Alkanoyl-6-O-ascorbic acid  esters의 제조 및 특성

한연규, 오성근, 김인경*, 조창기*

한양대학교 화학공학과, 한양대학교 섬유공학과*

The preparation of the alkanoyl-6-O-ascorbic acid  esters

and their properties

Youn-Kyu Han, Seong-Geun Oh, In-Kyoung Kim* and Chang-gi Cho*

Department of Chemical Engineering, Hanyang University,

Department of Textile Engineering, Hanyang University*

Introduction

L-ascorbic acid(vitamin C), a hydrophilic antioxidant and radical scavenger widely distributed in aerobic organisms, is known to protect cellular components against oxidative damage by free radicals and reactive oxygen-derived species. It is also an essential nutrient involved in many physiological functions. It readily undergoes two consecutive(yet reversible), one-electron oxidation processes to form the ascorbate radical, a relatively unreactive free radical, and is therefore considered an excellent reducing agent1: its aqueous solutions are easily oxidized, especially in the presence of trace of metals. In addition, there is considerable evidence that biological antioxidants including ascorbic acid play an important role in the prevention of a large number of chronic diseases such as cancer, heart disease, brain dysfunction, and AIDS2. And ascorbic acid is active in many biological processes; for example, it maintains proly hydroxlase in an active form by preserving its iron atom in the reduced Fe2+ state3, and it keeps collagen in a sufficiently hydroxylated form, preventing skin lesions and blood-vessel fragility that are so prominent in scurvy4.

But ascorbic acid is a water soluble molecule so that it has a limitation  for using with a oil. Although there are many reports showed methods to use with oil, they almost used a emulsion or a microemulsion. The ascorbic acid derivatives are therefore a very promising and important tool for the stabilization and protection of many different systems, such as biological materials, food, and artificial membranes used as drug carriers, under both the scientific and industrial points of view, and their properties need to be fully elucidated.

The stearoyl-6-O-ascorbic acid was synthesised by Swern et al5 in 1943, but its surface properties was not investigated. It consists of a hydrophilic group and a hydrophobic group. Therefore it can be adapted as a surfactant. We prepared octanoyl-, decanoyl-, lauroyl-, palmitoyl-, and stearoyl-6-O-ascorbic acid and studied their surface properties. The pH and temperature effect have been partially investigated as well.

Experimental

Sulfuric acid, sodium chloride, diethyl ether, petroleum ether and magnesium sulfate were purchased from Duksan pharmaceutical Co. LTD. L-ascorbic acid, stearic acid, palmitic acid, lauric acid, decanoic acid ,octanoic acid and barium chloride were from Aldrich. The double distilled deionized water were used for measuring of surface tension. 1H NMR as well as FT-IR and UV/vis, was carried out in the University of Hanyang. Elemental analysis was carried out in the University of Kyungpook. The surface tensions were measured by a surface tensiometer(Kr□ss K10ST, Germany) using a platinum-iridium plat at various temperatures and pHs. Since ascorbic acid is very sensitive to atmospheric oxygen, we performed all synthesis using inert nitrogen gas. The platinum-iridium plat before measuring the surface tension is washed with acetone  to remove organic impurities and water. Finally it was purified by burning with the gas torch.

Result and Discussion

All the vitamin C derivatives were synthesized according to  the procedure already in the literature1, which involves the reaction in concentrated sulfuric acid between stearic acid and the primary -OH group in position 6 of L-ascorbic acid2.

Preparation of (A) Stearoyl 6-O-ascorbic acid - A 100ml sample if 95% sulfuric acid was placed in a 2-neck flask, and a gentle stream of bubbling nitrogen was passed for 30min under magnetic stirring at room temperature to remove oxygen. Then 8.8g of L-ascorbic acid(50mmol) was added and completely dissolved, followed by 11.4g of stearic acid(50mmol). The flask equipped with a rubber stopper and kept in a nitrogen atmosphere. A oil bath at 45℃ was used to completely dissolve the stearic acid. After 16hr, the mixture was poured into  a 1000ml beaker containing 600ml of ice and stirred until it reached room temperature. The solution was then transferred to a separatory funnel and treated with diethyl ether several times. Should a thick emulsion be formed, Some brine was added in order to break the emulsion. The organic phase was then washed with distilled water, and the presence of sulfate ions was tested with barium chloride. the organic was suspended a anhydrous magnesium sulfate for 30min and then filtered. The white-yellow solid that was washed in petroleum ether and finally recrystallized three times from diethyl ether/petroleum ether(31%). TLC( Tetrahydrofuran/ethyl alcohol=50/1 ) showed the presence only one spot with Rf=0.775. Elemental analysis for stearate: C, calcd 65.13, found 65.11;H, calcd 9.56, found 9.58; O, calcd 25.31, found 25.31. NMR(□,  ppm; DMSO-d6): 0.8541(triplet, CH3); 1.2368(side chain -CH2-);  1.5254(triplet, CH2); 2.3133(triplet, CH2); 4.047-4.062(multiplet, O-CH2-CH); 4.6675(singlet, CH); 8.403(C2-OH); 11.115(C3-OH). UV/vis spectrum(□max, nm; CH3CN): 236, IR spectrum(KBr, cm-1): 3400-3000, 2918, 2849, 1732, 1690, 1472, 1292, 1179, 1115.

(B) Palmitoyl 6-O-ascorbic acid - palmitoyl 6-O-ascorbic acid(6.6g) was esterified by the procedure used palmitic acid(12.82g).

Anal. C, calcd 63.74, found 63.73; H, calcd 9.24, found 9.25; O, calcd

27.02, found 27.02

(C) Lauroyl 6-O-ascorbic acid - lauroyl 6-O-ascorbic acid(5.8g) was esterified by the procedure used lauric acid(10.01g).

Anal. C, calcd 60.32, found 60.30; H, calcd 8.43, found 8.42; O, calcd

31.25, found 31.28

(D) Decanoyl 6-O-ascorbic acid - decanoyl 6-O-ascorbic acid(5.45g) was esterified by the procedure used decanoic acid(8.61g).

Anal. C, calcd 58.17, found 58.18; H, calcd 7.93, found 7.92; O, calcd 33.80, found 33.80

(E) Octanoyl 6-O-ascorbic acid - octanoyl 6-O-ascorbic acid(4.87g) was esterified by the procedure used octanoic acid(7.21g).

Anal. C, calcd 55.62, found 55.61; H, calcd 7.83, found 7.84; O, calcd 37.05, found 37.05

Air-solution surface tension - Air-solution surface tensions □ were determined as a function of both surfactant concentration c and temperature(25℃∼40℃). Example □ vs c curve of Octanoyl 6-O-ascorbic acid, measured at 25℃, are shown in figure 2. The surface tensions at various pH were measured as well. Before measuring, the products has been purified by using column chromatography method and filtering.

The surface tension will decrease until a point is reached where the plot abruptly levels off; this point is taken to be the CMC. The sharp bend in the plot results from surfactant joining micelles rather than adsorbing at the air/water interface. Most of the products form a micelle according to surface tension data collected from 0.05mM to 10mM. Their value is on the order of 3∼4. The surface tension at the cmc( □cmc ) is a measure of the effectiveness of a surfactant. As expected, increasing the hydrocarbon chain decreases □cmc6. The efficiency factor pC20, the negative logarithm of the concentration of surfactant in the bulk phase required to produce a 20-dyn/cm reduction in the surface or interfacial tension of the solvent, showed the effiency increases with increases in the hydrophobic character of the surfactant7.

The changes in □ for pre-cmc concentrations can be described using the Gibbs isotherm, and estimated for as(the area per molecule) at the cmc. It is calculated from the relation

[eqn: a ^ s = { 10^16} over {N GAMMA }]

where N=Abogardo's number and □ is in mol/cm2.

References

1. Buettner, G.R., Jurkiewiez, B.A., 1996. Chemistry and biochemistry of ascorbic acid. In: Cadenas, E., Paker, L.(Eds.), Handbook of Antioxidants. Marcel Dekker, New York, pp. 91-115.

2. Rose, R.C.; Bode, A.M. FASEB J. 1993, 7, 1135. Ames, B.N.; Shigenaga, M.K.; Hagen, T.M. Proc. Nat. Acad. Sci. U.S.A. 1993, 90, 7915

3. Stryer, L. Biochemistry, 4th ed.; W.H. Freeman & Co.; New York, 1995, 454.

4. Liu, Z. L.;Wang, P. F.; Liu, Y. C. Sci. China Ser. B 1992, 35, 1307.

5. Swern, D.; Stirton, A. J.; Turer, J.; Wells, P. A. Oil & Soaps 1943, 224.

6. Rosen, milton J. Surfactants and interfacial phenomena 2nd ed.; John & Sons, Inc. 1989, 226.

7. Rosen, milton J. Surfactants and interfacial phenomena 2nd ed.; John & Sons, Inc. 1989, 213.

Fig. Synthetic scheme of alkanoyl-6-O- ascorbic  acid

Fig . the surface tension of octanoyl 6-O-ascorbic acid at 25℃