Amphiphilic Amino Acid Research Articles

Counterion dependent hydrogelation of amino acid based amphiphiles: switching from non-gelators to gelators and facile synthesis of silver nanoparticles

With the growing importance of hydrogels in scientific applications, the search for low molecular weight hydrogelators (LMWH), with simultaneous logical structural correlation is continuously increasing. In the present work, counterion variation of amino acid based amphiphiles was done to qualitatively evaluate its contribution towards hydrogelation. Further changes in the molecular skeleton of the amino acid amphiphile were done along with counterion variation to establish the importance of π–π interactions of aromatic planar ring in hydrogelation. An efficient conversion of a non-gelator to gelator molecule was achieved simply by changing the counterions to aromatic carboxylates. Role of the counterion in the mechanism of gelation process through the self-assembly of amino acid based amphiphiles has been discussed. The formation of supramolecular structures during hydrogelation was investigated by FESEM, CD, FT-IR, luminescence, 2D-NOESY and rheological studies. Interestingly, the L-tryptophan containing amphiphile hydrogelators were further utilized for synthesis of Ag nanoparticles under mild conditions without any need for high temperature, alkaline medium and external reducing agent. The nanoparticles obtained were characterized by UV-Vis, TEM, AFM and XRD experiments.

Synthesis of amphiphilic amino acid C 60 derivatives and their protective effect on hydrogen peroxide-induced apoptosis in rat pheochromocytoma cells

Water-soluble fullerene derivatives have often been used as effective scavengers for reactive oxygen species. Three amphiphilic amino acid C 60 derivatives, β-alanine C 60 derivative, cystine C 60 derivative and arginine C 60 derivative have been synthesized and characterized. Because of the hydrophobic interaction, the amino acid C 60 derivatives could self-assemble to form spherical aggregates. Moreover, the cystine C 60 derivative and arginine C 60 derivative could further form multilayer vesicles because of hydrogen bonding. Experiments demonstrate that the aggregate morphology could impact the protective effect of amino acid C 60 derivatives on apoptosis. These compounds were able to penetrate the cell membrane and reduce the accumulation of reactive oxygen species and cellular damage caused by hydrogen peroxide in PC12 cells. The results suggest that amino acid C 60 derivatives have the potential to prevent oxidative stress-induced cell death without evident toxicity. Hence, we hypothesize that the protective effect of amino acid C 60 derivatives on hydrogen peroxide induced apoptosis is related to many factors, such as scavenger activity and morphology of the aggregate.

Single particle and collective hydration dynamics for hydrophobic and hydrophilic peptides

We have conducted extensive molecular dynamics simulations to study the single particle and collective dynamics of water in solutions of N-acetyl-glycine-methylamide, a model hydrophilic protein backbone, and N-acetyl-leucine-methylamide, a model (amphiphilic) hydrophobic peptide, as a function of peptide concentration. Various analytical models commonly used in the analysis of incoherent quasielastic neutron scattering (QENS), are tested against the translational and rotational intermediate scattering function, the mean square displacement of the water molecule center of mass, and fits to the second-order rotational correlation function of water evaluated directly from the simulation data. We find that while the agreement between the model-free analysis and analytical QENS models is quantitatively poor, the qualitative feature of dynamical heterogeneity due to caging is captured well by all approaches. The center of mass collective and single particle intermediate scattering functions of water calculated for these peptide solutions show that the crossover from collective to single particle-dominated motions occurs at a higher value of Q for high concentration solutions relative to low concentration because of the greater restriction in movement of water molecules due to confinement. Finally, we have shown that at the same level of confinement of the two peptides, the aqueous amphiphilic amino acid solution shows the strongest deviation between single particle and collective dynamics relative to the hydrophilic amino acid, indicating that chemical heterogeneity induces even greater spatial heterogeneity in the water dynamics.

Catalysis of a Peptidic Micellar Assembly Covalently Immobilized within Mesoporous Silica Channels: Importance of Amphiphilic Spatial Design

A mesostructured silica/organic composite 1-MS, constructed from a rodlike micelle of amino acid amphiphile 1 that has a condensable head group and that can be used as a template, was found to be able to catalyze the acetalization of cyclohexanone, in ethanol at 25 degrees C (50% in 12 h), whereas no reaction took place with unfunctionalized mesoporous silica. In sharp contrast, hydrolytic removal of the C16 alkyl tail of immobilized 1 resulted in the complete disappearance of the catalytic activity, which suggests the importance of a hydrophobic inner domain for the admission of cyclohexanone. Unsupported peptide amphiphile 2, under identical conditions to those above, was inefficient for acetalization regardless of the absence (2% in 24 h) or presence of mesoporous silica (7% in 24 h). Reference composite 2-MS, which is a noncovalently immobilized peptidic micelle, was virtually inactive (1% in 24 h). These observations indicate the importance of covalent immobilization of the peptidic rod micelle for catalysis. Mesostructured silicate 3-MS hybridized with a nonpeptidic, ammonium ion amphiphile (3) showed a certain catalytic activity, but the yield (12% in 24 h) of the acetal was much lower than that achieved by using 1-MS as the catalyst. Amorphous silica with immobilized 1 on its surface was much less active than 1-MS for acetalization (5% in 24 h).

Fluorinated Amphiphilic Amino Acid Derivatives as Antioxidant Carriers: A New Class of Protective Agents

The use of classical antioxidants is limited by their low bioavailabilities, and therefore, high doses are usually required to display significant protective activity. In a recent article (J. Med. Chem. 2003, 46, 5230) we showed that the ability of the alpha-phenyl-N-tert-butylnitrone (PBN) to restore the viability of ATPase-deficient human skin fibroblasts was greatly enhanced by grafting it on a fluorinated amphiphilic carrier. With the aim of extending this concept to other antioxidants, we present here the design, the synthesis, and the physicochemical measurements of a new series of fluorinated amphiphilic antioxidant derivatives. The hydroxyl radical scavenging activity and the radical reducing potency of these newly designed compounds were respectively demonstrated in an ABTS competition and an ABTS(*+) reduction assay. We also showed that the protective effects of amphiphilic antioxidants derived from PBN, Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) or lipoic acid (5-[1,2]-dithiolan-3-ylpentanoic acid) in primary cortical mixed cell cultures exposed to oxidotoxins are greatly improved compared to their parent compounds in the following rank-order: (1) PBN, (2) Trolox, and (3) lipoic acid. In contrast, the protective activity of indole-3-propionic acid was slightly decreased by grafting it on the amphiphilic carrier. Similar observations were made in in vivo experiments using aquatic invertebrate microorganisms, called rotifers, which were exposed to lethal concentrations of nonselective (H(2)O(2)) and mitochondria-selective (doxorubicin) oxidotoxins. The conclusion of these studies is that fluorinated amphiphilic PBN, Trolox, and lipoic acid derivatives exhibit very potent protective activities in in vitro and in vivo experiments. The findings demonstrated herein therefore strongly suggest that the amphiphilic character enhances the bioavailability of the antioxidants and allows for a selective targeting of mitochondria.

First report on the efficacy of l-alanine-based in situ-forming implants for the long-term parenteral delivery of drugs

The recent advent of biotechnologies has led to the development of labile macromolecular therapeutic agents that require complex formulations for their efficient administration. This work reports a novel concept for the systemic, sustained delivery of such agents. The proposed approach is based on the spontaneous self-assembly of low-molecular weight amphiphilic amino acid derivatives in a hydrophobic pharmaceutical vehicle. The injectable, in situ-forming organogels were obtained by mixing N-stearoyl l-alanine (m)ethyl esters with a vegetable oil and a biocompatible hydrophilic solvent. The gels’ in vivo-delivering properties were evaluated in rats with leuprolide, a luteinizing hormone-releasing hormone agonist used in prostate cancer, endometriosis and precocious puberty treatment. Following subcutaneous injection, the gels degraded and gradually released leuprolide for 14 to 25 days. Drug release was accompanied by sustained castration lasting up to 50 days, as assessed by testosterone levels. This study demonstrates that in situ-forming implants based on l-alanine derivatives represent a novel injectable platform for the controlled delivery of hydrophilic compounds, which is simpler than currently available implant and microsphere technologies.

IRRAS Studies on Chain Orientation in the Monolayers of Amino Acid Amphiphiles at the Air−Water Interface Depending on Metal Complex and Hydrogen Bond Formation with the Headgroups

Monolayers of N-octadecanoyl-L-alanine at the air-water interface on pure water and metal ion containing subphases have been studied using polarized infrared reflection-absorption spectroscopy (IRRAS). The metal complex and hydrogen bond formation with the headgroups give rise to a change in chain order depending on metal ion in the subphase. On pure water and Ag(+)-/Pb(2+)-containing subphase, the antisymmetric CH(2) stretching band intensity [nu(a)(CH(2))] undergoes a slower increase than the symmetric one [nu(s)(CH(2))] below the Brewster angle, so the intensity ratios of nu(a)(CH(2))/nu(s)(CH(2)) are less than 1 in the cases of Ag(+) and Pb(2+). Beyond the Brewster angle, the nu(a)(CH(2)) band intensities are substantially reduced in comparison with the nu(s)(CH(2)) ones in the cases of pure water and Ag(+), but the nu(a)(CH(2)) bands still remain negative-oriented in the presence of Pb(2+). These unusual spectral features indicate that the alkyl chains take a preferential orientation with their C-C-C planes parallel to the water surface. The parallel packing of the alkyl chains results from the intermolecular hydrogen bonds C=O...H-N between the neighboring amide groups, strengthened by the metal complex of covalent interaction. On the Ca(2+)-/Cu(2+)-containing subphase, the corresponding polarized spectra display a usual behavior. The alkyl chains are roughly estimated to be inclined around 35-40 degrees from the surface normal on the assumption of chain segment orientation for the monolayers in the liquid-expanded phase. The chain conformation and tilt are closely related to the formation of intramolecular hydrogen bonds and the ionic interaction of the metal complex in the cases of Ca(2+) and Cu(2+).

Amphiphilic amino acid copolymers as stabilizers for the preparation of nanocrystal dispersion

The recent advance of particle size engineering in nanometer ranges has widened the formulation opportunities of relatively water-insoluble drugs. However, the ‘nanoformulation’ suffers from a lack of systematic understanding about the requirements of polymeric stabilizers. Furthermore, the polymers that can be used for the preparation of nanocrystals are so limited that finding a proper stabilizer for a given formulation is often difficult. In this study, amino acid copolymers whose properties can systematically be tailored are developed, and their morphological and compositional effects are investigated. Copolymers containing lysine (K) as their hydrophilic segments, and phenylalanine (F) or leucine (L) as their hydrophobic segments successfully produce stable nanocrystals (200–300 nm) in water, while copolymers of K and alanine (A) could not generate nanosized particles. Not the morphology but the hydrophobicity of copolymers seems to be a critical parameter in the preparation of drug nanocrystals by wet comminution. The effective stabilization performance of copolymers requires the hydrophobic moiety content to be higher than 15 mol%. Comminution for only 5 min is long enough for nanocrystal preparation, and the crystallinity of drug is found intact after the processing.

Molecular and biochemical characterization of LeCRK1, a ripening-associated tomato CDPK-related kinase

A cDNA clone (LeCRK1), encoding a novel isoform of calcium-dependent protein kinase (CDPK), was isolated by screening a tomato (Lycopersicon esculentum) cDNA library. The protein derived from the full-length sequence indicated that it belongs to the family of CDPK-related kinases (CRKs) and the predicted amino acid sequence shows a modular organization of the protein consisting of different characteristic domains. The kinase domain of LeCRK1 shares a high degree of similarity with the catalytic domain of CDPKs. In contrast to canonical members of the family, LeCRK1 has a degenerate sequence in the C-terminal calmodulin-like domain. LeCRK1 protein was shown to be a functional kinase, but, consistent with the lack of calcium-binding activity, its autophosphorylation activity did not require calcium. LeCRK1 harbours an amphiphilic amino acid region revealed to be a functional calmodulin-binding site by in vitro assay. A putative myristoylation/palmitoylation sequence has been identified at the N-terminus. Expressing an LeCRK1::GFP fusion protein in the protoplast resulted in its targeting to the plasma membrane. Site-directed mutagenesis of critical amino acids of the myristoylation/palmitoylation consensus sites led to the accumulation of the mutated protein in the cytoplasm, suggesting that the native protein is anchored to the plasma membrane by acylated residues. Expression studies revealed significant accumulation of LeCRK1 transcripts during fruit ripening, although transcripts were also detected in stem, leaf, and flower. LeCRK1 mRNA level in leaves was slightly induced by ethylene and salicylic acid, and upon mechanical wounding and cold treatment. It is noteworthy that LeCRK1 mRNAs were undetectable in different tomato-ripening natural mutants such as NR, Rin, and Nor, suggesting a role in the ripening process.

Molecular pair potential of chiral amino acid amphiphile in Langmuir monolayers on the basis of an atomistic model

In the present work, the pair potential of enantiomeric N-palmitoyl aspartic acid amphiphile monolayer at the air/water interface is calculated based on an atomistic model. The molecular structure and partial charges are calculated using two semi empirical (PM3, AM1) and one empirical (Gasteiger and Marcili) methods. A distance-dependent dielectric function is used to represent the interfacial dielectric constant at the aqueous subphase. The present study indicates that a pair of molecules have favorable interaction at specific ranges of mutual orientations. Other orientations are favorable but at larger separations. Favorable electrostatic interaction at a specific combination of orientation and short separations of the head groups significantly contribute to the total energy. The curvature of the domain boundary is suggested to be driven by the favorable arrangement which is dependent on the pair potential of molecules. The use of charges obtained by the PM3 and GM do not lead to a significant variation of the orientation-dependent features, while the AM1 predicts higher partial charges and interactions are stronger than the former two methods. However, orientation-dependent features remain the same. The variations in the LJ parameters and charges indicate that the conclusions made are insensitive to the choice of parameters. The mutual favorable interaction predicted by calculation agree with the handedness of curvature of domains.

Detection of NH Stretching Signals from the Monolayers of Amino Acid Amphiphiles at the Air−Water Interface and Change of Hydrogen Bond Depending on Metal Ion in the Subphase: Infrared Reflection−Adsorption Spectroscopy

The monolayers of amino acid amphiphiles at the air−water interface on pure water and ion-containing subphases have been studied using infrared reflection−absorption spectroscopy. It is the first time that the NH stretching signals are detected from the monolayers at the air−water interface with this technique. In the presence of Ag+ and Pb2+ in the subphases, the covalent interaction between metals and carboxylate groups is predominant in the form of chelating and/or bridging bidentate coordination, and the intermolecular hydrogen-bonding interaction between the adjacent molecules through amide groups is increased in comparison with that in the case of pure water. In the presence of Ca2+ and Cu2+, the ionic interaction between metal ions and carboxylate groups is preferred, and the intramolecular hydrogen bond is developed. The hydrogen bond in the monolayers at the air−water interface undergoes an obvious change in type and strength depending on metal ion in the subphase.

Calmodulin-binding proteins in bryophytes: identification of abscisic acid-, cold-, and osmotic stress-induced genes encoding novel membrane-bound transporter-like proteins

Plant responses to environmental stresses are mediated in part by signaling processes involving cytosolic Ca 2+ and a Ca 2+-binding protein, calmodulin. Screening with radiolabeled calmodulin of a cDNA library of the moss Physcomitrella patens resulted in identification of genes encoding novel membrane transporter-like proteins, MCamb1 and MCamb2. These proteins each had a central hydrophobic domain with two putative membrane spans and N- and C-terminal hydrophilic domains, and showed sequence similarity to mammalian inward rectifier potassium channels. Calmodulin binds to MCamb1 and MCamb2 via interaction with basic amphiphilic amino acids in the C-terminal domain. Levels of MCamb1 and MCamb2 transcripts increased dramatically following treatment with low temperature, hyperosmotic solutes, and the stress hormone abscisic acid, all of which were previously shown to increase cellular tolerance to freezing stress. These results suggest that calmodulin participates in cellular signaling events leading to enhancement of stress resistance through regulation of novel transporter-like proteins.

Characterization of a Novel Plant PP2C-like Protein Ser/Thr Phosphatase as a Calmodulin-binding Protein

Protein phosphatases regulated by calmodulin (CaM) mediate the action of intracellular Ca2+ and modulate functions of various target proteins by dephosphorylation. In plants, however, the role of Ca2+ in the regulation of protein dephosphorylation is not well understood due to a lack of information on characteristics of CaM-regulated protein phosphatases. Screening of a cDNA library of the moss Physcomitrella patens by using 35S-labeled calmodulin as a ligand resulted in identification of a gene, PCaMPP, that encodes a protein serine/threonine phosphatase with 373 amino acids. PCaMPP had a catalytic domain with sequence similarity to type 2C protein phosphatases (PP2Cs) with six conserved metal-associating amino acid residues and also had an extra C-terminal domain. Recombinant GST fusion proteins of PCaMPP exhibited Mn2+-dependent phosphatase activity, and the activity was inhibited by pyrophosphate and 1 mm Ca2+ but not by okadaic acid, orthovanadate, or beta-glycerophosphate. Furthermore, the PCaMPP activity was increased 1.7-fold by addition of CaM at nanomolar concentrations. CaM binding assays using deletion proteins and a synthetic peptide revealed that the CaM-binding region resides within the basic amphiphilic amino acid region 324-346 in the C-terminal domain. The CaM-binding region had sequence similarity to amino acids in one of three alpha-helices in the C-terminal domain of human PP2Calpha, suggesting a novel role of the C-terminal domains for the phosphatase activity. These results provide the first evidence showing possible regulation of PP2C-related phosphatases by Ca2+/CaM in plants. Genes similar to PCaMPP were found in genomes of various higher plant species, suggesting that PCaMPP-type protein phosphatases are conserved in land plants.

Influence of surface hydrophobicity of substrates on the self-organization of chiral molecule

The interfacial-dependent self-organization of a chiral amino acid amphiphile was observed on various substrates with different surface hydrophobicity. Three kinds of organizations are induced at different substrates, which appear as helical aggregates on hydrophobic surface (e.g. highly oriented pyrolytic graphite), molecular flat layer on hydrophilic surface, (e.g. mica) and a transition state between them on silicon wafer. The strength of the interactions between the helical aggregates formed in solution and the substrate is the key factor to the self-organization of the chiral molecule. The results reported here provide insight into how the surface hydrophobicity of substrates affects the self-organization of chiral molecules.

Prediction of the handedness of the chiral domains of amphiphilic monolayers: monolayers of amino acid amphiphiles

The effect of chiral interaction on the morphology of the condensed phase domains of amino acid amphiphiles has been investigated theoretically in the present paper. The amphiphiles considered are N-palmitoyl aspartic acid, N-stearoyl serine methyl ester, N-palmitoyl-allo-threonine methyl ester and N-stearoyl-allo-threonine methyl ester. The domains of these amphiphiles show pronounced curvatures in the experiment, which are known to be driven by the chirality of the molecules concerned. We use the previously derived relations to calculate the intermolecular potential of a pair of chiral molecules of general shape, which is dependent on the intermolecular separation and the mutual orientation of the molecules. It is observed that the chiral interaction favors a mutual azimuthal orientation between the molecules in the unit cell for all amino acid amphiphiles considered. However, the azimuthal projections of the molecules within the domain are parallel as observed in the experiment. The energy to obtain parallel arrangement between the molecules is of the same order of the hydrogen bonding energy in the amino acid residues. Thus, it is suggested that the hydrogen bond cycles present among the molecules within the domain prevent the tendency of intermolecular twist due to chiral interaction. However, at the interface between the condensed phase and the fluid phase, the hydrogen bonding energy is not as strong as that within the domain as well as less direction specific. It is expected that the chiral interaction is dominating at the interface resulting the curved shape of the domain. The mutual orientation driven by chirality is cooperative in nature i.e. the mutual orientations at short and long axis of the unit cell occur in the same direction and does not oppose each other. The directions of the orientations of the molecules with respect to a reference molecule have the same senses (handedness) as those of the aggregates. The theoretically predicted senses of all four amino acid amphiphiles considered in the present study agree fairly well with the experimentally observed handedness using Brewster angle microscopy.

Interfacial-Dependent Morphologies in the Self-Organization System of Chiral Molecules Observed by Atomic Force Microscopy

The interfacial-dependent self-organization of a chiral amino acid amphiphile is observed by atomic force microscopy (AFM). This chiral amino acid amphiphile possesses an amide group and two carboxyl groups that can form intermolecular hydrogen bonds in different forms, which corresponding to the two energy-minimum states exist in the aggregates of chiral molecules in one enantiomeric form. Three kinds of organizations were induced at different interfaces, which appear as helical aggregates on hydrophobic surface (e.g., highly oriented pyrolytic graphite (HOPG)), molecular flat layer, and superstructure layer on hydrophilic surface (e.g., mica). Moreover, these two energy-minimum states can also coexist in the same layer of the superstructure on an ordered hydrophobic surface made of the linearly aligned hydrocarbon chains. These phenomena experimentally prove the existence of double energy-minimum states in the chiral molecules that are in one enantiomeric form. The results presented here provide insight...

Preparation and properties of vesicles from condensable amphiphilic amino acids

AbstractThree double‐chain amphiphiles with amino acid groups as hydrophilic moiety were synthesized. These amphiphiles can be easily dispersed in buffer solution to form transparent dispersion. Examination of the dispersion by transmission electron microscopy (TEM) showed the formation of stable vesicular aggregates, which was also confirmed by the ability to encapsulate water‐soluble dyes. Since amino acid groups are located on the surface of the vesicles, water‐soluble carbodiimide can induce the condensation of these groups to form peptide. The phase transition temperatures of these vesicles were estimated by differential scanning calorimetry (DSC), and a decrease of phase transition temperature was observed after polycondensation due to the disturbance of the ordered arrangement of the hydrophobic chains. The leakage rate of the vesicles before and after condensation was studied by monitoring the increase of fluorescence intensity of water‐soluble dye. These vesicles belong to the least permeable ones and the leakage rate can be controlled by varying the degree of condensation or the temperature.

Special Surface and Aggregation Behavior of a Novel Amino Acid Amphiphile

An amino acid amphiphile, N-(4-decyloxy-2-hydroxybenzylidene)glycine, was synthesized. Surface tension measurement, dynamic light scattering, and transmission electron microscopy were used to inves...

Microscopic study of chiral interactions in langmuir monolayer: monolayers of N-palmitoyl aspartic acid and N-stearoyl serine methyl ester

In the present paper, we theoretically investigate the effect of chiral interaction on the morphology of the domain in the condensed phase of amino acid amphiphiles like N-palmitoyl aspartic acid and N-stearoyl serine methyl ester. We derived relations to calculate the intermolecular potential of a pair of chiral molecules, which is dependent on the intermolecular separation and the mutual orientation of the molecules. The calculation is carried out based on an effective pair potential description of the groups attached to the chiral center. It is observed that the intermolecular interaction is most favorable at a twist between the azimuthal orientations of the same type of enantiomeric molecules and the energy difference between the twisted arrangement and the parallel arrangement (of neighboring molecules) is of the same order as the intermolecular hydrogen bond energy of the corresponding amino acid head groups. Thus, the observed curvature of the domain shape of amino acid amphiphiles is suggested to be due to the twist at the condensed phase/fluid interface where the interresidue (amino acid) hydrogen bond cycle is not as regular as within the condensed phase. Secondly, we also calculated the interaction energy of an external molecule with all molecules within a circular domain, having the same azimuthal orientation. It is observed that the interaction energy is rather anisotropic for enantiomers in the compressed state (smaller intermolecular separation). The interaction energy has a symmetric profile around the domain composed of a pair of mirror image isomers. Thus, the excess free energy at the condensed phase/fluid interface could be anisotropic for domains where the molecules have parallel arrangement throughout the lattice and this effect may act in concert with the intrinsic tendency to have an intermolecular twist due to chirality. The possibility of the relationship between the molecular twist and the handedness of the curvature of the domain is also discussed.

ChemInform Abstract: Spontaneous Formation of Polypeptides in the Interfacial Thin Films of Amphiphilic Amino Acid Esters: Acceleration of the Polycondensation and Control of the Structure of Resultant Polymers

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