Characterizing How Acidic pH Conditions Affect the Membrane- Disruptive Activities of Lauric Acid and Glycerol Monolaurate Elba R. Valle-Gonza ́ lez, †,‡ Joshua A. Jackman, †,‡ Bo Kyeong Yoon, †,‡ Soohyun Park, †,‡ Tun Naw Sut, †,‡ and Nam-Joon Cho* ,†,‡,§ † School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore ‡ Centre for Biomimetic Sensor Science, Nanyang Technological University, 50 Nanyang Drive, 637553, Singapore § School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, 637459, Singapore * S Supporting Information ABSTRACT: Fatty acids and monoglycerides are single-chain lipid amphiphiles that interact with phospholipid membranes as part of various biological activities. For example, they can exhibit membrane-disruptive behavior against microbial pathogens on the human skin surface. Supported lipid bilayers (SLBs) provide a useful experimental platform to characterize these membrane- disruptive behaviors, although related studies have been limited to neutral pH conditions. Herein, we investigated how lauric acid (LA) and glycerol monolaurate (GML) interact with SLBs and cause membrane morphological changes under acidic pH conditions that are representative of the human skin surface. Although LA induces tubule formation under neutral pH conditions, we discovered that LA causes membrane phase separation under acidic pH conditions. By contrast, GML induced membrane budding in both pH environments, although there was more extensive membrane remodeling under acidic pH conditions. We discuss these findings in the context of how solution pH affects the ionization states and micellar aggregation properties of LA and GML as well as its effect on the bending stiffness of lipid bilayers. Collectively, the findings demonstrate that solution pH plays an important role in modulating the interaction of fatty acids and monoglycerides with phospholipid membranes, and hence influences the scope and potency of their membrane-disruptive activities. ■ INTRODUCTION The self-assembly of lipid molecules plays an important role in driving membrane organization 1,2 and facilitating biomedical applications such as drug delivery 3 and antimicrobial medicine. 4 Among the different types of lipids, fatty acids, and monoglycerides are single-chain lipid amphiphiles that exhibit broad-spectrum antimicrobial activity 5,6 and destabilize the cellular membranes of microbial pathogens. 7,8 Often referred to as “antimicrobial lipids”, they are an integral part of the innate immune system on the human skin surface where their main function is to inhibit pathogens and regulate microbial populations. 9-11 Within this scope, it is known that medium-chain saturated fatty acids and monoglyceride derivatives have particularly high antibacterial activities. 12 Lauric acid (LA; C12:0) is regarded as the most inhibitory saturated fatty acid against Gram-positive bacteria. 13 Its monoglyceride derivative, glycerol monolaurate (GML), also has potent antibacterial effects; GML demonstrates greater potency (lower effective concentration), albeit against a narrower spectrum of susceptible bacteria. 13-16 These inhibitory effects have motivated experimental efforts to characterize how antimicrobial lipids destabilize cellular membrane targets. Conventionally, the effects of antimicrobial lipids on cellular membranes have been assessed posttreatment by electron microscopy (EM) techniques, which enable visualization of membrane morphological changes and intracellular dam- age. 17, 18 However, real-time monitoring of membrane interactions is not possible with EM, and similar challenges also exist for atomic force microscopy approaches. 19 To address these shortcomings, membrane fluidity and ion leakage assays have been conducted on bacterial cells posttreatment 20 along with transcriptomic-level analysis to characterize cellular responses. 21 Such approaches have enabled a deeper under- standing of how membrane properties and cellular functions are affected by antimicrobial lipids. Complementing these approaches, there is significant opportunity to characterize the real-time interactions between antimicrobial lipids and phospholipid membranes by employing model systems in Received: July 26, 2018 Revised: October 14, 2018 Published: October 20, 2018 Article pubs.acs.org/Langmuir Cite This: Langmuir 2018, 34, 13745-13753 © 2018 American Chemical Society 13745 DOI: 10.1021/acs.langmuir.8b02536 Langmuir 2018, 34, 13745-13753 Downloaded via NANYANG TECHNOLOGICAL UNIV on April 17, 2021 at 11:34:59 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.