Lipidated Lysine and Fatty Acids Assemble into Protocellular Membranes to Assist Regioselective Peptide Formation: Correlation to the Natural Selection of Lysine over Nonproteinogenic Lower Analogues

Abstract

The self-assembly of prebiotically plausible amphiphiles (fatty acids) to form a bilayer membrane for compartmentalization is an important factor during protocellular evolution. Such fatty acid-based membranes assemble at relatively high concentrations, and they lack robust stability. We have demonstrated that a mixture of lipidated lysine (cationic) and prebiotic fatty acids (decanoic acid, anionic) can form protocellular membranes (amino acid-based membranes) at low concentrations via electrostatic, hydrogen bonding, and hydrophobic interactions. The formation of vesicular membranes was characterized by dynamic light scattering (DLS), pyrene and Nile Red partitioning, cryo-transmission electron microscopy (TEM) images, and glucose encapsulation studies. The lipidated nonproteinogenic analogues of lysine (Lys), such as ornithine (Orn) and 2,4-diaminobutyric acid (Dab), also form membranes with decanoate (DA). Time-dependent turbidimetric and 1H NMR studies suggested that the Lys-based membrane is more stable than the membranes prepared from nonproteinogenic lower analogues. The Lys-based membrane embeds a model acylating agent (aminoacyl-tRNA mimic) and facilitates the colocalization of substrates to support regioselective peptide formation via the α-amine of Lys. These membranes thereby assist peptide formation and control the positioning of the reactants (model acylating agent and −NH2 of amino acids) to initiate biologically relevant reactions during early evolution.

Molecular-level insights into a tripolyphosphate and pyrophosphate templated membrane assembly

“…The critical vesicle concentrations (CVCs) of the TTAC–TPP and TTAC–PPi systems were measured by pyrene I 1 / I 3 and merocyanine 540 assays. 55,60 The CVC of TTAC–TPP (∼0.75 mM) was lower than the CVC of TTAC–PPi (∼1.25 mM) (Fig. S23 and S24, ESI†).…”

“…It was reported that the NR maxima in vesicles are around 624 nm while in micelles they are around 640 nm. 54,55 These experiments suggest that TPP and PPi assist in the formation of vesicular structures. Apart from the emission maxima, the emission intensity was also monitored.…”

“…58,59 To investigate the mechanism of the phosphate-templated assembly, we measured the critical micelle concentration (CMC) of TTAC using pyrene I 1 /I 3 and merocyanine 540 assays. 55,60 The CMC of TTAC was found to be around 3.0 mM (Fig. S21 and S22, ESI †), 61 which suggests that under the experimental condition (B1.5 mM), it likely exists as monomers.…”

Self-assembled prebiotic amphiphile-mixture exhibits tunable catalytic properties

“…[16][17][18] Model amphiphiles such as fatty acids, 19,20 cyclophospholipids, 21 and amino-acid-based amphiphiles might have played important roles during protocell formation. [22][23][24][25][26] Protocell membrane assembly could provide a unique environment, that may be considered a "reaction territory" to support primitive catalytic reactions. The self-assembly of the amphiphiles and the formation of protocellular membranes could have supported primitive organocatalysis.…”

“…The self-assembly of the amphiphiles and the formation of protocellular membranes could have supported primitive organocatalysis. 22,27,28 Since amino acids and small peptides were proposed as prominent ancient organocatalysts, 3 we hypothesize that amino acid-based primitive amphiphiles might have supported membrane-surfaceassisted catalytic reactions. Therefore, it is interesting to establish the prebiotic synthesis of amino acid-based amphiphiles, their catalytic efficacy, and substrate selectivity, if any!…”