Ultralong Fatty Acyl Derivatives As Occlusive Structure Lipids for

Research Article pubs.acs.org/journal/ascecg

Ultralong Fatty Acyl Derivatives As Occlusive Structure Lipids for Cosmetic Applications: Synthesis and Characterization Bianca Pérez,† Pallav Bulsara,‡ Anthony Vincent Rawlings,§,○ Wei Wei,† Mads Mørk Jensen,∥,⊥ Zegao Wang,⊥ Jason Dickens,‡ Shuai Zhang,⊥ Russell P. Elliot,‡ Marianne Glasius,∥,⊥ Mingdong Dong,⊥ Martyn Clarke,‡ and Zheng Guo*,† †

Department of Engineering, Aarhus University, Gustav Wieds Vej 10C, Aarhus 8000, Denmark Skin Health Innovation, GlaxoSmithKline Consumer Healthcare, 184 Liberty Corner Road, Warren, New Jersey 08844, United States § AVR Consulting Ltd, 26 Shavington Way, Northwich, Cheshire CW9 8FH, United Kingdom ⊥ Interdisciplinary Nanoscience Center, Aarhus University, Gustav Wieds Vej 14, Aarhus 8000, Denmark ∥ Department of Chemistry, Aarhus University, Langelandsgade 140, Aarhus 8000, Denmark ‡

ABSTRACT: Finding sustainable and commercially viable sources of occlusive materials, as an alternative to petroleum, is of great interest. Inspired by the fundamental role of long chain fatty acids in maintaining skin barrier, ultralong fatty acyl derivatives with diverse structures (varied acyl chain length and different polar head; i.e. glycerol, ethylene glycol, and diethylene glycol) were synthesized. These molecules can be feasibly obtained via enzymatic esterification of fatty acids or fractionated from commercial glycerides mixture via short path distillation. The molecular packing behaviors of compounds were characterized via differential scanning calorimetry, Fourier transform infrared, and Langmuir isotherm measurements. The structure−property relationship study reveals that a glycerol molecule monoacylated with an ultralong fatty acyl is the derivative which entails the most occlusive properties of the series of ultralong chain fatty acid derivatives. Fast Fourier transform filtering (FFTF) analysis of atomic force microscopy images verified a homogeneous monolayer packing of glyceryl monobehenate monolayer, and the water vapor transmission study demonstrated that the formulation of glyceryl monobehenate at 3% w/w in a phospholipid-containing emulsion generates an occlusive film significantly superior to a 3% w/w petrolatum formulation. This work demonstrated that natural glyceryl monobehenate can be a novel source of sustainable occlusive structuring agents and green replacements for petrolatum. KEYWORDS: Glyceryl monobenhenate, Petrolatum, Stratum corneum, Orthorhombic lateral packing, Occlusive materials, Water vapor transmission rate

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“brick wall” structure where the bricks refer to the corneocytes and the mortar between the bricks is the lipid-rich matrix.5−7 This lipid matrix contains mainly ceramides, cholesterol, and fatty acids of varying chain lengths (C16−C26).8 These molecules organize themselves in two lamellar phases; namely the long periodicity phase (LPP) and short periodicity phase (SPP). The LPP and SPP are approximately 13 and 6 nm in thickness, respectively, as determined by X-ray diffraction.5,9 Moreover, perpendicular to the lamellar phase, SC lipids organize in orthorhombic, hexagonal, and/or fluid packed structures. Healthy skin lipids are predominantly organized in orthorhombic packing states, the significance of which is that the tighter packing behavior in the orthorhombic states and

INTRODUCTION Petrolatum is a mixture of hydrocarbons derived from petroleum that has been identified as a potentially carcinogenic ingredient.1,2 This toxicity is associated with polycyclic aromatic compounds which can be removed by full refining of petrolatum.3,4 If the petrolatum detailed refining history is unknown the material is classified as carcinogenic in Europe.1 In addition, petrolatum has poor sensory attributes that can adversely affect patient and consumer compliance rates. Thus, it is important to find consumer acceptable, sustainable, green, and commercially viable moisturizer alternatives, that better mimic the skins’ own lipid organization and physical properties. The stratum corneum (SC), the outermost layer of the skin, provides a barrier between the external environment and the deeper layers of the skin offering protection from penetration of irritant chemicals and microbes while reducing transepidermal water loss (TEWL).5 The structure of SC can be described as a © XXXX American Chemical Society

Received: August 22, 2016 Revised: September 10, 2016

A

DOI: 10.1021/acssuschemeng.6b02021 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

way of the TLC plate; and then, CHCl3:MeOH:acetic acid 97.5:2.5:1 to run the full plate. Retention factors (RF) were respectively calculated. Short Path Distillation (SPD). A KD5 system (UIC GmbH, Alzenau-Hörstein, Germany) was used for fractionation of glycerides by SPD operation. This equipment consists of a feeding tank, a cylindrical body surrounded by a heating jacket with a rotor and a condenser inside, a residual and a distillate receiver and two vacuum pumps including a diffusion pump. The thermal separation was carried out at 1 × 10−3 mbar and the feeding rate used was 100 mL/h. 185 °C was used for distillation of monoglycerides and 265 °C for distillation of diglycerides using the residue of the distillation at 185 °C as the feeding substrate. The triglycerides fraction was obtained as the residue of the distillation carried out at 265 °C. The fatty acid composition of CA8 is presented in Table 1.

higher level of organization of SC lipids, results in lower TEWL and improved barrier properties.10−12 However, in skin diseases such as atopic dermatitis, hexagonal packing are known to be present driven partly by a reduction in the abundance of long chain length fatty acids and ceramides in the SC.13−16 Furthermore, long chain fatty acids are known to be depleted from SC lipids in dry skin and have been demonstrated to play a crucial role in maintaining the orthorhombic packing of these lipids.7,8,13−16 Mixtures of ceramide/cholesterol/fatty acid containing, predominantly C16 and C18 carbon chain fatty acids, do not display orthorhombic packing.9,17,18 Therefore, the use of skin care formulations containing derivatives of longer chain fatty acids which are capable of mimicking the behavior of naturally occurring lipids in the SC has been proposed to lead to a more efficient occlusive barrier.19 A high melting point lipid that contains glycerides of long chain fatty acids is Compritol ATO 888 (CA8).20 CA8 is a waxy material widely used to modify drug release behavior from drug matrices.21 This material is a mixture of lipids containing 15− 23% of monoglycerides, 40−60% of diglycerides, and 21−35% of triglycerides which presents mainly behenoyl lipids (>83%) [CA8 Technical Data Sheet]. Therefore, it can be used as a source of ultralong chain fatty acids to obtain pure derivatives of ultralong chain fatty acids and posteriorly identify structural features relevant to generate occlusive materials for cosmetic purposes. Thus, the main objectives of this work were to (i) develop the reaction approaches for synthesis of structure diverse fatty acyl derivative using natural glyceryl behenate; (ii) study the physical properties and lipid phase packing behavior of behenoyl glycerides using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), Langmuir isotherm studies, and ultimately atomic force microscopy imaging; (iii) identify the structural functionality required in glyceryl ester derivatives that form orthorhombic packing; and, finally, (iv) demonstrate that naturally occurring molecules which can mimic the behavior and lateral packing structures of SC lipids and can be potential replacements for petrolatum.

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Table 1. Fatty Acid Composition of Compritol ATO 888 fatty acyl chain

percent (%) in Compritol ATO 888

C14:1 C16:0 C18:0 C20:0 C20:2 C20:4 C22:0 C22:1 C24:0

0.29 0.67 3.12 4.59 0.12 0.56 88.7 0.09 1.86

Synthesis of Behenoyl Glycerides and Analogues. Glycerol Monobehenate (1). In a jacketed reactor with a stirring magnet, 2.9 mmol of behenic acid and 29.4 mmol of glycerol were dissolved in 12 mL of t-butanol thermostabilized at 55 °C. The reaction was initiated by addition of 0.05g Novozym 435 and 0.1 g of activated molecular sieves.22 After 2 h, the enzyme was filtered off and t-butanol was removed by rotary evaporation. Subsequently, the resulting reaction mixture was dissolved in 300 mL of chloroform and the unreacted behenic acid and glycerol were removed by washing five times (5 × 200 mL) with a saturated solution of sodium carbonate. The resulting organic layer was washed 3 times with brine solution (3 × 200 mL), dried over anhydrous sodium sulfate, filtered, and evaporated down to dryness yielding a white solid with a melting point of 77 °C (RF = 0.21; % isolation yield = 49%): 1H NMR (400 MHz, CDCl3) δ 4.25− 4.09 (m, 2H), 3.97−3.89 (m, 1H), 3.73−3.55 (m, 2H), 2.35 (t, J= 7.2 Hz, 2H), 1.70−1.53 (m, 2H), 1.48−1.02 (m, 36H), 0.88 (t, J = 6.8 Hz, 3H). HPLC-ELSD: rt = 4.79 min (% area = 95). HRMS: calculated for C25H50O4Na [M + Na]+: 437.3601; found: 437.3597. Glycerol Dibehenate (2). A 5.4 mmol portion of behenic acid and 2.7 mmol of glycerol were added to a jacketed reactor at 90 °C under vacumm. When everything was dissolved, 0.27 g of Novozym 435 and 0.2 g of activated molecular sieves were added, and the reaction ran for 24 h. Subsequently, the reaction mixture was dissolved in 300 mL of chloroform and the enzyme was filtered off. The organic layer was washed with a saturated aqueous solution of sodium carbonate (5 × 200 mL) and brine solution (3 × 200 mL) and later dried over sodium sulfate anhydrous. Finally, the organic layer was evaporated to dryness. Column chromatography was required to isolate the desired Compound 2. Hexane:ethyl acetate 8:2 was used as eluent. This yielded a white solid with a melting point of 75 °C (RF = 0.68; % isolation yield = 24); 1H NMR (400 MHz, CDCl3) δ 4.33−3.98 (m, 5H), 2.28 (t, J = 7.6 Hz, 4H), 1.65−1.42 (m, 4H), 1.38−0.97 (m, 72H), 0.81 (t, J = 7.2 Hz, 6H); HPLC-ELSD: rt =11.05 min (% area = 93). HRMS: calculated for C47H92O5Na [M + Na]+: 759.6837; found: 759.6801. Acetylated Glycerol Monobehenate (3). A 2.3 mmol portion of glycerol monobehenate, 27.4 mmol of acetic anhydride, and 5.5 mmol of triethylamine were added in a round-bottom flask with a stirring magnet, and the reaction is conducted for 24 h at reflux (% isolation yield = 75). Following this step, the reaction mixture was dissolved in

EXPERIMENTAL SECTION

CA8 was donated by Gattefossé. Solvent and other chemicals were bought from Sigma-Aldrich except for myristic acid and palmitic acid which were bought from Bie & Berntsen A/S and Fluka, respectively. Novozym 435 (Candida antarctica lipase B) was provided by Novozymes A/S (Bagsvaerd, Denmark). Caprylic/capric triglyceride and phosphatidylcholine (mix of C16 and C18 saturated acyl chains) were bought from BASF, Florham Park, NJ, USA, and Lipoid, Newark, NJ, USA, respectively. Pentylene glycol was obtained from Symrise, Teterboro, NJ, USA. Mp Biomedicals, Solon, OH, was the supplier used for glycerin. Polymer mixtures were obtained from different suppliers: xanthan gum (CP Kelco, Leatherhead, Surrey, UK), sodium carbomer (3 V Inc., Georgetown, SC, USA), carbomer interpolymer type A (Lubrizol, Cleveland, OH, USA), hydroxyethycellulose (Ashland Inc., Covington, KY, USA). NMR spectra were acquired on a Bruker Avance III 400 spectrometer using as solvent deuterated chloroform. MS spectra were measured using a Bruker Maxis Impact electrospray ionization quadrupole time-of-flight mass spectrometer (ESI-QTOF-MS). HPLC analyses were run for the different glycerides using the following conditions: 70−30% of A in B (A = acetonitrile and B = isopropanol/hexane 2:1) in 35 min with a flow rate of 1 mL/ min using a C-18 column (150 × 4.0 mm; particle size, 5 μm) on a Thermo Scientific HPLC containing a Finnigan Surveyor LC pump plus also equipped with an evaporative light scattering detector (ELSD; SEDEX 80). Thin layer chromatography (TLC) analyses were done using as eluent, first, CHCl3:MeOH:H2O (64:10:1) to run half B


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2-Hydroxy-3-(palmitoyloxy)propyl Docosanoate (5e). White solid with melting point of 74 °C (RF = 0.68; % isolation yield = 15); 1H NMR (400 MHz, CDCl3) δ 4.23−4.06 (m, 2H), 2.35 (t, J = 7.6 Hz, 4H), 1.68−1.57 (m, 4H), 1.27 (m, 60H), 0.89 (t, J = 6.4 Hz, 6H); HPLC-ELSD: rt = 7.97 min (% area = 98). RMS: calculated for C41H80O5Na [M + Na]+: 675.5898; found: 675.5889. 2-Hydroxy-3-(stearoyloxy)propyl Docosanoate (5f). White solid with melting point of 64 °C (RF = 0.68; % isolation yield = 28); 1H NMR (400 MHz, CDCl3) δ 4.23−4.05 (m, 5H), 2.35 (t, J = 7.6 Hz, 4H), 1.67−1.58 (m, 4H), 1.27 (m, 71H), 0.89 (t, 6.4 Hz, 6H); HPLCELSD: rt = 8.55 min (% area = 97). HRMS: calculated for C43H84O5Na [M + Na]+: 703.6211; found: 703.6187. Ethylene Glycol Monobehenate (6). Compound 6 was synthesized following a similar procedure to that described for compound 1, but 58.8 mmol of ethylene glycol and 60 °C were used instead. This yielded a white solid (RF = 0.54; % isolation yield = 8) with a melting point of 71 °C; 1H NMR (400 MHz, CDCl3) δ 4.24−4.15 (m, 2H), 3.85−3.77 (m, 2H), 2.33 (t, J = 7.6 Hz, 2H), 1.94 (b, 1H), 1.69−1.55 (m, 2H), 1.48−1.03 (m, 36H), 0.89 (t, J = 6.4 Hz, 3H); HPLC-ELSD: rt = 4.54 min (% area = 98). HRMS: calculated for C24H48O3Na [M + Na]+: 407.3496; found: 407.3491. Diethylene Glycol Monobehenate (7). The same procedure described for compound 6 was followed but using 58.8 mmol of diethylene glycol as starting material instead. This resulted in a white solid (RF = 0.49; % isolation yield = 24) with melting point of 61 °C; 1 H NMR (400 MHz, CDCl3) δ 4.26 (t, J = 7.2 Hz, 2H), 3.79−3.65 (m, 4H), 3.61 (t, J = 4.4 Hz, 2H), 2.33 (t, J = 7.6 Hz, 2H), 1.82 (b, 1H), 1.70−1.55 (m, 2H), 1.47−1.03 (m, 36H), 0.89 (t, 6.4 Hz, 3H); HPLC- ELSD: rt = 4.64 min (% area = 96). HRMS: calculated for C26H52O4 [M + H]: 429.2938; found: 429.3942. Differential Scanning Calorimetry (DSC). DSC equipment PerkinElmer Cetus (Norwalk, USA) was used to analyze the thermal properties of the different behenoyl lipids. In an aluminum pan 8−12 mg of the lipid of interest were sealed and placed into the DSC system under a purging nitrogen atmosphere of 20 mL/min. A scan speed of 5 °C per min was used for cooling and heating runs in the temperature range between −60 to 90 °C. The DSC scans were analyzed using the MicroCal Origin 8.6 software. Fourier Transform Infrared Spectroscopy. FTIR spectra were acquired using a Q-interline FTLA2000-154 with PIKE Technologies MIRACLE single reflection horizontal ATR. All spectra were recorded at room temperature and derivatized using a Savitzky−Golay algorithm.23 Samples analyzed by FTIR were previously melted and cooled down. Langmuir Film and Atomic Force Microscopy. A similar procedure to that reported by Correa et al. was followed to carry out Langmuir−Blodgett studies.24 Monolayer measurements were performed out using an aqueous solution of 150 mM sodium chloride and 1 mM ethylenediaminetetraacetic acid (EDTA) tetrasodium salt at pH 5.5. Generally less than 20 μL of an approximately 2 mg/mL solution of lipid in chloroform:methanol (9:1) were spread on the aqueous phase surface and solvent allowed to evaporate for 20 min. Subsequently, the monolayer was compressed at a constant rate of ∼9 Å2/(chain min) until collapse. After the conditions were set, the deposition of monolayers was carried out on a mica plate, at a pressure just before the film has collapsed, for AFM studies. AFM measurements were conducted in ambient air by tapping-mode. A silicon tip on a micro cantilever (Olympus Inc., Japan) with spring constant 26 N/m and resonant frequency of 300 kHz was used for the measurements. Samples were analyzed in duplicate. Formulation Manufacture in a Simple Emulsion Vehicle. The simplified base formulation that was utilized to screen a number of putative occlusive lipids described in this report is detailed in Table 2. Manufacture was conducted on a 10 g scale using an IKA digital T25 Ultra Turrax homogenizer (Wilmington, NC, USA) equipped with a small-scale rotor stator attachment. The oil phase, with the putative occlusive agent of interest at 3% w/w, was heated to 85 °C, and the aqueous phase to 75 °C. The aqueous phase was added to the oil phase and the mixture homogenized at 13 000 rpm for several minutes until a temperature of 55 °C was reached. The polymers were then

200 mL of chloroform, and the organic layer was washed with a saturated solution of ammonium chloride (3 × 100 mL), a saturated solution of sodium carbonate (3 × 100 mL), and brine solution (3 × 100 mL). Finally, we dried the organic layer over anhydrous sodium sulfate, filtered, and evaporated down to dryness (RF = 0.79; % isolation yield = 75).1H NMR (400 MHz, CDCl3) δ 5.31−5.20 (m, 1H), 4.35−4.24 (m, 2H), 4.20−4.10 (m, 2H), 2.31 (t, J = 7.6 Hz, 2H), 2.08 (d, J = 4 Hz, 6H), 1.68−1.53 (t, 2H), 1.48−1.01 (m, 36H), 0.87 (t, J = 7.2 Hz, 3H); HPLC-ELSD: rt = 4.18 min (% area = 91). HRMS: calculated for C29H54O6Na [M + Na]+: 521.3813; found: 521.3815. Acetylated Glycerol Dibehenate (4). A similar procedure to that used to synthesize compound 3 was followed but using as starting material compound 2 (RF = 0.79, % isolation yield = 40%). 1H NMR (400 MHz, CDCl3) δ 5.32−5.20 (m, 1H), 4.37−4.09 (m, 5H), 2.32 (t, J = 7.6 Hz, 4H), 2.07 (s, 3H), 1.69−1.49 (m, 4H), 1.47−1.01 (m, 72H), 0.88 (t, J = 6.8 Hz, 6H); HPLC-ELSD: rt = 11.53 min (% area = 90). HRMS: calculated for C49H94O6Na [M + Na]+: 801.6943; found: 801.6924. Synthesis of Asymmetric Diglycerides of Behenic Acid (5a−f). A 3.5−6.9 mmol portion of a shorter chain fatty acid (C8−C18) and 70−138 mmol of glycerol were dissolved in 10 mL of t-butanol. Posteriorly, 0.05 g of Novozym 435 and 0.1 g of activate molecular sieves were added, and the reaction was run for 2 h at room temperature. Subsequently, the enzyme was removed by filtration, and the solvent was evaporated from the resulting mixture and the residue was dissolved in 200 mL DCM. Following, the DCM layer was washed with a saturated aqueous solution of 100 mL sodium carbonate (3−5 times) to remove the unreacted fatty acid. Later, the organic layer was washed with a saturated aqueous solution of sodium chloride (3 × 100 mL), dried over anhydrous sodium sulfate, filtered, and evaporated to dryness to obtain target monoglyceride. Finally, the desired diglycerides were obtained by making react the resulted monoglyceride, 0.5−0.65 mmol behenic acid, 0.5−0.65 mmol of EDC, and 0.2−0.3 mmol of DMAP at 40 °C for 45 min. After 45 min had elapsed, the reaction mixture was diluted in DCM (200 mL) and washed off with a saturated aqueous solution of ammonium chloride (3 × 100 mL), a saturated solution of sodium carbonate (3 × 100 mL), and brine solution (3 × 100 mL). Later, the organic layer was dried over anhydrous sodium sulfate, filtered, and evaporated to dryness; and the resulting solid was dissolved in hexane and the unreacted monoglyceride was washed off with 80% ethanol in water. If necessary, a column chromatography separation was carried out using as eluent (8:2 hexane:ethyl acetate). Spectroscopy data can be found below. 2-Hydroxy-3-(octanoyloxy)propyl Docosanoate (5a). White solid with a melting point of 57 °C (RF = 0.68; % isolation yield = 19); 1H NMR (400 MHz, CDCl3) δ 4.25−4.04 (m, 4H), 2.35 (t, J = 7.5 Hz, 4H), 1.68−1.52 (m, 4H), 1.36−1.20 (m, 42H), 0.95−0.83 (m, 6H); HPLC-ELSD: rt = 5.23 min (% area = 90.4). HRMS: calculated for C33H64O5Na [M + Na]+: 563.4646; found: 563.4652. 3-(Decanoyloxy)-2-hydroxypropyl Docosanoate (5b). White solid with melting point of 66 °C (RF = 0.68; % isolation yield = 18); 1H NMR (400 MHz, CDCl3) δ 4.24−4.03 (m, 5H), 2.35 (t, J = 7.6 Hz, 4H), 1.69−1.59 (m, 4H), 1.27 (m, 48H), 0.89 (t, J = 6,4 Hz, 6H); HPLC-ELSD: rt = 5.73 min (% area = 97). HRMS: calculated for C35H68O5Na [M + Na]+: 591.4959; found: 591.4953. 3-(Dodecanoyloxy)-2-hydroxypropyl Docosanoate (5c). White solid with melting point of 68 °C (RF = 0.68; % isolation yield = 15); 1 H NMR (400 MHz, CDCl3) δ 4.22−4.05 (m, 4H), 2.35 (t, J = 7.6 Hz, 4H), 1.67−1.58 (m, 4H), 1.27 (m, 51H), 0.89 (t, J = 6.4 Hz, 6H); HPLC-ELSD: rt = 6.15 min (% area = 96). HRMS: calculated for C37H72O5Na [M + Na]+: 619.5272; found: 619.5260. 2-Hydroxy-3-(tetradecanoyloxy)propyl Docosanoate (5d). White solid with melting point of 69 °C (RF = 0.68; % isolation yield = 18); 1 H NMR (400 MHz, CDCl3) δ 4.22−4.05 (m, 6H), 2.35 (t, J = 7.6 Hz, 4H), 1.67−1.60 (m, 4H), 1.27 (m, 56H), 0.93−0.82 (m, 6H); HPLC-ELSD: rt = 6.85 min (% area = 93). HRMS: calculated for C39H76O5Na [M + Na]+: 647.5585; found: 647.5572. C


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difference of fatty acid composition among the resulting glyceride fractions was observed. Synthesis of Behenoyl Lipids. Glycerol monobehenate 1 and glycerol dibehenate 2 were synthesized using Novozym 435 (Scheme 1).22 The acetylated glycerides 3−4 were

Table 2. Formulation Details for a Simplified Emulsion System Used to Evaluate Lipids INCI name

% w/w

caprylic/capric triglyceride putative occlusive agent phosphatidylcholine (mix of C16 and C18 saturated acyl chains) water pentylene glycol glycerin polymers

17.00 3.00a 1.50 62.86 5.00 10.00 0.64

Scheme 1. Synthesis of Derivatives of Behenic Acid: (a) Synthetic Pathway of Behenoyl Glycerides; (b) Synthetic Procedure Followed to Obtain Asymmetric Diglycerides Containing a C22 Fatty Acid Chain and a Shorter Fatty Acid Chain (C8−C18); and (c) Enzymatic Coupling Carried out to Obtain Ethylene Glycol Monobehenate or Diethyleneglycol Monobehenate

a

A petrolatum control cream was also prepared at 10% w/w. In this case the capric/caprylic triglycerides were lowered accordingly.

added and the system homogenized to 50 °C, after which the emulsion vessel was placed in a secondary vessel containing freshly drawn water to aid cooling while homogenizing to a temperature of 35 °C. The vessel containing the emulsion was weighed and losses attributable to water evaporation were corrected. A final brief homogenization was conducted and the systems stored at ambient laboratory conditions for a period of 24 h prior to further study. Water Vapor Transmission Rate. The WVTR was measured using a similar method to Pennick et al.25,26 with some modifications. Vitro-Skin (IMS Inc., Portland, ME, USA) discs were cut using a hole punch and uniform films (12.45 mm in diameter) were drawn down onto the rough side of preweighed Vitro-Skin sections using a pneumatic draw down bar (BYK drive automatic film applicator, Geretsried, Germany) with a 50 μm gap distance gauge block. Films of cream were applied first in one direction, and then a second film was applied perpendicular to the first after a period of 30 min to ensure a uniform coverage with the formulation of interest. The coated films were then placed on mesh shelves and allowed to partially dry for a period of 1 h at low RH conditions within a desiccator (26% RH (± 3% RH) and 21 °C (±2 °C)). The coated films were then reweighed and placed into WVTR cells (Payne cells, Surface Measurement Systems, Allentown, PA, USA), coated side up, over 190 μL of deionized water. The loaded cells were reweighed and then placed back in the desiccators. The cells were briefly removed from the desiccators and weighed periodically over a period of 3 h using a five point balance, with the first 45 min being considered an equilibration period. A WVTR value was calculated over a period of 45−180 min using eq 1: WVTR (g/m 2 h) =

water loss (g) (W0.75 − W3.0) area of membrane (m 2) × time (h)

(1)

The area of the membrane was 1.22 × 10−4 m2. W0.75 and W3.0 were the WVTR cell weights in grams at the 0.75 and 3 h time points, respectively. Samples were measured at least in triplicate. All samples were compared to a blank and 100% petrolatum controls. Petrolatum was chosen as the positive control as it has been consistently proven to be an occlusive agent in WVTR testing.25,27−29 Statistical Methods. All data was collected in Microsoft Excel before being transferred to GraphPad Prism 5 (GraphPad Software, San Diego, CA, U.S.A.). Data were checked for normality using the D’Agostino and Pearson omnibus normality test, then ANOVA tests with subsequent posthoc comparisons were made using the Bonferroni test, and Dunnet’s test. Statistical significance was set at 95%.

obtained through the reaction of the respective glyceride with acetic anhydride while refluxing. In addition, a series of asymmetric diglycerides containing a shorter fatty acid chain (C8−C18) and a behenoyl chain were synthesized to evaluate the influence of the chain length on the packing behavior of the studied lipids. First, the different monoglycerides were obtained by carrying out an esterification reaction using Novozym 435 and an excess of glycerol and later the resulting monoglycerides were respectively coupled to behenic acid using 1-ethyl-3-(3(dimethylamino)propyl)carbodiimide (EDC) and 4-dimethylaminopyridine (DMAP) as coupling reagents to synthesize the glyceryl derivatives 5a−f. All asymmetric diglycerides were obtained with isolation yields of up to 28% which was a result of the low reactivity of behenic acid (C22) compared to shorter fatty acids (C8−C18). However, no clear correlation was found between the yield of the reaction and the length of the different monoglycerides (C8−C18) previously synthesized. In addition, mixtures of 1,3- and 1,2-diglycerides were observed in the TLC

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RESULTS AND DISCUSSION Short Path Distillation (SPD). Table 1 displays the fatty acid composition of CA8 determined by GC-FID. Accordingly, this material contains mainly behenoyl glycerides. CA8 was fractionated using SPD. Based on HPLC analysis results, fractionated monoglycerides (FGMB), diglycerides (FGDB), and triglycerides (FGTB) were obtained with an approximate purity of 91%, 96%, and 99%, respectively; no significant D


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of hydrogen bond interaction, also generated a molecule with lower melting point (compound 3: mp = 53 °C) than the original molecule (compound 1: mp = 77 °C). In the case of the asymmetric diglycerides 5, molecules displayed melting points between 57 and 74 °C and the melting point of the derivatives were shown to slightly increase as one of the fatty acid chain increases in length from C8 to C18. The latter can be anticipated since the van der Waals interactions become stronger with increasing length of the fatty acid chain and more energy is required to break these noncovalent intermolecular interactions. In addition, as expected compounds 6 and 7 (mp = 71 and 61 °C, respectively) displayed lower melting points than FGMB (mp = 79 °C) and synthetic glycerol monobehenate 1 (mp = 77 °C) suggesting that less energy is required to overcome the molecular interaction and consequently melt compounds 6 and 7, correspondingly. Therefore, based on DSC data monoacylated glycerol might display better structure organization, thus, better packing behavior than the rest of the studied behenoyl lipids. FTIR Characterization. FTIR spectroscopy studies provided a better understanding of the intermolecular interactions and molecular alignment and organization of the different fatty acid derivatives.30 Accordingly, FT-IR can predict the transition between fully extended all-trans hydrocarbon chains to disordered chains packing upon conformation change since this yields a frequency increase in both symmetric and asymmetric stretching, at ∼2850 and 2920 cm−1, respectively.31 As shown in Table 3, all the behenoyl lipids displayed both peaks corresponding to the symmetric and asymmetric stretching of the fatty acyl chain at ∼2850 and 2920 cm−1, suggesting the presence of fully extended all-trans fatty acyl chain at room temperature. In addition, FTIR also provided an insight into the lateral acyl chain packing.32 FTIR spectroscopy can yield information about orthorhombic, hexagonal, or fluid packing of the lipids. The most densely packed is orthorhombic packing, where all alkyl chains display an all-trans conformation organized in a highly dense rectangular crystalline lattice. FTIR data suggested that FGMB, FGDB, compound 1, 6, and 7 exhibit orthorhombic packing. However, CA8, FGTB, and compound 2 show a mixture of hexagonal and orthorhombic packing since one of the peaks in the 740−700 cm−1 region of the FTIR spectrum is significantly smaller than the other (See Figure 1 for CA8 and FGTB results) and the rest of the molecules pack hexagonally. The former results can be explained by CA8 being a mixture of glycerides. HPLC analysis shows that it is composed of approximately 13% monoglycerides, 55% diglycerides, and 32% triglycerides. In the case of compound 2, it will be of interest to synthesize a diglyceride which is exclusively one of the isomers 1,2- or 1,3-substituted diglycerides to understand whether the mixture of diglycerides is generating a less tight packing behavior. COSMO-RS (conductor-like screening model for realistic solvents)33 predicts that 1,3-distearoyl glycerol at 20 °C presents a free energy of ΔG0 = −RT ln(10log P) = −73.80 kJ/mol and 1,2distearoyl glycerol at the same temperature a less negative ΔG0 = −RT ln(10log P) = −69.87 kJ/mol. The latter estimates that 1,2-distearoyl glycerol would display a tighter packing behavior and the same would be expected for 1,2-dibehenate glycerol. Thus, it seems plausible that the mixture of both isomers 1,2 and 1,3-diglycerides might be impairing better packing. In addition, multiacylation of the hydroxyl groups in the glycerol molecule seems to also have a negative impact on the way glycerides molecules pack since FTIR results for FGTB, and

analysis where the main product was found to be 1,3diglycerides which is in agreement with the 1,3 selectivity of CALB. Moreover, as expected when analyzing the different derivatives by reverse-phase HPLC, diglycerides containing longer fatty acyl chains displayed longer retention times (5f > 5e > 5d > 5c > 5b > 5a). In addition, to further understand the relevance of the polar head on the occlusive properties of the molecules, analogues of glycerol monobehenate were synthesized which contains ethylene glycol or diethylene glycol instead of glycerol as polar head (see compounds 6 and 7). Accordingly, compounds 6 and 7 were synthesized using Novozym 435. Differential Scanning Calorimetry (DSC) Characterization. All samples were heated, cooled down, and reheated, respectively, with a rate of 5 °C per minute and a temperature range between −60 and 90 °C. As shown in Table 3, all Table 3. FTIR Characterization Data of Behenoyl Lipids vibrational modes (3000−2800 cm−1 region)

compound

melting point (°C)

CA8

72

FGMB

79

FGDB

71−74

FGTB 1

68 77

2

75

3 4 5a 5b 5c 5d 5e 5f 6 7

53 69 57 66 68 69 64−74 64−69 71 61

FTIR suggested lateral packing (FTIR CH2 rocking mode) double peakorthorhombic double peakorthorhombic double peakorthorhombic single peak-hexagonal double peakorthorhombic double peakorthorhombic single peak-hexagonal single peak-hexagonal single peak-hexagonal single peak-hexagonal single peak-hexagonal single peak-hexagonal single peak-hexagonal single peak-hexagonal double peakorthorhombic double peakorthorhombic

vCH2 symmetric stretching

vCH2 asymmetric stretching

2849

2916

2849

2916

2849

2916

2849 2848

2196 2916

2849

2914

2848 2849 2849 2849 2849 2849 2849 2849 2849

2914 2914 2914 2912 2912 2913 2915 2916 2916

2849

2916

glycerides presented melting points below 80 °C. Monoglycerides from CA8 (FGMB) presented a higher melting point (mp = 79 °C) than diglycerides from CA8 (FGDB; mp = 71− 74 °C) and triglycerides from CA8 (FGTB; mp = 68 °C). According to DSC results, both FGMB and FGDB displayed two melting transitions suggesting the existence of a mixture of isomers; 1-monoglyceride and 2-monoglyceride, and 1,2diglyceride and 1,3-diglyceride, respectively. Furthermore, the melting point of the monoglycerides is higher than that of diglycerides of the same fatty acids. For instance, synthetic glyceryl monobehenate 1 presented a higher melting point (mp = 77 °C) than glyceryl dibehenate 2 (mp = 75 °C). This can be explained by the stronger hydrogen bond interactions and better alkanyl−alkanyl alignment between monoglyceride molecules, leading to a higher melting point. As a consequence, acetylation of compound 1, leading to the conversion of a hydroxyl group to a ester bond and the decrease of the number E


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Figure 1. Second derivate (Savitzky−Golay) of FTIR spectra between the region of 780−680 cm−1 of the different class of glycerides studied in this work at rt.

compound 3, and asymmetric diglycerides 5a−f suggest that all these molecules exhibit hexagonal packing. Conversely, monoacylation promotes tighter packing as demonstrated by FGMB, glycerol monobehenate 1, and compounds 6 and 7. Accordingly, the FTIR results suggest that glycerol monobehenate and dibehenate as well as ethylene glycol monobehenate might present occlusive characteristics. Thus, a skin care formulation system composed of these lipids may help to reduce TEWL. Langmuir Isotherm Studies. To better understand the packing behavior of behenoyl lipids, compounds 1, 2, 6, and 7 were further studied in the Langmuir−Blodgett apparatus and compared with CA8 and the less pure form of glycerol monobehenate (FGMB). Langmuir isotherm approach provides information about (i) the stability of the monolayer film that a compound can form (the higher final pressure, the higher stability); (ii) how well the lipids organize (the bigger the difference between initial and final area, the better compression characteristics the given lipid exhibits); and (iii) how densely molecules pack when restricted in a two-dimensional system (the smaller AreaFinal, the closer are the lipids chain to each other). These attributes are deemed important in generating an occlusive structure, in particular the ability to densely pack together, since this would suggest a greater ability to act as a diffusional barrier to water loss and generate an occlusive topical product. As shown in Figure 2 and Table 3, Langmuir isotherm results are in agreement with FTIR data since glyceryl dibehenate 2 results in a less well packed monolayer than the monobehenate compounds evaluated (FGMB and compounds 1, 6, 7). This is further demonstrated by the collapsing pressures and the difference between the initial and final area per chain of the studied lipids (e.g., monoglycerides present one chain and diglycerides two chains); one of lowest collapsing pressure and the largest difference between areas is seen with glycerol dibehenate 2 (PressureFinal = 40 mN/m and AreaFinal−Initial = 16 Å2/chain) among the evaluated compounds. Moreover, CA8 displayed the worst packing behavior of all

Figure 2. Surface pressure−area isotherms of lipids on aqueous surface containing 150 mM NaCl and 1 mM EDTA at pH 5.5.

studied materials as it presented the biggest difference between the initial and final area per chain (AreaInitial−Final = 35 Å2/ chain). On the other hand, glycerol monobehenate 1 displayed the lowest AreaFinal (AreaFinal = 17 Å2/chain) and the highest collapsing pressure (PressureFinal = 57 mN/m) suggesting a tighter lateral packing and higher stability. However, at the collapsing pressure, the final area should be equal or approximate to the cross section of the molecule (i.e., the cross-section of the cylindrical fatty acyl chain is 19 Å2)34 and the final area per chain of compound 1 resulted 17 Å2 which is lower than expected. This result suggests that the glycerol monobehenate 1 might have started to form double-layers at a pressure higher than 30 mN/m or molecules may not be homogeneously distributed in liquid expanding/condense phase and some aggregate or cluster may form. In addition, as observed in Figure 2 and Table 4, FGMB presented a less stable and less organized monolayer than compound 1. The latter is expected since FGMB is a less pure form of glycerol monobehenate containing other fatty acyl glycerides which might be affecting the ability of FGMB to present a more stable F


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CA8 were significantly superior to the blank control. Accordingly, all synthesized and fractionated compounds performed better than CA8 (WVTR = 101 g/(m2 h)) as predicted by Langmuir isotherm studies. Comparing FGMB, which displayed a WVTR = 51 g/(m2 h), to FGDB (WVTR = 76 g/(m2 h); P < 0.0001) and FGTB (WVTR = 94 g/(m2 h); P < 0.0001), it can be observed that as the number of hydrogen bond donors decrease, the material becomes less occlusive, thus, suggesting that the presence of hydrogen bond donors appears to result in a more occlusive formulation within this particular system. The latter is in agreement with DSC data which show that hydrogen bond interactions promoted a tighter packing behavior and a higher melting point. In addition, as predicted by FTIR and DSC data, compound 3 (WVTR = 90 g/(m2 h)) was significantly less occlusive than glycerol monobehenate 1 (WVTR = 64 g/(m2 h); P < 0.0001). In addition, FGMB presented a WVTR (51 g/(m2 h)) which is comparable to that of a similar formulation that instead contains 10% petrolatum (47 g/(m2 h)) and was significantly less than one containing 3% petrolatum (P < 0.0001; Table 5). Furthermore, FGMB displayed a lower WVTR than glycerol monobehenate 1 (WVTR = 64 g/(m2 h); P < 0.05) which suggests that a mixture of glycerides containing mainly glycerol monobehenate offers a more occlusive system than pure behenoyl lipid 1. Since compound 1 is composed of 96% glycerol monobehenate and FGMB is composed of approximately 88% of behenoyl lipids with the remaining 12% being glycerides of varied fatty acyl chain lengths (See Table 1), the better occlusive properties displayed by FGMB could suggest that the shorter fatty acyl lipids might be filling regions between the long-chain behenoyl lipids that may be less tightly packed. Furthermore, it is interesting to note that just like the FGMB, SC exhibit a range of lipid chain lengths and the occlusive properties present in the healthy SC may in part relate to this.40 Accordingly, the WVTR results illustrate how a holistic approach to understanding the physical and organizational properties of lipids, combining DSC, FTIR, and Langmuir studies is a successful approach that can be used to design and identify potential occlusive fatty amphiphiles that exhibit lateral packing characteristics similar to SC lipids. Atomic Force Microscopy Imaging. To further understand the difference observed between fractionated and synthesized glycerol monobehenate, compound 1, and

Table 4. Langmuir Film Results of Most Promising Behenoyl Lipids Langmuir isotherm studies compound

AreaInitial (Å2/chain)

AreaFinal (Å2/chain)

AreaInitial−Final (Å2/chain)

PressureFinal (mN/m)

CA8 FGMB FGDB 1 2 6 7

76 34 28 26 42 32 28

41 22 19 17 26 24 22

35 12 9 9 16 8 6

40 53 40 57 40 39 52

monolayer as shown by the final pressure and difference between initial and final area per chain of the evaluated material (FGMB: PressureFinal = 53 mN/m and AreaInitial−Final = 12 Å2/ chain) compared to the values obtained for compound 1 (PressureFinal = 57 mN/m and AreaInitial−Final = 9 Å2/chain). Moreover, Langmuir film studies showed that removal of a hydrogen bond donor does not favor the formation of monolayer as demonstrated by compound 6 which displayed a lower collapsing pressure (PressureFinal = 39 mN/m) as compared to compound 1 (PressureFinal = 57 mN/m). Nonetheless, adding a hydrogen bond acceptor as for the case of compound 7, which contains two units of ethylene glycol, favors the formation of the monolayer (PressureFinal = 52 mN/m and AreaFinal = 22 Å2/chain). In summary, Langmuir isotherm analysis results suggest that FGMB, compounds 1 and 7 are the most promising compounds of the evaluated series of behenoyl lipids for novel skin occlusive products. Water Vapor Transmission Rate. WVTR measurements were carried out using phospholipid-containing formulations made with the putative occlusive lipids. However, compound 6 and 7 were not further study due to potential toxicity of ethylene glycol containing agents on the skin.35,36 The WVTR measurements included a no treatment (blank) control and a 100% petrolatum positive control as the latter has consistently produced full occlusion in these types of tests.25,37−39 The results in the WVTR test are shown in Figure 3. WVTR is an in vitro method that estimates water permeation through a polymer film that mimics skin properties (Vitroskin). The polymer film is coated with an emulsion formulation incorporating the lipid of interest. All formulations except

Figure 3. WVTR results for fractionated and synthesized behenoyl lipids. G


Research Article

ACS Sustainable Chemistry & Engineering Table 5. Statistical Significant CA8 CA8 FGMB FGDB FGTB 1 3 P3 P10

****c **** N **** N ** ****

FGMB

FGDB

FGTB

1

3

P3a

P10b

****

**** ****

N **** ***

**** * N ****

N **** * N ****

** **** N N **** ****

**** N **** **** *** **** ****

**** **** * **** **** N

*** N * N ****

**** N N ****

**** **** ***

**** ****

****

P3= 3% petrolatum. bP10= %10 petrolatum. c* p < 0.05; **p < 0.01; ***p < 0.001; ****p < 0.0001. dN = no statistical significant (SS). All compounds displayed SS vs % 100 petrolatum (P < 0.0001). a

FGMB, were further evaluated by AFM imaging after film deposition on mica substrate (Figure 3). AFM results show that compound 1 and FGMB form films of 1.7 nm (P = 30 mN/m) and 2.2 nm thickness (P = 45 mN/m; near the collapsing pressure), respectively (Figure 4). No holes were observed for

organizing on the mica substrate, at P = 30 mN/m according to the Langmuir isotherm studies. Fast Fourier transform filtering (FFTF) of the AFM images was also used since this is a powerful tool for image analysis.27 Fourier images reflect repeated patterns as narrow peaks, the coordinates of which describe their periodicity and direction. Such peaks are easy to detect by image processing without any preknowledge of the features form or periodicity. Interestingly, as shown Figure 4, compound 1 showed a striped pattern after FFTF reflecting a property of the system rather than results of the imaging process.28 This type of striped pattern was previously observed for lignoceric acid (C24).29 However, FGMB did not display such a pattern. Accordingly, it could be inferred that in the case of FGMB, the heterogeneous composition in fatty acyl chain lengths of monoglycerides present in the mixture might be intercalating in between the molecules of glycerol monobehenates and may in turn help generate a more occlusive mixture of lipids.

■

CONCLUSIONS In summary, we have identified glyceryl monobehenate as a sustainable and commercially viable occlusive material that possesses physical attributes similar to petrolatum. Accordingly, this work describes the use of multiple techniques including FTIR, DSC, and Langmuir isotherm studies to measure the physical attributes of putative fatty amphiphiles. Also, it shows that fractionated and synthetic behenoyl monoglycerides decrease WVTRs which in turn, could potentially improve skin barrier function due to their ability to form tightly packed, lateral assemblies. FGMB is approximately 88% glyceryl monobehenate along with associated species of longer and shorter acyl chain length. These monoglycerides gave a marked reduction in the WVTR superior to a 3% petrolatum-containing formulation and was comparable to that of 10% petrolatum one. Mechanistically, the results suggest the ability of glycerides to act as hydrogen bond donors is important in establishing strong intermolecular interactions and in turn generate a more occlusive chemical species. Moreover, glycerol monobehenate has shown to be a highly promising skinlike fatty amphiphile that reduces water loss in vitro and has the potential to reduce water loss through the SC and thereby help in the treatment of xerosis, a skin condition that affects millions of people either chronically or acutely.

Figure 4. Topographical AFM image of behenoyl lipids deposited on mica: (a and b) AFM images of compound 1 and FGMB, respectively. (c and d) FFTF results of compound 1 and FGMB correspondingly.

the monolayer film of compound 1 near the collapsing pressure which impairs the precise measurement of the thickness of the film at this pressure (data not shown). This can suggest that compound 1 forms a highly uniform film at this pressure. The theoretical length of glycerol monobehenate is 3.1 nm from the terminal alkyl carbon to the oxygen of the primary alcohol when the carbons in the fatty acyl chain are all-trans to each other. Thus, it could be inferred that at P = 30 mN/m (i) the alkyl chain of glycerol monobehenate are not oriented perpendicular to the mica but rather tilted relative to the surface of the hydrophilic mica substrate and/or41 (ii) the fatty acyl chain is not in an all-trans conformation, e.g. the end of the hydrocarbon chain is tilted.30 In addition, for the case of FGMB the thickness difference could be attributed to the different fatty acyl chains present in the system (See Table 1).42 Nonetheless, in the case of compound 1 it is expected that the molecules are tilted since they are in the tilted condensed phase, still

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +4587155528. H


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ACS Sustainable Chemistry & Engineering Funding

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This work was fully funded by GlaxoSmithKline Consumer Healthcare. Notes

The authors declare no competing financial interest. ○ A.V.R. is a consultant to GlaxoSmithKline Consumer Healthcare.

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Ultralong Fatty Acyl Derivatives As Occlusive Structure Lipids for

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