and re-adsorption of amphipathic α-helices provide lipoprotein stability during remodeling reactions in plasma. We have also studied the interfacial properties of two amphipathic β strand (AβS) consensus peptides of apoB (P27 and P12) on DD/W and triolein/water (TO/W) interfaces (30Wang L. Small D.M J. lipid Res. 2004; 45: 1704-1715Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar). Unlike CSP peptide, AβS peptides show stronger binding to these oil/water interfaces, and the bound peptides are difficult to force off the surface under compression. Furthermore, AβS peptides are almost purely elastic on DD/W, TO/W, and A/W interfaces when oscillated at periods from 4 to 128 s and from 6 to 25% deformation. In this report, we examined the interfacial properties of native apoA-I and CSP at a more physiological TO/W interface. We measured and compared the adsorption isotherms, desorption behaviors, and the elasticity properties of both the protein and the peptide. Based on our results, we propose a probable mechanism for how apoA-I behaves at a hydrophobic interface, partly detaching at lower surface pressure and fully detaching at high pressures. Such information will be helpful to understand the structure of apolipoproteins on lipoprotein surfaces and the structural changes that occur during remodeling of lipoproteins in plasma. The CSP peptide ((PLAEELRARLRAQLEELRERLG)2-NH2), which contains two 22-amino acid tandem repeats derived from the consensus sequence of the tandem repeats of human apoA-I, apoA-IV, and apoE (28Wang L. Atkinson D. Small D.M. J. Biol. Chem. 2003; 278: 37480-37491Abstract Full Text Full Text PDF PubMed Scopus (32) Google Scholar, 29Nolte R.T. Atkinson D. Biophys. J. 1992; 63: 1221-1239Abstract Full Text PDF PubMed Scopus (152) Google Scholar), was synthesized at Quality Controlled Biochemical Inc., using a Biosearch9050 Plus Continuous Flow Synthesizer, and purified >97+%. Stock solution of CSP (9–10 mg/ml) was prepared in ultra-filtered water obtained from a Hydro (Research Triangle Park, NC) Picosystem. The peptide has been shown to exhibit ∼90% α-helical conformation in solution (16Chao, Y. (2002) Conformational Studies of a Consensus Sequence Peptide (CSP) and a Real Sequence Peptide (RSP) of Apolipoproteins by Circular Dichroism and X-ray Crystallography. Ph.D. thesis, Department of Physiology and Biophysics, Boston University School of Medicine, Boston, MAGoogle Scholar, 28Wang L. Atkinson D. Small D.M. J. Biol. Chem. 2003; 278: 37480-37491Abstract Full Text Full Text PDF PubMed Scopus (32) Google Scholar, 29Nolte R.T. Atkinson D. Biophys. J. 1992; 63: 1221-1239Abstract Full Text PDF PubMed Scopus (152) Google Scholar) and to interact with DMPC to form well defined phospholipid bilayer discoidal complexes. To measure the interfacial tension of the TO/W interface with CSP, varied amounts of peptide stocks were added to the aqueous phase to obtain different peptide concentrations (from 7 × 10–9 m to 5 × 10–6 m). The pH of the aqueous phase was kept at pH 7.4 with phosphate buffer (2 mm). Triolein (>99% pure) was purchased from NU-CHEK PREP, Inc. (Elysian, MN), and its interfacial tension was 32 mN/m. All other reagents were of analytical grade. Human apoA-I was isolated and purified as described previously (31Wetterau J.R. Jonas A. J. Biol. Chem. 1982; 257: 10961-10966Abstract Full Text PDF PubMed Google Scholar). Unfolded human plasma apoA-I (0.1 mg/ml) was dialyzed against 6 m guanidine in the cold room overnight, then dialyzed against 6 m urea, 4 m guanidine, and 2 m guanidine overnight separately, and then dialyzed against pH 7.4 phosphate buffer (0.02 m) for 2 days. An Amicon was used to concentrate the apoA-I solution. Its purity was assessed by running a SDS-PAGE mini-gel; its folded state was checked by CD spectroscopy, and the apoA-I concentration was measured by protein assay. For interfacial tension measurements on the TO/W interface, varied amounts of apoA-I stocks were added to the aqueous phase to obtain different protein concentrations (from 1.7 × 10–8 m to 1.4 × 10–6 m). The pH of the aqueous phase was kept at pH 7.4 with phosphate buffer (2 mm). The interfacial tension of the TO/W interface in the presence of different amounts of CSP or apoA-I in the aqueous phase was measured with an I. T. CONCEPT (Longessaigne, France) Tracker oil-drop tensiometer (32Labourdenne S. Gaudry-Rolland N. Letellier S. Lin M. Cagna A. Esposito G. Verger R. Rivière C. Chem. Phys. Lipids. 1994; 71: 163-173Crossref Scopus (90) Google Scholar). 8-μl triolein drops were formed in gently stirred pH 7.4 phosphate buffer (6.0 ml) containing a given amount of apoA-I or CSP peptide. The interfacial tension was recorded continuously until it approached an equilibrium. The surface pressure Π was obtained from γ of the interface without CSP or apoA-I (γ0) minus the surface tension of the interface with CSP or apoA-I (γ), i.e. Π = γ0 – γ. All experiments were carried out at 25 ± 0.1 °C in a thermostated system. Once the interfacial tension curve approached an equilibrium, the oil drop (8 μl) was compressed by rapidly decreasing the volume by about 25% (2 μl), 12% (1 μl), or 6% (0.5 μl). In some experiments with apoA-I, larger compression was achieved by decreasing the oil drop volume by up to 5 μl (62%). The sudden decrease in volume instantaneously decreased the drop surface area and resulted in a sudden compression causing the tension to decrease abruptly. The oil drop was held at this reduced volume for about 5 or 10 min, and the surface tension was recorded continuously. If peptide or protein molecules readily desorbed from the surface, the surface tension rose back toward the equilibrium value, and this is called the desorption curve. If peptide or protein molecules stayed on the interface, γ would not change. To expand the interface, the decreased volume of the oil drop was rapidly increased by about 25% (2 μl), 12% (1 μl), or 6% (0.5 μl) back to its initial volume (8 μl), respectively. In cases following larger compression of apoA-I, the decreased volume of the oil drop was increased by up to 5 μl back to its initial volume (8 μl). As a result the surface area increased, and the tension abruptly If molecules from the phase to to the formed surface, then the surface tension would drop back toward the equilibrium, and this is called the curve. The 25% desorption and adsorption were by a = The and are to desorption and adsorption (28Wang L. Atkinson D. Small D.M. J. Biol. Chem. 2003; 278: 37480-37491Abstract Full Text Full Text PDF PubMed Scopus (32) Google Scholar). To estimate the maximum pressure (ΠMAX) that the peptide or protein molecule could withstand without being ejected from the surface, a of experiments were carried out in which the oil drop volume was decreased abruptly to the surface pressure decrease the to a given value, and then the interfacial tension γ was for The in tension was then against A positive indicated that the bound peptide or protein molecules desorbed from the surface. If was no in γ, it indicated that the peptide or protein molecule on the surface without In a indicated that the peptide or protein molecules in the solution were to onto the that that was not at equilibrium. The of the was to a The at = the Π at which peptide or protein molecules showed no adsorption or this is the apoA-I consists of 10 tandem-repeating amphipathic α-helical it from the interface under compression. ΠMAX of apoA-I the pressure at which bound apoA-I to be off the interface. To estimate the pressure at which the molecule of apoA-I is ejected from the interface, we used the used to estimate ΠMAX that we used larger and of the interface. We then compared the re-adsorption curve the desorption with the initial adsorption curve and all cases the were almost under the the re-adsorption curve was to the initial adsorption curve for in the interfacial tension it indicated that the compression used the entire molecule of apoA-I into the aqueous For these cases we against and the to a The at = the Π at which the apoA-I molecule to be ejected from the interface for which we the compression was and to a lower the re-adsorption curve was faster than the initial adsorption curve in the γ for This indicated that only part of apoA-I was from the surface and could very rapidly when the surface was expanded. The from cases formed a with a different ΠMAX was the Π to part of apoA-I from the of ΠMAX and of apoA-I at the TO/W interface. of γ during compression against the pressure compression. only part of apoA-I was off the interface. The of the at γ = the of ΠMAX of apoA-I was off the interface and to from solution. The of the at γ = the of of apoA-I. when compressed to pressures above 14.8 mN/m, part of apoA-I was off the interface, whereas only at pressures above mN/m apoA-I to be ejected from the the tension approached an equilibrium value, the volume of the triolein drop was oscillated at different periods from 4 to 128 s and different of about or As the volume was oscillated in a interfacial area A and surface tension γ were recorded continuously and the phase between compression and The interfacial elasticity was derived = The elasticity part and the elasticity part were obtained J. D. R. at Scholar, J. Cagna A. and 1996; Scopus Google Scholar). Continuous were carried out 10 s an 8-μl oil drop was formed in CSP or apoA-I solution. 10 s to form the drop the to the volume was oscillated in experiments at different periods and 16 s) and different of about or continuously until the interfacial tension an equilibrium. This a of the surface tension surface and of CSP on (TO/W) a of interfacial tension (γ) of the TO/W interface with different amounts of CSP in the aqueous The γ of TO/W interface was about 32 mN/m. different amounts of CSP into the aqueous phase × 10–9 m to × 10–6 the CSP molecule onto the TO/W interface and lowered γ to an equilibrium At the most concentration × 10–9 curve γ showed a very decrease about s then a and then a very decrease an equilibrium at about 19 mN/m, which pressure on the surface. As the CSP concentration increased, the and the of the of γ At the highest concentration × 10–6 m) shown in γ not show the and rapidly at a very and mN/m at which surface In our previous (27Datta G. Chaddha M. Hama S. Navab M. Fogelman A.M. Garber D.W. Mishra V.K. Epand R.M. Epand R.F. Lund-Katz S. Phillips M.C. Segrest J.P. Anantharamaiah G.M. J. Lipid Res. 2001; 42: 1096-1104Abstract Full Text Full Text PDF PubMed Google