J. Phys. Chem. B 2009, 113, 6321–6327
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Catanionic Vesicles Formed with Arginine-Based Surfactants and 1,2-Dipalmitoyl-sn-glycero-3-phosphate Monosodium Salt Neus Lozano,† Aurora Pinazo,† Camillo La Mesa,‡ Lourdes Perez,† Patrizia Andreozzi,‡ and Ramon Pons*,† Departament de Tecnologia Quı´mica i de Tensioactius, Institut de Quı´mica AVanc¸ada de Catalunya (IQAC), CSIC, Jordi Girona 18-26, E-08034 Barcelona, Spain, and Dipartimento di Chimica, UniVersita` degli Studi “La Sapienza”, Piazzale Aldo Moro 5, I-00185 Rome, Italy ReceiVed: December 4, 2008; ReVised Manuscript ReceiVed: March 17, 2009
We report on mixing an anionic diacyl phospholipid (1,2-dipalmitoyl-sn-glycero-3-phosphate monosodium salt, DPPA) with either monoacyl and diacyl arginine-based surfactants. These mixtures are part of the rich family of pseudo-triple-chain and pseudo-tetra-chain catanionic mixtures, respectively. Vesicle size and ζ-potential were measured at several mixing ratios. Additional information on counterion binding, vesicle size, and integrity was obtained from ion selective electrode and Cryo-TEM measurements. Addition of positively charged surfactants to DPPA results in an increase of vesicle size. However, ζ-potential shows different trends, depending on whether water or acid media are used as solvent. In the latter, ζ-potential values progressively approach 0 upon addition of amino acid based surfactants. In water, surprisingly, ζ-potential values become more negative. The results are discussed in terms of modifications in counterion binding and vesicle size. 1. Introduction In search for new antimicrobial compounds to be used in food, pharmaceutical, and cosmetic applications,1,2 a novel class of cationic arginine glyceride conjugates was developed.3-5 Preliminary results on their physicochemical and biological properties showed that such novel compounds combine the advantages of glycerides and lipoamino acids.6 These surfactants consist of two aliphatic chains with arginine as polar head, linked through ester bonds to the glycerol backbone. They can be considered structural analogues of phospholipids and exhibit similar properties, as well as low toxicity profiles and antimicrobial activity.7,8 Compared with conventional cationic surfactants, the arginine glyceride conjugates are considered soft preservative cationic surfactants. We have already shown9 that arginine-based surfactants present two acid-base equilibriums, due to the presence of guanidinium and NR amino groups, respectively. Such equilibriums are among the driving forces controlling the aggregation of arginine-based surfactants. We previously elucidated the relationships between surface activity and biological properties of 1,2-dimyristoyl-rac-glycero3-O-(NR-acetyl-L-arginine) hydrochloride (1414RAc) and 1,2dilauroyl-rac-glycero-3-O-(NR-acetyl-L-arginine) hydrochloride (1212RAc).10 The former is the analogue of 1212RAc with two more methylene groups per chain. In this work, we report on the stability of vesicular formulations based on mixtures containing the aforementioned synthetic surfactants and 1,2dipalmitoyl-sn-glycero-3-phosphate monosodium salt (DPPA) in variable amounts. In cationic/anionic (catanionic) mixtures, the strong interactions between head groups, modulated by tail-tail ones, lead to strong association and a high degree of nonideality.11 There * Corresponding author. Tel: (34) 93 400 61 50. Fax: (34) 93 204 59 04. E-mail:
[email protected]. † CSIC. ‡ Universita` degli Studi “La Sapienza”.
exists now a good wealth of information concerning mixtures of cationic and anionic surfactants. When the systems are equimolar mixtures, they are defined as catanionic surfactants. Catanionic mixtures can be prepared by mixing a cationic surfactant with an anionic one. The process produces, in addition to the catanionic compound, the release of the respective counterions. Catanionic mixtures have also been prepared as true binary salts, by using hydroxyl- and hydronium-based surfactants.12 Catanionic mixtures follow the sequence micelles f swollen micelles f cylindrical micelles f vesicles f precipitate13,14 on increasing the minority surfactant ion concentration up to charge neutralization. The surfactant ions used to prepare catanionic mixtures can be single or multiple chain ones. Mixing oppositely charged single chain surfactant ions gives pseudo-double-chain catanionic surfactants.15-17 However, a large amount of the literature deals with single-double chain surfactants, forming pseudo-triple-chain catanionic mixtures.18-23 At present, the phase behavior and the fluorescence characterization of pseudo-tetra-chain catanionic surfactant (DDABAOT) systems have been reported by Caria et al.24 and Karukstis et al.,25 respectively. The ζ-potential of catanionic mixtures has been studied by several authors.26-28 Findings show that, on increasing the relative proportions of cationic surfactant, a charge reversal occurs symmetrically around the equimolar mixture. This process is accompanied by a vesicle growth around the neutralization point. The ζ-potential can also be modulated by the presence of salts that reduce its absolute value.29 Changes in ζ-potential correlate with changes in acid-base interactions, which essentially affect the stability of vesicular systems.30 Such changes may stabilize the vesicular entities, because of relevant changes in their surface charge density and double layer thickness, as well. It is expected, on these grounds, that the presence of arginine glyceride conjugates plays a pivotal role in the stabilization of mixed vesicles and in the control of the related size and dispersity.
10.1021/jp810671p CCC: $40.75 2009 American Chemical Society Published on Web 04/14/2009
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J. Phys. Chem. B, Vol. 113, No. 18, 2009
Lozano et al.
CHART 1: Chemical Structures of the Cationic and Anionic Amphiphiles Tested in This Work
The outline of this paper is as follows. We present, first, studies on vesicle size and ζ-potential of systems made of cationic surfactant and DPPA as a function of the surfactant mole fraction in the mixture and pH. ζ-Potential trends are then discussed and rationalized in terms of changes in counterion binding degree and in particle size. 2. Experimental Section 2.1. Materials. The monoacyl (140RAc) and diacyl (1212RAc, 1414RAc) glycerol arginine-based surfactants were synthesized according to the method of Pe´rez et al.31 Their purity, higher than 99%, was checked by HPLC, C13 NMR, and elemental analysis. LAM (Nr-lauroylarginine methyl ester hydrochloride) was synthesized according to the work of Infante et al.32 and was obtained with purity higher than 99%. 1,2-Dipalmitoyl-snglycero-3-phosphate monosodium salt (DPPA) was a generous gift from LIPOID GmbH, with a purity of 99.8%. The chemical structures of all surfactants used in this contribution and of the phospholipid are shown in Chart 1.Water was obtained using an Milli-QPLUS 185 water system (resistivity ) 18.2 MΩ cm). 2.2. Preparation of Vesicles. Cationic surfactant/DPPA aqueous dispersions were prepared by weighing separately aqueous DPPA and aqueous cationic surfactant at the desired concentration. The samples were heated at 90 °C for 15 min before mixing to obtain readily vesicle dispersions. DPPA and cationic dispersions were mixed with vigorous stirring at 90 °C, obtaining the catanionic systems. The resulting dispersions were cooled down to room temperature within 10 min. According to Pe´rez et al., the products undergo thermal hydrolysis at high temperatures and long times (4% hydrolysis after 24 h at 40 °C in the worst case).31 The chemical stability of DPPA in the experimental conditions was checked by C13 NMR, and no hydrolysis was detected (lower detection limit
