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1. Binding of Dipeptides to Fatty Acid Membranes Explains Their Colocalization in Protocells but Does Not Select for Them Relative to Unjoined Amino Acids

   Mengjun Xue, Roy A. (July 2021, The journal of physical chemistry)

   null (Ed.)

   Dipeptides, which consist of two amino acids joined by a peptide bond, have been shown to have catalytic functions. This observation leads to fundamental questions relevant to the origin of life. How could peptides have become colocalized with the first protocells? Which structural features would have determined the association of amino acids and peptides with membranes? Could the association of dipeptides with protocell membranes have driven molecular evolution, favoring dipeptides over individual amino acids? Using pulsed-field gradient nuclear magnetic resonance, we find that several prebiotic amino acids and dipeptides bind to prebiotic membranes. For amino acids, the side chains and carboxylate contribute to the interaction. For dipeptides, the extent of binding is generally less than that of the constituent amino acids, implying that other mechanisms would be necessary to drive molecular evolution. Nevertheless, our results are consistent with a scheme in which the building blocks of the biological polymers colocalized with protocells prior to the emergence of RNA and proteins. 

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2. Semipermeable Mixed Phospholipid-Fatty Acid Membranes Exhibit K+/Na+ Selectivity in the Absence of Proteins

   https://doi.org/10.3390/life10040039  

   Zhou, Xianfeng; Dalai, Punam; Sahai, Nita (April 2020, Life)

   Two important ions, K+ and Na+, are unequally distributed across the contemporary phospholipid-based cell membrane because modern cells evolved a series of sophisticated protein channels and pumps to maintain ion gradients. The earliest life-like entities or protocells did not possess either ion-tight membranes or ion pumps, which would result in the equilibration of the intra-protocellular K+/Na+ ratio with that in the external environment. Here, we show that the most primitive protocell membranes composed of fatty acids, that were initially leaky, would eventually become less ion permeable as their membranes evolved towards having increasing phospholipid contents. Furthermore, these mixed fatty acid-phospholipid membranes selectively retain K+ but allow the passage of Na+ out of the cell. The K+/Na+ selectivity of these mixed fatty acid-phospholipid semipermeable membranes suggests that protocells at intermediate stages of evolution could have acquired electrochemical K+/Na+ ion gradients in the absence of any macromolecular transport machinery or pumps, thus potentially facilitating rudimentary protometabolism. 

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3. Synthesis and Characterization of Amino Acid Decyl Esters as Early Membranes for the Origins of Life

   https://doi.org/10.3390/membranes12090858  

   Lago, Isabella; Black, Lissa; Wilfinger, Maximillian; Maurer, Sarah E. (September 2022, Membranes)

   Understanding how membrane forming amphiphiles are synthesized and aggregate in prebiotic settings is required for understanding the origins of life on Earth 4 billion years ago. Amino acids decyl esters were prepared by dehydration of decanol and amino acid as a model for a plausible prebiotic reaction at two temperatures. Fifteen amino acids were tested with a range of side chain chemistries to understand the role of amino acid identity on synthesis and membrane formation. Products were analyzed using LC-MS as well as microscopy. All amino acids tested produced decyl esters, and some of the products formed membranes when rehydrated in ultrapure water. One of the most abundant prebiotic amino acids, alanine, was remarkably easy to get to generate abundant, uniform membranes, indicating that this could be a selection mechanism for both amino acids and their amphiphilic derivatives. 

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4. Synthesis and Characterization of 2-Decenoic Acid Modified Chitosan for Infection Prevention and Tissue Engineering

   https://doi.org/10.3390/md19100556  

   Wells, Carlos Montez; Coleman, Emily Carol; Yeasmin, Rabeta; Harrison, Zoe Lynn; Kurakula, Mallesh; Baker, Daniel L.; Bumgardner, Joel David; Fujiwara, Tomoko; Jennings, Jessica Amber (October 2021, Marine Drugs)

   Chitosan nanofiber membranes are recognized as functional antimicrobial materials, as they can effectively provide a barrier that guides tissue growth and supports healing. Methods to stabilize nanofibers in aqueous solutions include acylation with fatty acids. Modification with fatty acids that also have antimicrobial and biofilm-resistant properties may be particularly beneficial in tissue regeneration applications. This study investigated the ability to customize the fatty acid attachment by acyl chlorides to include antimicrobial 2-decenoic acid. Synthesis of 2-decenoyl chloride was followed by acylation of electrospun chitosan membranes in pyridine. Physicochemical properties were characterized through scanning electron microscopy, FTIR, contact angle, and thermogravimetric analysis. The ability of membranes to resist biofilm formation by S. aureus and P. aeruginosa was evaluated by direct inoculation. Cytocompatibility was evaluated by adding membranes to cultures of NIH3T3 fibroblast cells. Acylation with chlorides stabilized nanofibers in aqueous media without significant swelling of fibers and increased hydrophobicity of the membranes. Acyl-modified membranes reduced both S. aureus and P.aeruginosa bacterial biofilm formation on membrane while also supporting fibroblast growth. Acylated chitosan membranes may be useful as wound dressings, guided regeneration scaffolds, local drug delivery, or filtration. 

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5. Hybrid Protocells Based on Coacervate‐Templated Fatty Acid Vesicles Combine Improved Membrane Stability with Functional Interior Protocytoplasm

   https://doi.org/10.1002/smll.202406671  

   Lee, Jessica; Pir_Cakmak, Fatma; Booth, Richard; Keating, Christine_D (October 2024, Small)

   Abstract Prebiotically‐plausible compartmentalization mechanisms include membrane vesicles formed by amphiphile self‐assembly and coacervate droplets formed by liquid–liquid phase separation. Both types of structures form spontaneously and can be related to cellular compartmentalization motifs in today's living cells. As prebiotic compartments, they have complementary capabilities, with coacervates offering excellent solute accumulation and membranes providing superior boundaries. Herein, protocell models constructed by spontaneous encapsulation of coacervate droplets by mixed fatty acid/phospholipid and by purely fatty acid membranes are described. Coacervate‐supported membranes form over a range of coacervate and lipid compositions, with membrane properties impacted by charge–charge interactions between coacervates and membranes. Vesicles formed by coacervate‐templated membrane assembly exhibit profoundly different permeability than traditional fatty acid or blended fatty acid/phospholipid membranes without a coacervate interior, particularly in the presence of magnesium ions (Mg²⁺). While fatty acid and blended membrane vesicles are disrupted by the addition of Mg²⁺, the corresponding coacervate‐supported membranes remain intact and impermeable to externally‐added solutes. With the more robust membrane, fluorescein diacetate (FDA) hydrolysis, which is commonly used for cell viability assays, can be performed inside the protocell model due to the simple diffusion of FDA and then following with the coacervate‐mediated abiotic hydrolysis to fluorescein. 

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