Effect of Methyl Group on Acyclic Serinol Scaffold for Tethering Dyes on the DNA Duplex Stability

Acyclic serinol derivatives are useful scaffolds for tethering dyes within DNA duplexes. Here we synthesised an inverse l ‐threoninol (il ‐threoninol) scaffold and compared its effect on DNA duplex stability to other acyclic artificial nucleic acid scaffolds that are based on d ‐threoninol, l ‐threoninol, and serinol. When planar trans‐azobenzene was incorporated into the DNA duplex through a single bulge‐like motif (the wedge), the il ‐threoninol scaffold stabilised the duplex most efficiently. When scaffolds were incorporated in complementary positions (dimer motif) or in three adjacent positions (cluster motif), d ‐threoninol was the most stabilising. CD spectra indicated that the effect of scaffold on the duplex stability was closely related to the winding induced by each scaffold. When trans‐azobenzene was photo‐isomerised to non‐planar cis‐azobenzene, il ‐threoninol destabilised the duplex most strongly, irrespective of the number of artificial residues incorporated. The properties of the il ‐threoninol scaffold make it a useful tether for dyes or other functionalities.     

Synthesis of S‐Adenosyl‐L‐homocysteine Capture Compounds for Selective Photoinduced Isolation of Methyltransferases

Understanding the interplay of different cellular proteins and their substrates is of major interest in the postgenomic era. For this purpose, selective isolation and identification of proteins from complex biological samples is necessary and targeted isolation of enzyme families is a challenging task. Over the last years, methods like activity‐based protein profiling (ABPP) and capture compound mass spectrometry (CCMS) have been developed to reduce the complexity of the proteome by means of protein function in contrast to standard approaches, which utilize differences in physical properties for protein separation. To isolate and identify the subproteome consisting of S‐adenosyl‐L ‐methionine (SAM or AdoMet)‐dependent methyltransferases (methylome), we developed and synthesized trifunctional capture compounds containing the chemically stable cofactor product S‐adenosyl‐L ‐homocysteine (SAH or AdoHcy) as selectivity function. SAH analogues with amino linkers at the N6 or C8 positions were synthesized and attached to scaffolds containing different photocrosslinking groups for covalent protein modification and biotin for affinity isolation. The utility of these SAH capture compounds for selective photoinduced protein isolation is demonstrated for various methyltransferases (MTases) acting on DNA, RNA and proteins as well as with Escherichia coli cell lysate. In addition, they can be used to determine dissociation constants for MTase–cofactor complexes.     

Application of low loading of collagen in electrospun poly[(l‐lactide)‐co‐(ε‐caprolactone)] nanofibrous scaffolds to promote cellular biocompatibility

Electrospinning of various polymers has been used to produce nanofibrous scaffolds that mimic the extracellular matrix and support cell attachment for the potential repair and engineering of nerve tissue. In the study reported here, an electrospun copolymer of l ‐lactide and ε‐caprolactone (67:33 mol%) resulted in a nanofibrous scaffold with average fibre diameter and pore size of 476 ± 88 and 253 ± 17 nm, respectively. Blending with low loadings of collagen (<2.5% w/w) significantly reduced the average diameter and pore size. The uniformity of fibre diameter distributions was supported with increasing collagen loadings. The nanofibrous scaffolds significantly promoted the attachment and proliferation of olfactory ensheathing cells compared to cells exhibiting asynchronous growth. Furthermore, analysis of cell health through mitochondrial activity, membrane leakage, cell cycle progression and apoptotic indices showed that the nanofibrous membranes promoted cell vigour, reducing necrosis. The study suggests that the use of more cost‐effective, low loadings of collagen supports morphological changes in electrospun poly[(l ‐lactide)‐co‐(ε‐caprolactone)] nanofibrous scaffolds, which also support attachment and proliferation of olfactory ensheathing cells while promoting cell health. The results here support further investigation of the electrospinning of these polymer blends as conduits for nerve repair. © 2013 Society of Chemical Industry     

Synthesis of PEGylated poly(amino acid) pentablock copolymers and their self‐assembly

Pentablock copolymers with an ABCBA architecture were synthesized by ring‐opening polymerization of N‐carboxyanhydrides of l ‐leucine and γ‐benzyl l ‐glutamate using an α, ω‐diamino poly(ethylene glycol) (PEG) as macroinitiator. Three different PEGs with molecular weights of 2000, 4600 and 10 000 Da were used and the poly(amino acid) (PAA) block lengths were set to a combined 10 and 40, respectively, repeat units for p(l ‐Leu) and 40 repeat units for p(l ‐Glu). The molecular architecture of the resulting pentablock copolymers was determined by the order of monomer addition. The living character of the N‐carboxyanhydride ring‐opening polymerization enables the formation of multiblock copolymers. The degree of polymerization for the PAA blocks matched the monomer/initiator ratio. A structural switch element, which controls the hydrophilicity of the pentablock copolymers, was incorporated in the form of the p(l ‐Glu) blocks. The pentablock copolymers became water soluble after hydrolyzing the benzyl ester protective groups. The pentablock copolymers self‐assembled into polymeric aggregates ranging in size between 160 and 340 nm. Hydrogels formed readily if the central PEG block had a molecular weight of at least 4600 Da and the terminal A‐blocks consisted of p(l ‐Leu). SEM images confirmed the size ranges of the polymeric aggregates and showed non‐distinct spherical aggregates. © 2016 Society of Chemical Industry     

Molecular Recognition of Two 2,4‐syn‐Functionalized (S)‐Glutamate Analogues by the Kainate Receptor GluK3 Ligand Binding Domain

The kainate receptors are the least studied subfamily of ionotropic glutamate receptors. These receptors are thought to have a neuromodulatory role and have been associated with a variety of disorders in the central nervous system. This makes kainate receptors interesting potential drug targets. Today, structures of the ligand binding domain (LBD) of the kainate receptor GluK3 are only known in complex with the endogenous agonist glutamate, the natural product kainate, and two synthetic agonists. Herein we report structures of GluK3 LBD in complex with two 2,4‐syn‐functionalized (S)‐glutamate analogues to investigate their structural potential as chemical scaffolds. Similar binding affinities at GluK3 were determined for the 2‐(methylcarbamoyl)ethyl analogue (K_i=4.0 μM ) and the 2‐(methoxycarbonyl)ethyl analogue (K_i=1.7 μM ), in agreement with the similar positioning of the compounds within the binding pocket. As the binding affinity is similar to that of glutamate, this type of Cγ substituent could be used as a scaffold for introduction of even larger substituents reaching into unexplored binding site regions to achieve subtype selectivity.     

A robust affinity chromatography system based on ceramic monoliths coated with poly(amino acid)‐based polymeric constructs

Traditional chromatographic separation systems are disadvantaged by low flow rates, a high pressure drop across the column, low capacity and poor reusability. Searching for more efficient separation systems we introduced the use of a ceramic monolith as robust support in bioseparations. A coating consisting of l ‐asparagine as ligand, poly(l ‐lysine) as spacer arm and a commercial poly(ethylene acrylic acid) film forming copolymer network (Michem 4983‐40R) was developed as a coating for these ceramic monoliths. Poly(l ‐lysine) was synthesized by ring‐opening polymerization of ε‐trifluoroacetyl‐l ‐lysine N‐carboxyanhydride and coupled to a commercial film‐forming poly(ethylene acrylic acid) network. This construct was then ‘decorated’ with l ‐asparagine via the terminal amino functional groups of poly(L‐lysine) and coated onto the ceramic monolith to selectively bind l ‐asparaginase. Adsorption/elution experiments showed reversible binding between l ‐asparagine and l ‐asparaginase, and the subsequent release of l ‐asparaginase, and between 83% and 94% of the active enzyme was recovered by elution with d ‐asparagine and NaCl solutions. The functional activity of the eluted l ‐asparaginase was verified by a Nessler's assay. While traditional separation processes (adsorption and elution) using gel bead packings take many hours, the ceramic monolith system achieves the same of level of separation in about 1 h. This new system served as a proof of concept for its application in protein separation and purification. This work paves the way to a better understanding of the use of ceramic monoliths as stationary phase coated with a stable polymer construct for more robust and efficient supports in affinity chromatography. © 2020 Society of Industrial Chemistry     

An Iterative O‐Methyltransferase Catalyzes 1,11‐Dimethylation of Aspergillus fumigatus Fumaric Acid Amides

S‐adenosyl‐l ‐methionine (SAM)‐dependent methyltransfer is a common biosynthetic strategy to modify natural products. We investigated the previously uncharacterized Aspergillus fumigatus methyltransferase FtpM, which is encoded next to the bimodular fumaric acid amide synthetase FtpA. Structure elucidation of two new A. fumigatus natural products, the 1,11‐dimethyl esters of fumaryl‐l ‐tyrosine and fumaryl‐l ‐phenylalanine, together with ftpM gene disruption suggested that FtpM catalyzes iterative methylation. Final evidence that a single enzyme repeatedly acts on fumaric acid amides came from an in vitro biochemical investigation with recombinantly produced FtpM. Size‐exclusion chromatography indicated that this methyltransferase is active as a dimer. As ftpA and ftpM homologues are found clustered in other fungi, we expect our work will help to identify and annotate natural product biosynthesis genes in various species.     

Synthesis and Pharmacological Evaluation of Enantiomerically Pure GluN2B Selective NMDA Receptor Antagonists

To determine the eutomers of potent GluN2B‐selective N‐methyl‐d ‐aspartate (NMDA) receptor antagonists with a 3‐benzazepine scaffold, 7‐benzyloxy‐3‐(4‐phenylbutyl)‐2,3,4,5‐tetrahydro‐1H‐3‐benzazepin‐1‐ols (S)‐ 2 and (R)‐ 2 were separated by chiral HPLC. Hydrogenolysis and subsequent methylation of the enantiomerically pure benzyl ethers of (S)‐ 2 and (R)‐ 2 provided the enantiomeric phenols (S)‐ 3 and (R)‐ 3 [3‐(4‐phenylbutyl)‐2,3,4,5‐tetrahydro‐1H‐3‐benzazepine‐1,7‐diol] and methyl ethers (S)‐ 4 and (R)‐ 4 . All enantiomers were obtained with high enantiomeric purity (≥99.7 % ee). The absolute configurations were determined by CD spectroscopy. R‐configured enantiomers turned out to be the eutomers in receptor binding studies and two‐electrode voltage clamp experiments. The most promising ligand of this compound series is the R‐configured phenol (R)‐ 3 , displaying high GluN2B affinity (K_i=30 nm ), high inhibition of ion flux (IC₅₀=61 nm ), and high cytoprotective activity (IC₅₀=93 nm ). Whereas the eudismic ratio in the receptor binding assay is 25, the eudismic ratio in the electrophysiological experiment is 3.     

Bis‐(1,2,3,4‐tetrahydroisoquinolinium): A Chiral Scaffold for Developing High‐Affinity Ligands for SK Channels

N‐Methyl‐bis‐(1,2,3,4‐tetrahydroisoquinolinium) analogues derived from AG525 (1,1′‐(propane‐1,3‐diyl)‐bis‐(6,7‐dimethoxy‐2‐methyl‐1,2,3,4‐tetrahydroisoquinoline)) stereoisomers and tetrandrine, a rigid bis‐(1,2,3,4‐tetrahydroisoquinoline) analogue with an S,S configuration, were synthesized and tested for their affinity for small‐conductance calcium‐activated potassium channel (SK/K_Ca2) subtypes using radioligand binding assays. A significant increase in affinity was observed for the quaternized analogues over the parent 1,2,3,4‐tetrahydroisoquinoline compounds. Interestingly, the impact of stereochemistry was not the same in the two groups of compounds. For quaternized analogues, affinities of S,S and R,R isomers for SK2 and SK3 channels were similar and in both cases higher than that of the meso derivative. Among the bis‐tetrahydroisoquinoline compounds, the S,S isomers exhibited high affinity, while the R,R and meso isomers had similarly lower affinities. Furthermore, the SK2/SK3 selectivity ratio was slightly increased for quaternized analogues. Bis‐(1,2,3,4‐tetrahydroisoquinolinium) represents a new scaffold for the development of high‐affinity ligands for SK channel subtypes.     

Efficient Biosynthesis of (R)‐ or (S)‐2‐Hydroxybutyrate from l‐Threonine through a Synthetic Biology Approach

An efficient multi‐enzyme cascade reaction for the synthesis of (R)‐ or (S)‐2‐hydroxybutyric acid [(R)‐ or (S)‐2‐HB] from l ‐threonine was developed by using recombinant Escherichia coli cells expressing separately or co‐expressing l ‐threonine deaminase from Escherichia coli K‐12 (ilvA), formate dehydrogenase (FDH) from Candida boidinii and l ‐lactate dehydrogenase (l ‐LDH) from Oryctolagus cuniculus or d ‐lactate dehydrogenase (d ‐LDH) from Staphylococcus epidermidis ATCC 12228. Up to 750 mM of l ‐threonine were completely transformed to (R)‐ or (S)‐2‐HB in optically pure form (>99% ee) with high isolated yields. This one‐pot multi‐enzyme transformation provides a new practical method for the synthesis of these important optically pure compounds.

     

Preparation and characterization of nanohydroxyapatite strengthening nanofibrous poly(L‐lactide) scaffold for bone tissue engineering

A biomimetic nanofibrous poly(L ‐lactide) scaffold strengthened by nanohydroxyapatite particles was fabricated via a thermally induced phase separation technique. Scanning electron microscopy results showed that nanohydroxyapatite particles uniformly dispersed in the nanofibrous poly(L ‐lactide) scaffold (50–500 nm in fiber diameter) with slight aggregation at a high nHA content, but showed no influence on the interconnected macroporous and nanofibrous structure of the scaffold. The nanofibrous poly(L ‐lactide) scaffold presented a specific surface area of 34.06 m² g^?1, which was much higher than that of 2.79 m² g^?1 for the poly(L ‐lactide) scaffold with platelet structure. Moreover, the specific surface area of the nanofibrous scaffold was further enhanced by incorporating nanohydroxyapatite particles. With increasing the nanohydroxyapatite content, the compressive modulus and amount of bovine serum albumin adsorbed on the surface of the nanofibrous composite scaffold were markedly improved, as opposed to the decreased crystallinity. In comparison to poly(L ‐lactide) scaffold, both the nanofibrous poly(L ‐lactide) and poly(L ‐lactide)/nanohydroxyapatite scaffolds exhibited a faster degradation rate for their much larger specific surface area. The culture of bone mesenchymal stem cell indicated that the composite nanofibrous poly(L ‐lactide) scaffold with 50 wt % nanohydroxyapatite showed the highest cells viability among various poly(L ‐lactide)‐based scaffolds. The strengthened biomimetic nanofibrous poly(L ‐lactide)/nanohydroxyapatite composite scaffold will be a potential candidate for bone tissue engineering. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Amphiphilic star‐shaped poly(ε‐caprolactone)‐block‐poly(l‐lysine) copolymers with porphyrin core: Synthesis,self‐assembly,and cell viability assay

Star‐shaped copolymers poly(ε‐caprolactone)‐bolck‐poly(ε‐benzyloxycarbonyl‐l ‐lysine) (SPPCL‐b‐PZLLs) with porphyrin core were synthesized by a sequential ring‐opening polymerization (ROP) of CL and Nε‐Benzyloxycarbonyl‐l ‐lysine N‐Carboxyanhydride. After the deprotection of benzyloxycarbonyl groups in polylysine blocks, the star‐shaped amphiphilic copolymers SPPCL‐b‐PLLs were obtained. These amphiphilic copolymers can self‐assemble into micelles or aggregates in aqueous solution. Investigation shows that the morphology of micelles/aggregates varied according to the change of pH values of media, indicating the pH‐responsive property of SPPCL‐b‐PLL copolymers. Furthermore, associated with conjugated porphyrin cores, the SPPCL‐b‐PLL copolymers micelles showed a certain degree of Photodynamic Therapy (PDT) effects on tumor cells, suggesting its potential application as carrier for hydrophobic drug with additional therapeutic ability of inherent porphyrin segments. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40097.     

Discovery, synthesis, and in vitro evaluation of West Nile virus protease inhibitors based on the 9,10-dihydro-3H,4aH-1,3,9,10a-tetraazaphenanthren-4-one scaffold

West Nile virus (WNV), a member of the Flaviviridae family, is a mosquito‐borne pathogen that causes a great number of human infections each year. Neither vaccines nor antiviral therapies are currently available for human use. In this study, a WNV NS2B–NS3 protease inhibitor with a 9,10‐dihydro‐3H,4aH‐1,3,9,10a‐tetraazaphenanthren‐4‐one scaffold was identified by screening a small library of non‐peptidic compounds. This initial hit was optimized by solution‐phase synthesis and screening of a focused library of compounds bearing this scaffold. This led to the identification of a novel, uncompetitive inhibitor ( 1a40 , IC₅₀=5.41±0.45 μM ) of WNV NS2B–NS3 protease. Molecular docking of this chiral compound onto the WNV protease indicates that the S enantiomer of 1a40 appears to interfere with the productive interactions between the NS2B cofactor and the NS3 protease domain; (S)‐ 1a40 is a preferred isomer for inhibition of WNV NS3 protease.     

Application of the guanidine-acylguanidine bioisosteric approach to argininamide-type NPY Y₂ receptor antagonists

Strongly basic groups such as guanidine moieties are crucial structural elements, but they compromise the drug‐likeness of numerous biologically active compounds, including ligands of G‐protein‐coupled receptors (GPCRs). As part of a project focused on the search for guanidine bioisosteres, argininamide‐type neuropeptide Y (NPY) Y₂ receptor (Y₂R) antagonists related to BIIE0246 were synthesized. Starting from ornithine derivatives, N^G‐acylated argininamides were obtained by guanidinylation with tailor‐made mono‐Boc‐protected N‐acyl‐S‐methylisothioureas. The compounds were investigated for Y₂R antagonism (calcium assays), Y₂R affinity, and NPY receptor subtype selectivity (flow cytometric binding assays). Most of the N^G‐substituted (S)‐argininamides showed Y₂R antagonistic activities and binding affinities similar to those of the parent compound, whereas N^G‐acylated or ‐carbamoylated analogues with a terminal amine were superior (Y₂R: K_i and K_B values in the low nanomolar range). This demonstrates that the basicity of the compounds, although 4–5 orders of magnitude lower than that of guanidines, is sufficient to form key interactions with acidic amino acids of the Y₂R. The acylguanidines bind with high affinity and selectivity to Y₂R over the Y₁, Y₄, and Y₅ receptors. As derivatization of the amino group is tolerated, these compounds can be considered building blocks for the preparation of versatile fluorescent and radiolabeled pharmacological tools for in vitro studies of the Y₂R. The results support the concept of bioisosteric guanidine–acylguanidine exchange as a broadly applicable approach to retain pharmacological activity despite decreased basicity.     

Evaluation of polycaprolactone − poly‐D,L‐lactide copolymer as biomaterial for breast tissue engineering

The potential of the copolymer polycaprolactone‐co‐ poly‐d ,l ‐lactic acid (PCLLA ) as a biomaterial for scaffold‐based therapy for breast tissue engineering applications was assessed. First, the synthesized PCLLA was evaluated for its processability by means of additive manufacturing (AM ). We found that the synthesized PCLLA could be fabricated into scaffolds with an overall gross morphology and porosity similar to that of polycaprolactone. The PCLLA scaffolds possessed a compressive Young's modulus (ca 46 kPa ) similar to that of native breast (0.5 ? 25 kPa ), but lacked thermal stability and underwent thermal degradation during the fabrication process. The PCLLA scaffolds underwent rapid degradation in vitro which was characterized by loss of the scaffolds' mechanical integrity and a drastic decrease in mass‐average molar mass (M _w) and number‐average molar mass (M _n) after 4 weeks of immersion in phosphate buffer solution maintained at 37 °C. The tin‐catalysed PCLLA scaffold was also found to have cytotoxic effects on cells. Although the initial mechanical properties of the PCLLA scaffolds generally showed potential for applications in breast tissue regeneration, the thermal stability of the copolymer for AM processes, biocompatibility towards cells and degradation rate is not satisfactory at this stage. Therefore, we conclude that research efforts should be geared towards fine‐tuning the copolymer synthesizing methods. © 2016 Society of Chemical Industry     

Synthesis and properties of non‐cytotoxic poly(l‐malic acid acetate‐co‐l‐lysine ester)s with thermo‐sensitivity as hydrophobic drug nanocarrier

In this investigation, a group of poly(l ‐malic acid acetate‐co‐l ‐lysine ester)s (PMALs) with excellent thermo‐sensitivity and non‐cytotoxicity were prepared by an optimized synthetic route from natural l ‐malic acid and l ‐lysine. The structure and properties of PMALs including monomers were systematically characterized by FTIR, ¹H NMR, UV, gel permeation chromatograph, scanning electron microscope, contact angle measurement, cell counting kit assess (CCK‐8), and confocal laser scanning microscopy (CLSM). Three PMALs show a reversible lower critical solution temperature of 8–36 °C depending on their chemical structure. The contact angle measurement revealed a considerable discrepancy in the hydrophilicity/hydrophobicity of PMALs and further influence on their thermo‐sensitivity. The viability of HeLa cells exposed to 0.2–100 μg/mL PMALs solution was found to be in a range 80–103% after 24, 48, and 72 h of incubation, indicating no cytotoxicity. Moreover, a spherical nanocarrier with core‐shell structure was facilely fabricated via the thermo‐sensitivity of PMALs and hydrophobicity of drug. CLSM observations manifested that the hydrophobic‐curcumin‐enwrapped nanocarriers can clearly internalize into the cellular inside. The sustained release of curcumin from nanocarriers in vitro provided a possibility of depressing fast hydrolytic degradation at physiological pH or other side‐effects. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45984.     

Novel Dioxane and Morpholino Nucleotide Analogues: Syntheses and RNA-Hybridization Properties

A novel class of nucleotide analogues with a dioxane ring as central scaffold has been developed. Synthetic routes in two diastereomeric series were realized, and the final thymidine analogues were synthesized with common functionalities for the automated oligonucleotide synthesis. The chemical space of the initially derived nucleotides was expanded by changing the central dioxane to analogous morpholine derivatives. This opens up the possibility for further derivatization by attaching different substituents at the morpholine nitrogen. The novel nucleotide building blocks were incorporated into double-stranded RNA sequences, and their hybridization properties investigated by melting-temperature analysis. Both scaffolds, dioxanes and morpholines, had an equal impact on double-strand stability, but T_m values differed depending on the chirality in the six-membered ring.     

Design,Synthesis and Evaluation of Triazole‐Pyrimidine Analogues as SecA Inhibitors

SecA, a key component of the bacterial Sec‐dependent secretion pathway, is an attractive target for the development of new antimicrobial agents. Through a combination of virtual screening and experimental exploration of the surrounding chemical space, we identified a hit bistriazole SecA inhibitor, SCA‐21, and studied a series of analogues by systematic dissections of the core scaffold. Evaluation of these analogues allowed us to establish an initial structure–activity relationship in SecA inhibition. The best compounds in this group are potent inhibitors of SecA‐dependent protein‐conducting channel activity and protein translocation activity at low‐ to sub‐micromolar concentrations. They also have minimal inhibitory concentration (MIC) values against various strains of bacteria that correlate well with the SecA and protein translocation inhibition data. These compounds are effective against methicillin‐resistant Staphylococcus aureus strains with various levels of efflux pump activity, indicating the capacity of SecA inhibitors to null the effect of multidrug resistance. Results from studies of drug‐affinity‐responsive target stability and protein pull‐down assays are consistent with SecA as a target for these compounds.     

In vitro Synthesis of New Cyclodepsipeptides of the PF1022‐Type: Probing the α‐D‐Hydroxy Acid Tolerance of PF1022 Synthetase

The nonribosomal peptide synthetase PF1022‐synthetase (PFSYN) synthesises the cyclooctadepsipeptide PF1022 from the building blocks D ‐lactate, D ‐phenyllactate and N‐methylleucine. The substrate tolerance of PFSYN for hydroxy acids was probed by in vitro screening of a set of aliphatic and aromatic α‐D ‐hydroxy acids with various structural modifications in the side chain. Thus, new PF1022 derivatives for example, propargyl‐D ‐lactyl‐PF1022 and β‐thienyl‐D ‐lactyl‐PF1022 were generated. The promiscuous behaviour of PFSYN towards aliphatic and aromatic α‐D ‐hydroxy acids is considerably larger than that of related enniatin synthetase (ESYN) and thus gives rise to the enzymatic generation of various new PF1022 derivatives.     

Nuclear Magnetic Resonance Investigation of the Effect of pH on Micelle Formation by the Amino Acid‐Based Surfactant Undecyl l‐Phenylalaninate

Micelle formation by the anionic amino acid‐based surfactant undecyl l ‐phenylalaninate (und‐Phe) was investigated as a function of pH in solutions containing either Na⁺, l ‐arginine, l ‐lysine, or l ‐ornithine counterions. In each mixture, the surfactant's critical micelle concentration (CMC) was the lowest at low pH and increased as solutions became more basic. Below pH 9, surfactant solutions containing l ‐arginine and l ‐lysine had lower CMC than the corresponding solutions with Na⁺ counterions. Nuclear magnetic resonance (NMR) diffusometry and dynamic light scattering studies revealed that und‐Phe micelles with Na⁺ counterions had hydrodynamic radii of approximately 15 Å throughout the investigated pH range. Furthermore, l ‐arginine, l ‐lysine, and l ‐ornithine were found to bind most strongly to the micelles below pH 9 when the counterions were cationic. Above pH 9, the counterions became zwitterionic and dissociated from the micelle surface. In und‐Phe/l ‐arginine solution, counterion dissociation was accompanied by a decrease in the hydrodynamic radius of the micelle. However, in experiments with l ‐lysine and l ‐ornithine, micelle radii remained the same at low pH when counterions were bound and at high pH when they were not. This result suggested that l ‐arginine is attached perpendicular to the micelle surface through its guanidinium functional group with the remainder of the molecule extending into solution. Contrastingly, l ‐lysine and l ‐ornithine likely bind parallel to the micelle surface with their two amine functional groups interacting with different surfactant monomers. This model was consistent with the results from two‐dimensional ROESY (rotating frame Overhauser enhancement spectroscopy) NMR experiments. Two‐dimensional NMR also showed that in und‐Phe micelles, the aromatic rings on the phenylalanine headgroups were rotated toward the hydrocarbon core of micelle.