Amphiphilic compound, phospholipid structure, and method for producing phospholipid structure

Abstract

To provide an amphiphilic compound capable of suppressing phase separation of a phospholipid membrane and stabilizing a phospholipid structure.SOLUTION: The present invention provides an amphiphilic compound for use as a stabilizer of two or more phospholipid compounds having different phase transition temperatures, the amphiphilic compound being represented by the following general formula (1).SELECTED DRAWING: Figure 2B

Description

[Technical Field]

[0001] The present invention relates to an amphiphilic compound, a phospholipid structure, and a method for producing the phospholipid structure. [Background technology]

[0002] BACKGROUND ART Conventionally, a technique of encapsulating a drug in a phospholipid structure such as a liposome is known as an example of a drug delivery system.

[0003] For example, Patent Document 1 discloses liposomes using phospholipids such as DPPC (dipalmitoylphosphatidylcholine) and DOPC (dioleoylphosphatidylcholine) as lipid formulations for encapsulating anti-infective drugs. It is expected that the properties of the phospholipid structures can be adjusted in various ways by mixing two or more types of phospholipid compounds with various different properties. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2019-52170 Summary of the Invention [Problem to be solved by the invention]

[0005] However, phospholipid structures containing two or more phospholipid compounds with different phase transition temperatures are unstable and may undergo phase separation and deformation in the phospholipid membrane.

[0006] The present invention has been made in view of the above circumstances, and aims to provide an amphipathic compound that can suppress phase separation of phospholipid membranes and stabilize phospholipid structures, a phospholipid structure containing the amphipathic compound, and a method for producing the same. [Means for solving the problem]

[0007] In order to solve the above problems, one aspect of the present invention includes the following aspects.

[0008] [1] A compound represented by the following general formula (1): An amphiphilic compound for use as a stabilizer for two or more phospholipid compounds having different phase transition temperatures. [ka] (In the general formula (1), R1 is hydrogen or a cationic peptide consisting of 1 to 10 amino acid residues, and R2 and R3 are hydrophobic alkyl groups. However, this does not include compounds represented by the following general formula (2).) [ka]

[0009] [2] The amphiphilic compound according to [1], wherein the compound represented by the general formula (1) is any one of the compounds represented by the following formulae (3) to (5): [ka] [ka] [ka]

[0010] [3] A phospholipid structure comprising the amphiphilic compound according to [1] or [2] and two or more types of phospholipid compounds having different phase transition temperatures.

[0011] [4] The phospholipid structure according to [3], containing the amphiphilic compound in an amount of 0.6 to 5 mol %.

[0012] [5] The phospholipid structure according to [3] or [4], further comprising cholesterol.

[0013] [6] The phospholipid structure according to any one of [3] to [5], wherein the two or more types of phospholipid compounds include a phospholipid compound having a phase transition temperature of 25°C or less and a phospholipid compound having a phase transition temperature of more than 25°C.

[0014] [7] The phospholipid structure according to any one of [3] to [6], wherein the two or more types of phospholipid compounds include dioleoylphosphatidylcholine (DOPC) and dipalmitoylphosphatidylcholine (DPPC).

[0015] [8] The phospholipid structure according to any one of [3] to [7], which is a liposome comprising a phospholipid bilayer membrane.

[0016] [9] The phospholipid structure according to any one of [3] to [8], which contains one or more members selected from the group consisting of amino acids, proteins, viruses, and nucleic acids.

[0017]

[10] A method for producing a phospholipid structure, comprising a step of mixing an aqueous phospholipid solution containing two or more types of phospholipid compounds having different phase transition temperatures with an amphipathic compound represented by the general formula (1) described in [1] or an amphipathic compound represented by any one of the general formulae (3) to (5) described in [2]. [Effects of the Invention]

[0018] According to the present invention, it is possible to provide an amphipathic compound that can suppress phase separation of a phospholipid membrane and stabilize a phospholipid structure, a phospholipid structure containing the amphipathic compound, and a method for producing the same. [Brief explanation of the drawings]

[0019] [Figure 1A] 1 shows a schematic diagram and an atomic force microscope image of a phospholipid thin film to which no cationic pep has been added. [Figure 1B] Schematic diagram and atomic force microscope image showing a phospholipid thin film mixed with cationic pep. [Figure 2A]This is a microscopic image of liposomes to which no cationic pep has been added. [Figure 2B] This is a microscopic image of liposomes to which cationic pep has been added. [Figure 3] 2C is a time course image of the liposome shown in FIG. 2B. [Figure 4A] 1 is a microscopic image showing liposomes positioned on a hydrophilized SiO 2 substrate in the presence of 0.5 mM Asp4. [Figure 4B] 1 is a microscopy image showing liposomes positioned on poly(diallyldimethylammonium chloride)-coated SiO 2 substrates in the presence of 0.5 mM Asp4. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, the amphipathic compound, the phospholipid structure containing the amphipathic compound, and the method for producing the phospholipid structure according to the present invention will be described with reference to embodiments, although the present invention is not limited to the following embodiments.

[0021] <Amphiphilic compounds> The amphipathic compound of this embodiment is an amphipathic compound for use as a stabilizer (stabilizer) for two or more types of phospholipid compounds having different phase transition temperatures, and has a structure represented by the following general formula (1):

[0022] [ka] (In the formula (1), R1 is hydrogen or a cationic peptide consisting of 1 to 10 amino acid residues, and R2 and R3 are hydrophobic alkyl groups, except for compounds represented by the following general formula (2).)

[0023] [ka] Phospholipid structures such as liposomes containing two or more phospholipid compounds with different phase transition temperatures are prone to phase separation. By mixing the amphipathic compound of the present embodiment with two or more phospholipid compounds with different phase transition temperatures (in other words, by incorporating the amphipathic compound into the phospholipid structure), phase separation of the phospholipid membrane can be suppressed and the phospholipid structure can be stabilized. That is, by mixing two or more phospholipid compounds with different phase transition temperatures with the amphipathic compound of the present embodiment, a phospholipid structure in which the phospholipid compounds are mixed substantially uniformly can be produced. In addition, by adding the amphipathic compound of the present embodiment to two or more phospholipid compounds in which phase separation has already occurred, phase separation can be eliminated and a uniform membrane without component bias can be formed.

[0024] The hydrophobic alkyl groups R2 and R3 in formula (1) interact (associate) with the hydrophobic lipid moiety of the phospholipid compound to form a lipid bilayer membrane together, while the cationic peptide R1 is configured to interact (bind, accept) with anionic compounds such as acidic amino acids, proteins, viruses, and nucleic acids.

[0025] When the cationic peptide R1 interacts with an anionic compound, the anionic compound is encapsulated inside the phospholipid structure. Therefore, the phospholipid structure containing the amphipathic compound of this embodiment can be suitably used as a drug delivery system for encapsulating anionic compounds. Hereinafter, the above-mentioned amphipathic compound is also referred to as a "cationic pep." Note that the compound encapsulated inside the phospholipid structure is not limited to anionic compounds. Hydrophilic compounds can be encapsulated inside the phospholipid structure, and hydrophobic compounds can be encapsulated in the membrane of the hydrophobic part of the phospholipid structure.

[0026] R1 is hydrogen when the number of amino acid residues is 0. The number of amino acid residues in R1 is preferably 2 to 5.

[0027] The hydrophobic alkyl groups of R2 and R3 may be appropriately selected so as to be easily associated with the phospholipid compound, depending on the structure of the phospholipid compound. The alkyl group is preferably linear or linear with a partial branch, and although there is no limitation on its length, it is preferably an alkyl group having 10 to 20 carbon atoms. The alkyl groups of R2 and R3 may have a substituent. The number of carbon atoms of the alkyl group does not include the number of carbon atoms of the substituent.

[0028] More specifically, the amphipathic compound is preferably any of the compounds represented by the following formulas (3) to (5): The compounds represented by formulas (3) and (4) have NBD (nitrobenzoxadiazole) at the end of R3, which functions as a fluorescent label, but the amphipathic compound may or may not have NBD.

[0029] [ka]

[0030] [ka]

[0031] [ka]

[0032] The amount of amphipathic compound to be mixed with two or more types of phospholipid compounds having different phase transition temperatures is not particularly limited, and for example, the phospholipid structure obtained by mixing may contain the amphipathic compound in an amount of 0.1 to 50 mol%, 0.6 to 50 mol%, 0.6 to 5 mol%, 0.6 to 1.5 mol%, 0.7 to 50 mol%, 0.7 to 5 mol%, or 0.7 to 1.5 mol%. For example, as in the examples below, when an aqueous dispersion is prepared by dispersing 0.08 mM (mol / L) DOPC, 0.08 mM DPPC, 0.04 mM cholesterol, and 0.0015 mM amphiphilic compound (cationic pep), the phospholipid structures contain 0.74 mol% of the amphiphilic compound (0.08, 0.08, 0.04, and 0.0015 divided by 0.2015, which is the sum of these). The amphiphilic compound of this embodiment can suppress phase separation of the phospholipid membrane and maintain the morphology of the phospholipid structures, even in a small amount less than 1 mol%.

[0033] The amphiphilic compound of this embodiment makes it possible to produce phospholipid structures (liposomes, vesicles, etc.) that are stable against various changes in the external environment, and is useful for developing biofunctional materials such as encapsulating agents for biomolecules over a long period of time.

[0034] (phospholipid compounds) Two or more types of phospholipid compounds having different phase transition temperatures can be selected as appropriate, but compounds that have been used to form lipid bilayer membranes or liposomes using conventional techniques can be preferably selected.

[0035] Two or more phospholipid compounds having different phase transition temperatures may be combined in a manner that causes phase separation when mixed alone. Even in such a combination, mixing with an amphiphilic compound having the structure represented by the above general formula (1) can suppress phase separation and stabilize the phospholipid structure.

[0036] The two or more types of phospholipid compounds may include a phospholipid compound having a phase transition temperature of 25°C or lower and a phospholipid compound having a phase transition temperature of more than 25°C.

[0037] The phospholipid compound may contain two or more species selected from the group consisting of DPPC (dipalmitoylphosphatidylcholine), DOPC (dioleoylphosphatidylcholine), and DSPC (1,2-distearoyl-sn-glycero-3-phosphocholine). The phospholipid compound may contain DPPC and DOPC.

[0038] <Phospholipid structure> The phospholipid structure of this embodiment is a phospholipid structure obtained by mixing the above-mentioned amphipathic compound with the phospholipid compound, and contains the above-mentioned amphipathic compound and two or more types of phospholipid compounds having different phase transition temperatures. The phospholipid structure is not particularly limited as long as it has a phospholipid membrane, and examples thereof include a vesicle, which is a bag-like structure, and a liposome containing a phospholipid bilayer membrane.

[0039] The vesicle may be a unilamellar vesicle made of one lipid bilayer membrane, or a vesicle made of multiple lipid bilayer membranes.

[0040] The phospholipid structures may contain components other than the above-mentioned amphipathic compound and phospholipid compound. Examples of other components include, but are not limited to, lipid membrane components such as cholesterol and drugs encapsulated therein. Examples of drugs encapsulated therein include one or more drugs selected from the group consisting of amino acids, proteins, viruses, and nucleic acids. These compounds are preferably anionic compounds. The substances encapsulated in the phospholipid structures are not limited to drugs, and various substances such as antigens used in vaccines, cosmetic ingredients, plant protection agents, and chemicals used in the construction field can be encapsulated in the phospholipid structures.

[0041] The proportion of the amphipathic compound contained in the phospholipid structure is not particularly limited, and may be, for example, 0.1 to 50 mol%, 0.6 to 50 mol%, 0.6 to 5 mol%, 0.6 to 1.5 mol%, 0.7 to 5 mol%, or 0.7 to 1.5 mol%. When the phospholipid structure contains the amphipathic compound in these proportions, the shape of the phospholipid structure can be easily maintained.

[0042] The proportion of the phospholipid compound in the phospholipid structure is not particularly limited, but may be, for example, 10 to 99.4 mol %, 50 to 99.4 mol %, or 70 to 90 mol %.

[0043] The proportion of membrane components consisting of lipids other than phospholipid compounds, such as cholesterol, in the phospholipid structure is not particularly limited, but can be, for example, 0 to 40 mol%. Generally, when cholesterol is contained in a phospholipid structure, membrane deformation due to phase separation is likely to occur, but by containing the above-mentioned amphipathic compound in the phospholipid structure, membrane deformation can be suppressed even in phospholipid structures containing cholesterol.

[0044] <Method of Producing Phospholipid Structures> The method for producing a phospholipid structure of this embodiment is a method for producing the above-mentioned phospholipid structure, and includes a step of mixing an aqueous phospholipid solution containing two or more types of phospholipid compounds having different phase transition temperatures with the above-mentioned amphipathic compound represented by general formula (1).

[0045] The water used for the phospholipid aqueous solution containing the phospholipid compound may be pure water or may contain ions as appropriate. When the phospholipid compound is not dissolved in water, the "phospholipid aqueous solution" can also be called a "phospholipid dispersion."

[0046] The pH of the aqueous phospholipid solution can be selected from the range of about 4 to 10. The pH is near neutral (pH 7), preferably 6.5 to 7.5, and more preferably 6.8 to 7.2.

[0047] The temperature of the aqueous phospholipid solution is not particularly limited as long as it is 100°C or less, but may be, for example, approximately 37°C, which corresponds to the in vivo environment, or approximately 25°C, which corresponds to room temperature.

[0048] Typically, when an aqueous phospholipid solution containing the phospholipid compound is mixed with an amphipathic compound, the phospholipid compound and the amphipathic compound associate with each other to form a stable phospholipid structure, which has a layer formed approximately uniformly in the aqueous solution.

[0049] A substantially uniformly formed layer refers to a lipid bilayer membrane with a substantially uniform composition and membrane shape with little bias in composition. For example, it refers to a state in which the two or more phospholipid compounds and amphipathic compounds are distributed substantially uniformly to form a substantially flat membrane. It also refers to a state in which the bilayer membrane has a large phospholipid structure, such as a vesicle or liposome structure formed by a lipid bilayer membrane with a diameter of 5 to 50 μm.

[0050] For example, when an amphipathic compound represented by the general formula (1) is added to an aqueous phospholipid solution containing two or more types of phospholipid compounds having different phase transition temperatures as described above to form a phospholipid structure, the phospholipid structure forms a substantially uniform layer in the aqueous solution.

[0051] In the aqueous phospholipid solution, the concentration of the phospholipid structure is preferably 1% by mass or less, and more preferably about 0.1% by mass.

[0052] The method for producing a phospholipid structure of the present embodiment may include a step of adding the anionic compound to allow the small-particle-sized liposomes to incorporate a component containing the anionic compound.

[0053] When the anionic compound is added to form liposomes with small particle sizes, the anionic compound is incorporated into the liposomes, but other molecules present in the vicinity are also incorporated into the liposomes. This effect can be used to incorporate the anionic compound and other molecules, particles, etc. into the liposomes.

[0054] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the invention described in the claims, and it goes without saying that these modifications are also included within the scope of the present invention. [Example]

[0055] The effects of the present invention will be made clearer by the following examples and comparative examples. Note that the present invention is not limited to the following examples, and can be practiced by making appropriate changes within the scope of the present invention.

[0056] (Synthesis of amphiphilic compounds) The amphiphiles used were synthesized by Fmoc solid phase peptide synthesis. A condensation reagent cocktail of HBTU (2.28 g, 6.01 mmol) and HOBt-HO (0.94 g, 6.12 mmol) was prepared immediately prior to synthesis. A cleavage cocktail of a mixture of DMF (12 mL), DIEA (2.24 mL) and NMP (12 mL), TIS (125 μL), TFA (4.75 mL) and water (125 μL) was prepared immediately prior to synthesis.

[0057] Fmoc-NH-SAL resin (0.08 mmol) in a polypropylene tube was soaked in DMF (2 mL) at 25°C for 3 hours. After removing the DMF, piperidine in DMF (20%, 2 mL) was added and mixed on a vortex device for 1 minute. After removing the reaction solution, piperidine in DMF (20%, 2 mL) was added, and the reaction tube was shaken at 25°C for 10 minutes. After evacuating the reaction mixture, the resin was washed with DMF (2 mL, 5 times), CHCl (2 mL, 3 times), and DMF (2 mL, 3 times). To the resin was added the Fmoc-protected amino acid (0.24 mmol) dissolved in condensation-reagent cocktail (560 μL) and a mixture of DIEA and NMP (560 μL). After shaking at 25 °C for 20 min, the reaction mixture was evacuated and the resin was washed with DMF (2 mL, 5 times), CHCl (2 mL, 3 times), and DMF (2 mL, 3 times). The piperidine-based Fmoc deprotection and Fmoc-protected amino acid coupling reactions were repeated in the designated order. After the final Fmoc deprotection and washing, acetic anhydride in CHCl (25%, 2 mL) was added to the resin, and the reaction tube was shaken at 25 °C for 10 min. After removing the reaction solution, the resin was washed with CHCl (2 mL, 3 times), DMF (2 mL, 5 times), and CHCl (2 mL, 5 times). Cleavage cocktail (2.5 mL) was added to the resin, and the reaction tube was left at 25 °C for 90 min, with gentle shaking every 30 min. The solution was filtered and collected in a polypropylene centrifuge tube. The reaction tube was washed with TFA (500 μL, 3 times), which was also collected by filtration. EtO (40 mL) was added to the centrifuge tube, mixed on a vortex for 1 min, and centrifuged at 3500 × g for 5 min at 4 °C. The supernatant was then removed. This process was repeated three times, and the peptide was then dried under vacuum at 25°C for 2 hours, dispersed in water, and lyophilized.

[0058] The synthesized molecules were identified by MALDI-TOF MS or ESI-TOF MS. MALDI-TOF MS (2,5-dihydroxybenzoic acid, reflector positive): m / z calculated for Cationic pep (C 66 H 117 N 15 O 10 + ) 1190.05 (compound represented by the above formula (5)); found 1191.18, Cationic pep (with NBD, compound represented by the above formula (2)) (C 66 H 121 N 15 O 10 + ) 1280.911; found 1280.979, for CaRL Ole (The compound represented by the above formula (4), C 66 H 117 N 15 O 10 + ) 1275.677; found 1275.940, CaRL Br (The compound represented by the above formula (3), C 74 H 135 N 17 O 12 + ) 1454.08; found 1455.93

[0059] <Test Example 1: Inhibition of phase separation on phospholipid membranes by cationic pep> In this test example, lipid thin films were prepared by mixing DOPC, DPPC, cholesterol, and the cationic pep represented by formula (5) in a ratio of 4:4:2:0.1. As a comparative example, a lipid thin film was also prepared using only the three components of DOPC, DPPC, and cholesterol without adding cationic pep. The solvent was an aqueous dispersion, and the test was conducted at room temperature (25°C).

[0060] Figure 1 shows a schematic diagram and a microscope image of a phospholipid thin film containing cationic pep. In detail, Figure 1A shows a schematic diagram and an atomic force microscope image of a phospholipid thin film without cationic pep, and Figure 1B shows a schematic diagram and an atomic force microscope image of a phospholipid thin film with cationic pep. The scale bar in Figure 1 indicates 100 nm.

[0061] As shown in Figure 1A, in the phospholipid thin film without the cationic pep mixture, there is a domain (region) 100 where DPPC and cholesterol exist, shown in dark color, and a domain (region) 200 where DOPC exists, shown in light color, and it can be seen that DPPC and DOPC, which have different phase transition temperatures, are phase-separated.

[0062] In contrast, as shown in Figure 1B, in the phospholipid thin film mixed with Cationic Pep, no domains were observed, and the overall color was uniform, indicating that DOPC, DPPC, and cholesterol were distributed approximately uniformly without any bias.

[0063] These results demonstrate that phase separation can be suppressed by mixing two or more phospholipid compounds with different phase transition temperatures with cationic pep.

[0064] <Test Example 2: Inhibition of liposome membrane deformation by cationic pep> In this test example, the membrane deformation suppression effect of liposomes containing the cationic pep represented by the above formula (5) was confirmed. This test was carried out at room temperature (25°C).

[0065] A solution containing liposomes was prepared by the static hydration method. Specifically, 2 mM DOPC / CHCl3 solution (8 μL), 2 mM DPPC / CHCl3 solution (8 μL), 2 mM cholesterol / CHCl3 solution (4 μL), and 0.8 mM cationic pep / MeOH solution (0.5 μL) were added to a 1 mL microtube, and the mixture was dried overnight for at least 5 hours by rotating it in a centrifugal evaporator.

[0066] Then, Milli-Q (registered trademark, 10 μL) was added and the mixture was shaken in a constant temperature shaking bath at 45°C or higher for 5 minutes. Milli-Q (200 μL) was further added, the microtube was capped, and the mixture was shaken at 45°C or higher for 5 hours or more to prepare an aqueous dispersion containing liposomes. This aqueous dispersion contained DOPC at 0.08 mM, DPPC at 0.08 mM, cholesterol at 0.04 mM, and cationic pep at 0.0015 mM. As a comparative example, an aqueous dispersion system without cationic pep (0.8 mM cationic pep / MeOH solution) was also prepared.

[0067] FIG. 2A is a microscopic image of liposomes without cationic pep added, and FIG. 2B is a microscopic image of liposomes with cationic pep added. As shown in Figure 2A, in liposomes to which no cationic pep had been added, phase separation occurred, and three domains were formed, resulting in membrane deformation in which the liposome membrane protruded in three directions.

[0068] In contrast, as shown in Figure 2B, phase separation did not occur in liposomes to which cationic pep had been added, and as a result, the phospholipid membrane of the liposomes was spherical.

[0069] Figure 3 shows images of the liposomes shown in Figure 2B observed over time, showing the time that has elapsed since the liposomes shown in Figure 2B were added to the substrate. As shown in FIG. 3, the liposomes maintained their spherical shape even 15 minutes after the preparation of the aqueous dispersion containing liposomes.

[0070] These results demonstrate that the inclusion of cationic pep in liposomes can suppress membrane deformation of liposomes for a long period of time.

[0071] <Test Example 3: Inhibition of liposome membrane deformation by cationic peptide in the presence of anionic peptide> In this test example, we confirmed the membrane deformation inhibitory effect of liposomes containing DOPC, DPPC, cholesterol, and the cationic pep represented by formula (5) in the presence of an anionic peptide that easily induces phase separation through interaction with an amphiphilic compound having the structure represented by formula (1).The anionic peptide used was Asp4, which is a peptide bond of four aspartic acids. This test was carried out at room temperature of 25° C. Liposomes were prepared in the same manner as in Test Example 2 above.

[0072] Figure 4A is a microscopic image showing liposomes located on a hydrophilized SiO2 substrate in the presence of 0.5 mM Asp4, and Figure 4B is a microscopic image showing liposomes located on a poly(diallyldimethylammonium chloride)-coated SiO2 substrate in the presence of 0.5 mM Asp4.

[0073] As shown in Figure 4A and Figure 4B, liposomes containing cationic pep were spherical on both hydrophilized SiO2 substrates and poly(diallyldimethylammonium chloride)-coated SiO2 substrates. This indicates that cationic pep strongly suppresses membrane deformation of liposomes even in the presence of Asp4, which is likely to induce membrane deformation due to phase separation. [Industrial Applicability]

[0074] According to the present invention, by adding an amphipathic compound having a structure represented by the above formula (1) to a phospholipid structure containing two or more types of phospholipid compounds having different phase transition temperatures, phase separation of the phospholipid structure can be suppressed, and therefore the present invention is industrially applicable. [Explanation of symbols]

[0075] 100...domain containing DPPC and cholesterol, 200...domain containing DOPC

Claims

1. A compound represented by the following general formula (1): An amphiphilic compound for use as a stabilizer for two or more phospholipid compounds having different phase transition temperatures. 【Chemical Formula 1】 (In the general formula (1) above, R1 is hydrogen or a cationic peptide consisting of 1 to 10 amino acid residues, and R2 and R3 are hydrophobic alkyl groups. However, this does not include compounds represented by the following general formula (2).) 【Chemistry 2】

2. 2. The amphiphilic compound according to claim 1, wherein the compound represented by the general formula (1) is any one of compounds represented by the following formulas (3) to (5): 【Chemistry 3】 【Chemistry 4】 【Chemistry 5】

3. A phospholipid structure comprising the amphiphilic compound according to claim 1 or 2 and two or more types of phospholipid compounds having different phase transition temperatures.

4. The phospholipid structure according to claim 3, comprising 0.6 to 5 mol% of the amphiphilic compound.

5. The phospholipid structure of claim 3, further comprising cholesterol.

6. The phospholipid structure according to claim 3, wherein the two or more types of phospholipid compounds include a phospholipid compound having a phase transition temperature of 25°C or less and a phospholipid compound having a phase transition temperature of more than 25°C.

7. 6. The phospholipid structure of claim 5, wherein the two or more types of phospholipid compounds comprise dioleoylphosphatidylcholine (DOPC) and dipalmitoylphosphatidylcholine (DPPC).

8. The phospholipid structure according to claim 3, which is a liposome comprising a phospholipid bilayer membrane.

9. The phospholipid structure according to claim 8, comprising at least one member selected from the group consisting of amino acids, proteins, viruses, and nucleic acids.

10. A method for producing a phospholipid structure, comprising a step of mixing an aqueous phospholipid solution containing two or more types of phospholipid compounds having different phase transition temperatures with the amphipathic compound according to claim 1.

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