JOURNAL OF COLLOID AND INTERFACE SCIENCE 186, 271–279 (1997) ARTICLE NO. CS964646 Mixtures of Ascorbyl-stearate and Vitamin D 3 : A Monolayer Study at the Gas/ Water Interface GIULIA CAPUZZI,* KAVITA KULK ARNI,² J ACK E. FERNANDEZ,² F RANCO F. VINCIERI,‡ AND PIERANDREA LO NOSTRO * ,1 * Dipartimento di Chimica, Universita ` degli Studi di Firenze, 50121 Florence, Italy; ² Department of Chemistry, University of South Florida, Tampa, Florida 33620; and ‡ Dipartimento di Scienze Farmaceutiche, Universita ` degli Studi di Firenze, 50121 Florence, Italy Received May 13, 1996; accepted October 14, 1996 water phase and perform their radical scavenger activity. Vitamin C and vitamin D 3 perform different and important The study of the properties of aggregates formed by vitamin activities in living systems. Vitamin D 3 forms stable monomolecu- C derivatives and other important materials (vitamins, lar films at the gas/water interface and, at higher concentrations, drugs, fats, etc.) is an important issue for the food industry, produces large aggregates in water solutions. Because of their am- for pharmaceutical applications, and for the therapy of sev- phiphilic structure, vitamin C alkyl esters form monolayers, mi- eral diseases that are related to the activity of oxidizing celles, vesicles, or microemulsions in water, depending on composi- radical species (aging, ischemia–reperfusion, brain and tion, concentration, solvent, and temperature. In this work the kidney damage, inflammatory disorders, gastric ulcer, can- monolayer of 6-O-ascorbyl-stearate and vitamin D 3 was studied cer, radiation damage, xenobiotic-induced liver damage, to investigate the behavior of the two compounds and their mix- tures at the gas/water interface at different temperatures, at differ- atherogenesis, and AIDS) (2, 6–8). ent pH, and in the presence of Ca 2/ in the subphase. The results Ascorbic acid (see Fig. 1a) contains four hydroxyl show that stable Langmuir films are formed in all molar ratios, groups: the –OH in position 2 and 3 are both acidic (p K a with repulsive interactions between the two components, and that Å 11.6 for C 2 –OH and 4.2 for C 3 –OH), those in position the presence of calcium ions modifies the properties of the mono- 5 and 6 behave as secondary and primary alcoholic residues, layer.  1997 Academic Press respectively. The ascorbic acid ring is negatively charged in Key Words: ascorbyl-6-O-stearate; vitamin C; vitamin D 3 ; water, and a tautomeric equilibrium between C 1 |O and spreading monolayer( s ) ; miscibility; mixed monolayer( s ) . C 3 {OH groups occurs with the shift of the double bond. Ethers and esters of ascorbic acid in positions 2, 3, 5, and 6 can be easily synthesized according to different procedures INTRODUCTION (6–10). Figure 1b shows the minimized CPK model for 6- O-ascorbyl-stearate. Ascorbic acid, as well as its derivatives, are strong radi- The biological activity of vitamin D 3 is related to the cal scavengers (1), and have already had very interesting control of calcium and phosphorus metabolism. The biosyn- applications as protective agents for fats, vitamins, and thesis of vitamin D 3 begins with the transformation of cho- other degradable materials. Ascorbic acid is water soluble, lesterol into 7-dehydrocholesterol ( or provitamin D 3 ) and but the addition of one or two hydrocarbon chains to the then, through a UV-catalyzed photolysis, into previtamin D 3 . ring, as esters or ethers, produces amphiphilic molecules After spontaneous isomerization, previtamin D 3 is converted that manifest strong surface activity in aqueous solutions. into vitamin D 3 (or cholecalciferol, see Fig. 1c) and finally In this form, vitamin C can easily produce self-assembled into calcitriol, which is the active hormone. Vitamin D 3 supramolecular aggregates, such as monolayers, micelles, forms micellar-like aggregates in water solutions under vesicles, and microemulsions, and can readily penetrate physiological conditions, and produces stable monomolecu- biomembranes and perform its antioxidant function (2, 3– lar films at the air/water interface (11). 5). Assembled in these supramolecular structures, vitamin The biological activity of several important compounds C derivatives protect hydrophobic degradable materials. In is dramatically affected by their physical state. For instance, fact lipophilic molecules, such as tocopherols and linoleic many enzymes, such as porcine pancreatic lipase, soybean acid, are segregated and protected in the micellar hydropho- lipoxygenase, and phospholipase A 2 , perform their activity bic core while the ascorbic acid polar head groups face the more rapidly when they react with substrates dispersed in supramolecular aggregates such as micelles and microemul- sions (11, 12). Vitamin D itself must be metabolized by at 1 To whom correspondence should be addressed. E-mail: LONOSTRO@ CHUMA.CAS.USF.EDU. least two enzymes to exert its full biological activity (11). 271 0021-9797/97 $25.00 Copyright  1997 by Academic Press All rights of reproduction in any form reserved.