Surfactin derivatives and their salts

Abstract

The purpose of the present invention is to provide: a novel surfactin derivative and a salt thereof which are usable as surfactants even in relatively low-pH environments; and a surfactant including the surfactin derivative or salt thereof. The surfactin derivative or salt thereof according to the present invention is characterized by being represented by formula (I). [In formula (I), X1 represents leucine, etc.; X2 and X3 each represent a single bond or a linker group; R1 represents a C9-18 alkyl group; and R2 and R3 each represent OH or a strong-acid group selected from among sulfino, sulfo, and sulfate groups, with the proviso that R2 and / or R3 is the strong-acid group.]

Description

[Technical Field]

[0001] The present invention relates to a novel surfactin derivative or a salt thereof, and a surfactant containing the surfactin derivative or a salt thereof. [Background technology]

[0002] Surfactin is a peptide produced by Bacillus subtilis, and because it is a peptide, it is highly degradable and safe, and in addition, it is a biosurfactant that has a low critical micelle concentration (CMC) and exhibits strong surface activity (e.g., Non-Patent Document 1). Therefore, it is used as a surfactant in a variety of fields, from cosmetics to pharmaceutical preparations and antistatic agents (e.g., Patent Documents 1 to 3).

[0003] Surfactin also forms giant micelles in water, and forms nanodiscs when used in combination with phospholipids (Patent Document 4).

[0004] As shown below, surfactin functions as a surfactant because it has a long-chain aliphatic hydrocarbon group R as a hydrophobic group and side-chain carboxy groups of aspartic acid and glutamic acid as hydrophilic groups.

[0005] [ka] [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-128512 [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-146827 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-077236 [Patent Document 4] Japanese Patent Application Publication No. 2019-186835 [Non-patent literature]

[0007] [Non-Patent Document 1] Onaizi,SA et al.,Physicochem.Eng.Aspects,2012,vol.415,pp.388-393 Summary of the Invention [Problem to be solved by the invention]

[0008] As mentioned above, surfactin is known as an excellent naturally occurring surfactant. However, the present inventors have experimentally confirmed that sodium surfactin begins to become insoluble at a pH of about 6 and precipitates at pH levels below that. Because surfactin is a safe surfactant, it could be considered for use as an ingredient in cosmetics applied to the skin, but the pH of skin is said to be weakly acidic, around 5.0 to 5.5; for example, the pH of acidic lotions that have a strong astringent effect and are effective in tightening the skin is around 4 to 5. Surfactin, which becomes insoluble at a pH of about 6, cannot be used as an ingredient in such cosmetics. Therefore, an object of the present invention is to provide a novel surfactin derivative or a salt thereof that can be used as a surfactant even in a relatively low pH environment, and a surfactant containing the surfactin derivative or a salt thereof. [Means for solving the problem]

[0009] The present inventors have conducted extensive research to solve the above problems, and as a result have discovered that surfactin can be used as a surfactant even in a relatively low pH environment by utilizing the carboxyl group of surfactin to introduce a strong acid group, thereby completing the present invention. The present invention will now be described.

[0010] [1] A surfactin derivative or a salt thereof represented by the following formula (I): [ka] [In the formula, X 1 represents an amino acid residue selected from leucine, isoleucine, and valine; X 2 and X 3 represents a single bond or a linker group, R 1 is C 9-18 represents an alkyl group, R 2 and R 3 represents OH or a strong acid group selected from a sulfino group, a sulfo group, and a sulfate group; However, R 2 or R 3 At least one of the groups is a strong acid group.] [2] R 2 is a strong acid group, X 2 is a linker group, and R 3 is OH and X 3 is a single bond, and R 3 is a strong acid group, X 2 is a single bond, and R 2 is OH and X 3 The surfactin derivative or salt thereof according to the above [1], wherein is a linker group. [3] The surfactin derivative or salt thereof according to the above [1] or [2], which is a salt of the surfactin derivative represented by the above formula (I). [4] The surfactin derivative or salt thereof according to the above [1] or [2], which is an alkali metal salt of the surfactin derivative represented by the above formula (I). [5] A surfactant comprising the surfactin derivative or salt thereof according to any one of [1] to [4] above.

[0011] [6] Use of the surfactin derivative or a salt thereof according to any one of the above [1] to [4] as a surfactant. [7] Use of the surfactin derivative represented by the above formula (I) or a salt thereof as a surfactant. [8] R 2is a strong acid group, X 2 is a linker group, and R 3 is OH and X 3 is a single bond, and R 3 is a strong acid group, X 2 is a single bond, and R 2 is OH and X 3 The use according to the above [7], wherein R is a linker group. [9] The use according to the above [7] or [8], wherein the surfactant is a salt of the surfactin derivative represented by the above formula (I).

[10] The use according to the above [7] or [8], wherein the surfactant is an alkali metal salt of the surfactin derivative represented by the above formula (I).

[0012]

[11] A method for reducing the interfacial tension of a solution, comprising the step of dissolving the surfactin derivative or salt thereof according to any one of [1] to [4] above in the solution.

[12] A method for reducing the interfacial tension of a solution, comprising the step of dissolving the surfactin derivative represented by formula (I) or a salt thereof in the solution.

[13] R 2 is a strong acid group, X 2 is a linker group, and R 3 is OH and X 3 is a single bond, and R 3 is a strong acid group, X 2 is a single bond, and R 2 is OH and X 3 The method according to the above

[12] , wherein R is a linker group.

[14] The method according to

[12] or

[13] above, wherein the surfactant is a salt of the surfactin derivative represented by formula (I).

[15] The method according to

[12] or

[13] above, wherein the surfactant is an alkali metal salt of the surfactin derivative represented by the formula (I). [Effects of the Invention]

[0013] While sodium surfactin begins to become insoluble in a solution at a pH of about 6, the surfactin derivatives and salts thereof according to the present invention do not become insoluble at a pH of about 6 and can be used as surfactants up to a pH of about 2. Therefore, the present invention is extremely advantageous industrially as a technology that widens the range of applications of surfactin, which is a safe and excellent surfactant. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 shows the particle size distribution of particles contained in a dispersion of the sodium salt of a surfactin derivative according to the present invention, and photographs of the dispersion of the sodium salt of a surfactin derivative according to the present invention when it is not irradiated with ultraviolet light and when it is irradiated with ultraviolet light. [Figure 2] FIG. 2 shows the absorption spectrum and fluorescence spectrum of a dispersion of the sodium salt of the surfactin derivative according to the present invention. [Figure 3] FIG. 3 is a graph showing the relationship between the concentration and surface tension of a dispersion of the sodium salt of a surfactin derivative according to the present invention. [Figure 4] FIG. 4 is a graph showing the relationship between the pH and light transmittance of a dispersion of sodium surfactin or a sodium salt of the surfactin derivative according to the present invention. [Figure 5] FIG. 5 shows photographs of the appearance of a DMPC solution, a mixed solution in which sodium surfactin and a sodium salt of a surfactin derivative have been added to the DMPC solution, and the mixed solution after being irradiated with ultraviolet light. [Figure 6] FIG. 6 shows the particle size distribution of particles contained in the nanodisk liquid. [Figure 7] FIG. 7 is a photograph showing the appearance of a DMPC solution and a mixed solution obtained by adding sodium surfactin, a sodium salt of a surfactin derivative, and Sudan III, a hydrophobic dye, to the DMPC solution. [Figure 8] FIG. 8 shows the particle size distribution of particles contained in a nanodisc solution containing Sudan III, a hydrophobic dye. [Figure 9] FIG. 9 shows the measurement results of the fluorescence spectrum of a nanodisc solution containing the hydrophobic dye Sudan III. DETAILED DESCRIPTION OF THE INVENTION

[0015] The surfactin derivative according to the present invention has a structure represented by the above formula (I). In the present disclosure, the "compound represented by formula (x)" may be abbreviated as "compound (x)".

[0016] In formula (I), X 1 The amino acid residue as R may be either L- or D-form, but L-form is preferred. 1 The surfactant may be a mixture containing two or more surfactin derivatives having different surfactants.

[0017] In this disclosure, "C 9-18 The term "alkyl group" refers to a linear or branched monovalent saturated hydrocarbon group having 9 to 18 carbon atoms. Examples include n-nonyl, 6-methyloctyl, 7-methyloctyl, n-decyl, 8-methylnonyl, n-undecyl, 9-methyldecyl, n-dodecyl, 10-methylundecyl, n-tridecyl, 11-methyldodecyl, n-tetradecyl, n-pentadecyl, n-hexadecyl, n-heptadecyl, and n-octadecyl.

[0018] In this disclosure, a "single bond" refers to the covalent bond connecting the C(=O) and OH in a carboxy group.

[0019] In the present disclosure, the term "linker group" refers to an organic group that bonds the C(=O) derived from a carboxy group with the strong acid group, and has the effect of facilitating the introduction of a strong acid group into a carboxy group or increasing the degree of positional freedom of the introduced strong acid group. Specific linker groups are not particularly limited as long as they exhibit the above-mentioned effect, but examples thereof include C 1-6 Alkanediyl group, C 6-12Examples of the linker group include an aromatic hydrocarbon group, an amino group (-NH-), an ether group (-O-), a thioether group (-S-), an ester group (-O-C(=O)- or -C(=O)-O-), an amide group (-NH-C(=O)- or -C(=O)-NH-); and groups in which two or more and five or less of these groups are linked together. The linker group preferably has an amino group (-NH-) or an ether group (-O-) at the end on the carboxyl group side, since this can be bound to surfactin or its salt by amidation or esterification of the carboxyl group.

[0020] Furthermore, if the linker group contains a fluorescent group, the surfactin derivative (I) will have fluorescent coloring properties, which will be useful for structural analysis of aggregates of the surfactin derivative (I) in a solvent and for examining the dynamics of the surfactin derivative (I). Examples of the fluorescent group include a naphthalenediyl group, a coumarindiyl group, a benzofurazandiyl group, a fluoresceindiyl group, an acridinediyl group, and a pyrenediyl group.

[0021] R 2 or R 3 At least one of the two carboxy groups is a strong acid group selected from a sulfino group (-S(=O)OH), a sulfo group (-S(=O)OH), and a sulfate group (-OS(=O)OH). That is, in the surfactin derivative (I) according to the present invention, a strong acid group is introduced via a linker group into at least one of the two carboxy groups derived from surfactin.

[0022] The surfactin derivative (I) according to the present invention may be a salt. Examples of counter cations that form such salts include alkali metal ions such as lithium ion, sodium ion, and potassium ion, and ammonium ion. The counter cation is preferably an alkali metal ion, more preferably a sodium ion or potassium ion, and even more preferably a sodium ion.

[0023] The ammonium ion is NH4 +and quaternary ammonium ions. Examples of the substituents on the quaternary ammonium ions include organic groups such as alkyl groups such as methyl, ethyl, n-propyl, isopropyl, n-butyl, and t-butyl; aralkyl groups such as benzyl, methylbenzyl, and phenylethyl; and aryl groups such as phenyl, toluyl, and xylyl. Examples of the quaternary ammonium ions include tetramethylammonium ion, tetraethylammonium ion, and pyridinium ion.

[0024] When the surfactin derivative (I) according to the present invention contains one carboxy group and one strong acid group, both of these may be in the form of a salt, or only one of them may be in the form of a salt. When only one of them is in the form of a salt, it is considered that the strong acid group is in the form of a salt due to the difference in acidity. In the surfactin derivative (I) having two strong acid groups, both of the strong acid groups may be in the form of a salt, or only one of them may be in the form of a salt.

[0025] The surfactin derivative (I) or its salt can be easily synthesized using a water-soluble surfactin salt such as sodium surfactin or a solid acid-type surfactin. Acid-type surfactin can be obtained by dissolving a water-soluble surfactin salt such as sodium surfactin in water, adjusting the pH of the aqueous solution to less than 2 using hydrochloric acid or the like to cause precipitation, filtering, washing with water or the like, and then drying.

[0026] For example, a water-soluble surfactin salt or acid-type surfactin may be dissolved in a solvent and reacted with the compound having a strong acid group and a linker group in the presence of a condensing agent. Examples of the solvent that can be used include aprotic polar solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide; protic polar solvents such as water, methanol, and ethanol; ketone solvents such as acetone and methyl ethyl ketone; ester solvents such as ethyl acetate; halogenated hydrocarbon solvents such as dichloromethane, chloroform, and carbon tetrachloride; nitrile solvents such as acetonitrile; ether solvents such as diethyl ether and tetrahydrofuran; and mixed solvents thereof.

[0027] The number of strong acid groups introduced can be adjusted by adjusting the amount of the compound having a strong acid group and a linker group used. For example, the compound can be used in an amount of 0.8 to 2.5 times the molar ratio per mole of water-soluble surfactin salt or acid-type surfactin, and when one linker group is introduced, the molar ratio is preferably 0.9 to 1.1 times the molar ratio. Furthermore, to reduce the reactivity of the strong acid group, the strong acid group may be in the form of a salt. In this way, the number of strong acid groups introduced can be adjusted by the ratio of the compound to the water-soluble surfactin salt or acid-type surfactin. The counter cation constituting the salt is preferably the same as the counter cation constituting the salt of the target compound, surfactin derivative (I).

[0028] The condensing agent may be selected appropriately. For example, when introducing a strong acid group by amidating the carboxy group of surfactin, the amidation reaction between the carboxy group and the amino group can be selectively promoted by using a strong acid group-containing compound having an amino group at the linker end and a triazine condensing agent such as 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMT-MM).

[0029] Alternatively, the carboxyl group of surfactin may be converted into an active ester or acid chloride, and then reacted with a linker group having a strong acid group in the presence of a base. Alternatively, a linker group may be introduced first, and then the strong acid group may be bonded to the introduced linker group.

[0030] The reaction conditions may also be adjusted appropriately. For example, the reaction may be carried out until consumption of the raw material compound can be confirmed by thin layer chromatography or the like, and the reaction time may be determined by a preliminary experiment. For example, the reaction may be carried out at 0°C or higher and 150°C or lower for 5 minutes or longer and 24 hours or shorter.

[0031] After the reaction is completed, the target compound, surfactin derivative (I), can be purified by a conventional method, for example, by chromatography such as silica gel column chromatography.

[0032] According to the above reaction conditions, 1 or 2 moles of strong acid groups can be introduced per mole of surfactin. However, when 1 mole of strong acid groups is introduced per mole of surfactin, the reactivity of the aspartic acid side chain carboxy groups and glutamic acid side chain carboxy groups of surfactin remains almost unchanged, and therefore, in the obtained surfactin derivative (I), the introduction positions of the strong acid groups are considered to be approximately 1:1 between the aspartic acid side chain carboxy groups and the glutamic acid side chain carboxy groups.

[0033] Because surfactin precipitates from an aqueous solution of sodium surfactin at a pH of about 6 or less, sodium surfactin cannot be used in compositions with a pH of about 6 or less. In contrast, the surfactin derivative (I) and its salts according to the present invention do not precipitate until a pH of about 2, and can therefore be used in compositions with a pH of about 2 or more, even if the pH is 6 or less, and exhibit surface activity. Therefore, the surfactin derivative (I) and its salts according to the present invention can be used as surfactants in a wider range than surfactin, which is a safe and excellent surfactant. In the present disclosure, a surfactant refers to a compound that has hydrophilic and hydrophobic groups in its molecule and can reduce the interfacial tension of a solution by dissolving in the solution.

[0034] This application claims the benefit of priority to Japanese Patent Application No. 2020-69954, filed on April 8, 2020. The entire content of the specification of Japanese Patent Application No. 2020-69954, filed on April 8, 2020, is incorporated herein by reference. [Example]

[0035] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the following examples, and it is possible to carry out the invention by making appropriate modifications within the scope of the above and below-described aims, and all such modifications are included in the technical scope of the present invention.

[0036] Example 1: Synthesis of surfactin derivative salts having sulfonic acid groups Sodium surfactin (5 g, 4.8 mmol) was dissolved in water (50 mL), 11 N dilute hydrochloric acid (10 mL) was added, and the mixture was stirred at room temperature for 15 minutes to precipitate acid-form surfactin. The precipitated acid-form surfactin was collected by filtration, washed with water, and then dried overnight in a desiccator to obtain a white powder of acid-form surfactin (yield: 75%, yield: 3.6 g, 3.6 mmol). To improve water solubility, we investigated the incorporation of sodium 5-(2-aminoethylamino)-1-naphthalenesulfonate (Sodium 5-(2-aminoethylamino)-1-naphthalenesulfonate) into surfactin as a fluorescent chromophore with a sulfonic acid group. Acid-type surfactin (152 mg, 0.15 mmol), sodium 5-(2-aminoethylamino)-1-naphthalenesulfonate hydrate (46 mg, 0.16 mmol), and DMT-MM (44 mg, 0.16 mmol) were dissolved in dimethylformamide (15 mL) and stirred at room temperature for 24 h. Diethyl ether (50 mL) was added to the resulting solution, and the precipitated crude product was collected by filtration. The resulting product was purified by silica gel column chromatography (eluent: chloroform / methanol = 8 / 2) to obtain the desired surfactin derivative (yield: 73%, yield: 140 mg, 0.11 mmol). The resulting compound was analyzed by MALDI-TOFMS ("AutoFlex speed," Bruker). Similar to the raw material sodium surfactin, molecular ion peaks of surfactin derivatives with different alkyl chain lengths were observed at m / z = 1313.9, 1327.9, and 1341.9. This confirmed that the sodium salt of the following surfactin derivative, in which a naphthylsulfonic acid group was introduced into one of the two carboxyl groups of surfactin, was obtained. Although the molecular ion peak of a salt containing two sodium ions was observed in MALDI-TOFMS, the pH of the aqueous solution of the resulting surfactin derivative was 6.1, which is between the pKa values ​​of 5.8 and 6.8 of the two carboxyl groups of surfactin. Therefore, it was considered that the introduced naphthylsulfonic acid group was in the form of a sodium salt, and the remaining carboxyl group was in the form of an acid or sodium salt.

[0037] [ka]

[0038] The resulting compound was analyzed by reversed-phase high performance liquid chromatography under the following conditions. Column: "Inertsil SIL-100A 5 mm C18 column" manufactured by GL Sciences, 0.46 x 25 cm Eluent: Water / acetonitrile / trifluoroacetic acid = 99 / 1 / 0.1 for 5 minutes → 10 / 90 / 0.07 over 20 minutes Flow rate: 1mL / min Detection wavelength: 220nm As a result, the raw material surfactin was not detected in the obtained compound, and its retention time was shorter than that of the raw material surfactin, which revealed that the hydrophilicity was improved by the introduction of sulfonic acid groups.

[0039] Example 2: Synthesis of surfactin derivative salts having sulfonic acid groups Acid-form surfactin (400 mg, 0.40 mmol), sodium 5-(2-aminoethylamino)-1-naphthalenesulfonate hydrate (123 mg, 0.43 mmol), and DMT-MM (119 mg, 0.43 mmol) were dissolved in dimethylformamide (15 mL) and stirred at room temperature for 24 hours. Diethyl ether (30 mL) and acetone (10 mL) were added to the reaction solution, and the precipitated crude product was collected by filtration. Purification using an automated flash purification system ("IsoLera One" Biotage) yielded a powder containing a surfactin derivative with two sulfonate groups as the main product (yield: 82 mg). The obtained compound was analyzed by MALDI-TOFMS ("AutoFlex speed", Bruker). Molecular ion peaks of surfactin derivatives with different alkyl chain lengths were observed at m / z=1569.9, 1583.9, and 1597.9, confirming that the sodium salt of the following surfactin derivative, in which naphthylsulfonic acid groups were introduced into the two carboxyl groups of surfactin, was obtained.

[0040] [ka]

[0041] Test example 1: Confirmation of water solubility, etc. Purified water (5 mL) was placed in a test tube, and the sodium salt of the surfactin derivative (18 mg) obtained in Example 1 was added thereto and shaken. As a result, the sodium salt of the surfactin derivative was able to dissolve in water. Furthermore, the particle size distribution of the particles contained in the above aqueous solution was measured using a dynamic light scattering analyzer ("DLS-7000" manufactured by Otsuka Electronics Co., Ltd.). The results are shown in Figure 1. As shown in Figure 1, it was revealed that the above surfactin derivative sodium salt formed giant aggregates in water with an average particle size of 91.0±18.5 nm.

[0042] Test Example 2: Fluorescence evaluation In the vial, the concentration is 1.0 x 10 -4The sodium salt of the surfactin derivative of Example 1 was dissolved in water so that the concentration became M. The absorption spectrum of this solution was measured using an ultraviolet-visible spectrophotometer ("V-560" manufactured by JASCO Corporation). The results are shown in Figure 2. As shown in Figure 2, an absorption peak derived from the naphthyl group of the sodium salt of the surfactin derivative was observed at 339 nm. Furthermore, when the fluorescence spectrum was measured using a spectrofluorometer (FP-8500, manufactured by JASCO Corporation) with an excitation wavelength of 339 nm, a blue fluorescence peak was observed at 505 nm. Figure 1 shows photographs of the solution irradiated with and unirradiated with light of 339 nm wavelength. When irradiated with light of 339 nm wavelength, blue emission was observed. This clearly demonstrates that a fluorescent chromophore has been introduced into the surfactin derivative.

[0043] Test Example 3: Surface activity evaluation In the vial, the concentration is 2.5 x 10 -6 ~1.0×10 -3 The sodium salt of the surfactin derivative of Example 1 was dissolved in water so that the concentration became M. Each aqueous solution was transferred to a petri dish, and the surface tension of the sodium salt of the surfactin derivative relative to water was measured using a surface tension measuring device (DY-500, Kyowa Interface Science Co., Ltd.) using the Wilhelmy method. The results are shown in Figure 3. As shown in Figure 3, the sodium salt of the surfactin derivative of Example 1 was well dissolved in water at room temperature without precipitation, and reduced the surface tension. In other words, it exhibited interfacial activity. Furthermore, from the concentration at which the surface tension becomes constant in the graph of Figure 3, the critical micelle concentration (CMC) was calculated to be 7.7 x 10 -5 M, and the surface tension of water (γCMC) was determined to be 28.2 mN / m.

[0044] Test Example 4: Acid resistance of surfactin derivatives In a vial, the sodium salt of the surfactin derivative (10 mg) of Example 1 was dissolved in water (10 mL) to obtain an aqueous solution with a concentration of 1 mM. The pH of this aqueous solution was measured with a pH meter ("F-74" manufactured by HORIBA Corporation) and found to be 6.1. Dilute hydrochloric acid was gradually added to the above solution to evaluate the effect of pH change on solubility. The solubility was measured by measuring the transmitted light at 650 nm in the UV-visible spectrum using a UV-visible spectrophotometer ("V-560" manufactured by JASCO Corporation) and calculating the relative percentage of the transmitted light amount of the initial aqueous solution, with the amount of transmitted light being 100%. The pH of each aqueous solution was also measured using the pH meter. For comparison, the experiment was carried out in the same manner except that the sodium salt of the surfactin derivative was changed to sodium surfactin. The results are shown in Figure 4. As shown in the results in Figure 4, it was revealed that in the aqueous solution of sodium surfactin, precipitates began to form at around pH 6, whereas in the aqueous solution of the sodium salt of the surfactin derivative of Example 1, no precipitates formed and the solution remained transparent up to a pH of about 2. These results demonstrate that the introduction of sulfonic acid groups improves the acid resistance of surfactin, making it possible to use it as a surfactant even at lower pH levels.

[0045] Example 5: Preparation of Nanodiscs First, liposomes serving as nanodisc precursors were prepared. L-α-dimyristoylphosphatidylcholine (hereinafter abbreviated as "DMPC") was weighed into a test tube, and chloroform was added to dissolve the DMPC. Nitrogen gas was sprayed into the test tube while stirring with a vortex mixer, and the chloroform was removed, forming a lipid film on the wall of the test tube. After leaving the tube in a desiccator for 2 days, water was added and the mixture was hydrated at room temperature for 10 minutes. A 5 mM DMPC liposome solution was then prepared by stirring with a vortex mixer for 3 minutes. Separately, an aqueous surfactin solution was prepared. Sodium surfactin and the sodium salt of the surfactin derivative from Example 1 were weighed into a test tube, and a mixture of the two was prepared by adding water. The resulting DMPC liposome aqueous solution and surfactin mixture were mixed with water to adjust the concentrations of sodium surfactin and DMPC to 2.5 mM, and the sodium salt of the surfactin derivative to 0.025 mM. The resulting mixture was irradiated with 365 nm ultraviolet light. Photographs of each solution are shown in Figure 5. As shown in Figure 5, the DMPC liposome solution was in a suspended state, but the addition of surfactin sodium and the surfactin derivative sodium salt significantly reduced the turbidity and turned it into a transparent solution. This confirmed that nanodiscs were formed from DMPC, surfactin sodium, and the surfactin derivative sodium salt. It was also confirmed that when the resulting transparent nanodisk solution was irradiated with 365 nm ultraviolet light, it glowed blue.

[0046] The resulting nanodisk transparent solution was subjected to dynamic light scattering measurement to determine the diameter of the particles contained therein. The results are shown in Figure 6. As shown in Figure 6, the particle diameter of the formed nanodisks was estimated to be 7.8 ± 1.4 nm.

[0047] To clarify the detailed structure of nanodiscs, we investigated the encapsulation of hydrophobic dyes within nanodiscs. Specifically, to prepare liposomes, which serve as precursors to nanodiscs, DMPC and the hydrophobic dye Sudan III were weighed into a test tube and dissolved in chloroform. Nitrogen gas was sprayed into the test tube while stirring with a vortex mixer, and the chloroform was removed, forming a lipid film on the test tube wall. After leaving the mixture in a desiccator for 2 days, water was added and the mixture was hydrated at room temperature for 10 minutes. Subsequently, DMPC liposomes encapsulating Sudan III were prepared by vortex mixing for 3 minutes. Next, sodium surfactin and a sodium salt of a surfactin derivative were weighed into a test tube, and a mixture of the two was prepared by adding water. The resulting DMPC liposomes and the surfactin mixture were mixed with water to adjust the concentrations of sodium surfactin and DMPC to 2.5 mM, and the concentrations of sodium surfactin and the sodium salt of a surfactin derivative and Sudan III to 0.025 mM. As a result, the turbidity of the DMPC liposome solution decreased significantly, turning it into a clear red solution (Figure 7), confirming the formation of nanodisks.

[0048] The resulting transparent nanodisc solution was subjected to dynamic light scattering measurement to determine the diameter of the particles contained therein. The results are shown in Figure 8. As shown in Figure 8, the particle diameter of the formed nanodiscs was 7.2 ± 1.5 nm, and no change in particle diameter was observed before and after the encapsulation of Sudan III, suggesting that the nanodisc structure was maintained.

[0049] Fluorescence spectroscopy of this nanodisc solution confirmed that the fluorescence intensity was significantly lower than that without Sudan III (Figure 9). This result is thought to be due to the spatial proximity of Sudan III and the sodium salt of the surfactin derivative, which led to the progression of fluorescence resonance energy transfer. These results demonstrate that the sodium salt of the surfactin derivative is incorporated as a component of nanodiscs and can function as a fluorescent probe for the structural analysis of such molecular assemblies.

Claims

1. A surfactin derivative or a salt thereof, characterized by being represented by the following formula (I): 【Chemical 1】 [In the formula, X 1 represents an amino acid residue selected from leucine, isoleucine, and valine; X 2 and X 3 is a linker group, or R 2 or R 3 is OH, it represents a covalent bond connecting C(=O) and the OH, the linker group is a C 1-6 alkanediyl group, a C 6-12 aromatic hydrocarbon group, an amino group, an ether group, a thioether group, an ester group, an amide group, or a group in which two or more and five or less of these groups are linked together, and may further include a fluorescent group selected from a naphthalenediyl group, a coumarindiyl group, a benzofurazandiyl group, a fluoresceindiyl group, an acridinediyl group, and a pyrenediyl group; R 1 is C 9-18 represents an alkyl group, R 2 and R 3 represents OH or a strong acid group selected from a sulfo group and a sulfate group, However, R 2 or R 3 one of which is OH and the other is a strong acid group.]

2. The linker group is C 1-6 Alkanediyl group, C 6-12 2. The surfactin derivative or salt thereof according to claim 1, which comprises an aromatic hydrocarbon group, an amino group, an ether group, a thioether group, an ester group, an amide group, or a group in which two or more and five or less of these groups are linked together, and which contains a fluorescent group selected from a naphthalenediyl group, a coumarindiyl group, a benzofurazandiyl group, a fluoresceindiyl group, an acridinediyl group, and a pyrenediyl group.

3. 3. The surfactin derivative or salt thereof according to claim 1 or 2, which is a salt of the surfactin derivative represented by the above formula (I).

4. 3. The surfactin derivative or salt thereof according to claim 1 or 2, which is an alkali metal salt of the surfactin derivative represented by the above formula (I).

5. A surfactant comprising the surfactin derivative or a salt thereof according to any one of claims 1 to 4.

6. 5. Use of the surfactin derivative or a salt thereof according to any one of claims 1 to 4 as a surfactant.

7. 1. A method for reducing the interfacial tension of a solution, comprising: A method comprising the step of dissolving the surfactin derivative or a salt thereof according to any one of claims 1 to 4 in the solution.

Images