Terminal Carboxyl Groups Research Articles

Cancer immune therapy using engineered ‛tail-flipping’ nanoliposomes targeting alternatively activated macrophages

Alternatively-activated, M2-like tumor-associated macrophages (TAM) strongly contribute to tumor growth, invasiveness and metastasis. Technologies to disable the pro-tumorigenic function of these TAMs are of high interest to immunotherapy research. Here we show that by designing engineered nanoliposomes bio-mimicking peroxidated phospholipids that are recognised and internalised by scavenger receptors, TAMs can be targeted. Incorporation of phospholipids possessing a terminal carboxylate group at the sn-2 position into nanoliposome bilayers drives their uptake by M2 macrophages with high specificity. Molecular dynamics simulation of the lipid bilayer predicts flipping of the sn-2 tail towards the aqueous phase, while molecular docking data indicates interaction of the tail with Scavenger Receptor Class B type 1 (SR-B1). In vivo, the engineered nanoliposomes are distributed specifically to M2-like macrophages and, upon delivery of the STAT6 inhibitor (AS1517499), zoledronic acid or muramyl tripeptide, these cells promote reduction of the premetastatic niche and/or tumor growth. Altogether, we demonstrate the efficiency and versatility of our engineered “tail-flipping” nanoliposomes in a pre-clinical model, which paves the way to their development as cancer immunotherapeutics in humans.

A Fixing Composition Based on a Synthetic Divinyl Rubber with Terminal Carboxyl Groups and an Epoxy Oligomer with Enhanced Specific Functionality

A Fixing Composition Based on a Synthetic Divinyl Rubber with Terminal Carboxyl Groups and an Epoxy Oligomer with Enhanced Specific Functionality

Three New Stigmatellin Derivatives Reveal Biosynthetic Insights of Its Side Chain Decoration.

Myxobacteria generate natural products with unique chemical structures, which not only feature remarkable biological functions, but also demonstrate unprecedented biosynthetic assembly strategies. The stigmatellins have been previously described as potent inhibitors of the mitochondrial and photosynthetic respiratory chain and originate from an unusual polyketide synthase assembly line. While previous biosynthetic investigations were focused on the formation of the 5,7-dimethoxy-8-hydroxychromone ring, side chain decoration of the hydrophobic alkenyl chain in position 2 was investigated less thoroughly. We report here the full structure elucidation, as well as cytotoxic and antimicrobial activities of three new stigmatellins isolated from the myxobacterium Vitiosangium cumulatum MCy10943T with side chain decorations distinct from previously characterized members of this compound family. The hydrophobic alkenyl chain in position 2 of the herein described stigmatellins feature a terminal carboxylic acid group (1), a methoxy group at C-12′ (2) or a vicinal diol (3). These findings provide further implications considering the side chain decoration of these aromatic myxobacterial polyketides and their underlying biosynthesis.

Facile Preparation of a Transparent and Rollable Omniphobic Coating with Exceptional Hardness and Wear Resistance.

The encapsulating film for the touchscreen of a foldable smartphone consists of a flexible polymer layer covered by a hard coating and an antismudge coating, and the two coating layers are currently deposited via separate steps. This paper reports the preparation of a bilayer bifunctional coating via the deposition of a single polymer mixture. The base material for the coating is a ladder-like polysilsesquioxane (LASQ) that is derived from the sol-gel chemistry of 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane. Reacting a limiting amount of the liquid antismudge agent FP-COOH, which is a perfluorinated poly(propylene oxide) bearing a terminal carboxyl group, with LASQ yields m-LASQ-FP, a mixture of unreacted LASQ, and a graft copolymer LASQ-FP. m-LASQ-FP at a fluorine mass fraction of 6.0% is photocured to yield a coating with a surface energy of 12.3 ± 1.5 mJ/m2. At a thickness of 40 μm, the coating has at 500 nm a transmittance of >99% measured against its glass substrate, a remarkable nanoindentation hardness H value of 1.4 GPa, and a pencil hardness of > 9H. After being abraded for 300 strokes under a pressure of 26 kPa with steel wool, the coating exhibits no noticeable degradation in its ink contraction properties. At a thickness of 10 μm on a poly(ethylene terephthalate) film, the coating can undergo inward (on the inner surface of the bend) and outward bending to radii <1 and <2 mm, respectively, without cracking. Aside from being a superb candidate as a protective antismudge coating for foldable smartphones, this marvelous material should also have many other applications.

Effect of amine‐reactive elastomer on the properties of poly(lactic acid) films obtained by solvent‐cast 3D printing

AbstractBlends of poly(lactic acid) (PLA) and amine‐terminated butadiene‐acrylonitrile (ATBN) elastomer were prepared by solution with ATBN contents of 8–20 wt%. Films were molded by liquid deposition modeling 3D printing. Fourier transform infrared (FTIR) spectroscopy showed that the terminal carboxylic groups of the PLA chains reacted with the terminal amino groups of ATBN. Water was the byproduct of this reaction, causing PLA hydrolysis and accelerating the reaction. Thermogravimetric analysis (TGA) showed that components interaction caused a loss of PLA thermal stability. Scanning electron microscopy (SEM) of the blends revealed a porous morphology and no phase separation. There was change in the elongation when compared with neat PLA. Although the addition of a telechelic elastomer could improve PLA toughness, any benefits arising from such addition seem to be neutralized by PLA chain scission due to hydrolysis and porosity, resulting from the condensation reaction.

Peroxidase‐like activity of Fe3O4 nanoparticles and Fe3O4‐graphene oxide nanohybrids: Effect of the amino‐ and carboxyl‐surface modifications on H2O2 sensing

Magnetic, iron oxide nanoparticles and graphene‐iron oxide nanohybrids have been introduced as artificial peroxidase‐mimics. In the present paper, the impact of coating with different functional groups on the peroxidase activity of both nanoparticles and nanohybrids was studied. Magnetite nanoparticles and graphene‐iron oxide nanohybrids, with ‐amino or ‐carboxyl terminal groups were synthesized and fully characterized by Fourier transform‐infrared, thermogravimetric analysis, transmission electron microscopy (TEM), X‐ray diffraction, vibrating sample magnetometer, and zeta potential. All particle formulations were evaluated for peroxidase activity, by studying the oxidation reaction of the peroxidase substrate 3,3,5,5‐tetramethylbenzidine (TMB) by hydrogen peroxide (H2O2). It was found that all tested formulations retained their catalytic ability (&gt;70%) at 95 °C, at wide pH range, and were reusable with 60% of their catalytic activity after 10 uses. All nanomaterials were found to follow Michaelis–Menten kinetics and peroxidase activity varied with respect to electrostatic affinity between nanoparticles or nanohybrids and the peroxidase substrates, evidenced by differences in determined kinetic parameters. The Km values of all tested particles with H2O2 substrate were higher than those reported for horseradish peroxidase (HRP; 0.214–6.36 mM), while the calculated values of reaction rates (Vmax) for the studied nanoparticles and nanohybrids with TMB and H2O2 as substrates were comparable with those reported for HRP. Our findings confirm that iron oxide nanoparticles and graphene based metallic nanohybrids can be tailor‐made to possess improved peroxidase‐like activity. Such enhancements could further widen nanohybrids application in a wide range of biomedical applications, including H2O2 detection in a proof‐of‐concept assay, which confirmed low detection limits of the tested formulations (5.3–11.3 μM).

Flexible Sustained Ionogels with Ionic Hyperbranched Polymers for Enhanced Ion-Conduction and Energy Storage.

Flexible and mechanically robust gel-like electrolytes offer enhanced energy storage capabilities, versatility, and safety in batteries and supercapacitors. However, the trade-off between ion conduction and mechanical robustness remains a challenge for these materials. Here, we suggest that the introduction of ionic hyperbranched polymers in structured sustained ionogels will lead to both enhanced ion conduction and mechanical performance because of the hyperbranched polymers' ionically conductive groups and the complementary interfacial interactions with ionic liquids. More specifically, we investigate the effect of hyperbranched polymers with carboxylate terminal groups and imidazolium counterions with various ionic group densities on the properties of ionogels composed of coassembled cellulose nanofibers (CNFs) and cellulose nanocrystals (CNCs) as sustainable open pore frame for ionic liquid immersion. The addition of hyperbranched polymers leads to the formation of highly interconnected openly porous, lightweight, and shape-persistent materials by harnessing hydrogen bonding between the polymers and the CNFs/CNCs "frame". Notably, these materials possess a 2-fold improvement in ionic conductivity combined with many-fold increase in Young's modulus, tensile strength, and toughness, making them comparable to common reinforced nanocomposite materials. Furthermore, the corresponding thin-film gel supercapacitors possess enhanced electrochemical cycling stability upon repeated bending with an 85% capacitance retention after 10 000 cycles, promising new insight in the development of simultaneously conductive and flexible gel electrolytes with sustained performance.

Probing Ca2+-induced electron transfer on the surface of self-assembled monolayer using SECM

The electron transfer process of protein on the surface of biomembrane is closely related to various physiological phenomena occurring on the biomembrane. Calcium cations (Ca2+), as a ubiquitous messenger, induced electron transfer of negatively charged protein on the surface of negatively charged biomembrane is a problem worth exploring. In this work, the 3-Mercaptopropionic acid self-assembled monolayer (MPA SAM) with terminal carboxyl groups was constructed on gold surface as a simplified model for simulated surface of negatively charged biomembrane. The electron transfer of negatively charged redox ion induced by Ca2+ on the surface of MPA SAM was investigated using scanning electrochemical microscopy. It was found that Ca2+ can induce electron transfer and enhance the electron transfer of negatively charged redox ions. The result is attributed to the reduction of the energy barrier of the electron transfer of negatively charged redox ion on the surface of MPA SAM because Ca2+ neutralizes the electrostatic repulsion interaction between negatively charged redox ion and negatively charged carboxylate surface of MPA SAM. The conditions of lower pH also benefit the above-mentioned electron transfer, which further confirms the mechanism of Ca2+-induced electron transfer. This work may provide a reference for studying the electron transfer process on the surface of simulated biomembrane.

Scaling-up of robust nanofiltration membrane of ultrathin-film-composite structure

Ultrathin film composite nanofiltration membrane was prepared in large-scale (1 m × 50 m) using indigenously developed casting and coating units. A spongy, nano-porous, compaction tolerant substrate layer (middle layer) was formed through judicial selection of the concentrations of base polymer (PSF) and its relative proportion with the pore former (PVP). The substrate with symmetric surface pore morphology (uniform distribution of innumerable nano-pores devoid of macrovoids) facilitated the formation of ultrathin polyamide rejection layer much in the form of laminate (of near uniform thickness) with very limited penetration to the substrate pores. The polyamide layer thickness could be down to 40 nm for the NF/PS/8PVP membrane. XPS study revealed a co-existence of polyamide cross-linked network and linear portion comprised of many sub-units with terminal carboxylic acid groups, which impart surface hydrophilicity and potential. The membranes exhibit a trade-off relationship between flux and selectivity with a flux of 158 L M−2H−1 at 50 psi and Na2SO4 rejection of >99% for NF/PS/8PVP membrane.

Renewable branched-chain sulfonate surfactants by addition of sodium hydrogensulfite to alkyl oleate

Branched-chain surfactants, with unique properties that make them amenable to exclusive applications, are currently typically synthesized from petrochemically derived Guerbet alcohols. Natural oleic acid, as one of the renewable platform molecules, is attracting increasing interest for the synthesis of surfactants due to its CC double bond and terminal hydrophilic carboxyl group. Herein, we report the synthesis of oleic acid-based branched-chain anionic surfactants with a sulfonate head group through terminal esterification of oleic acid with various alcohols (methanol, ethanol, and butanol) and subsequent sulfonation of the CC double bond using sodium hydrogensulfite. The three resulting alkyl oleate sulfonates have been characterized by FTIR, HPLC, LC-MS, HR-MS, and NMR techniques. Their surfactant properties, including equilibrium surface tension, dynamic surface tension, foaming property, and wetting ability, have been thoroughly investigated and compared with those of methyl ester sulfonate (MES), a linear chain analogue of alkyl oleate sulfonates. The results showed that the synthesized surfactants exhibited superior defoaming capacities (foam height after 1200 s is approximately 5% of initial foam height) and wettabilities (8 s, measured by the canvas descending method), in line with expectation for branched-chain surfactants, indicating their potential as primary surfactants for industrial cleaning products.

A Photostable 1D Ruthenium−Zinc Coordination Polymer as a Multimetallic Building Block for Light Harvesting Systems

AbstractA luminescent ruthenium complex bearing two 2,6‐di(quinolin‐8‐yl)‐pyridine carboxylic acid (dqpCOOH) ligands has been successfully synthesised and fully characterised. The new metalloligand has been coordinated to zinc ions through the terminal carboxylate groups using a one‐step solvothermal method, to give a multimetallic photoactive 1D coordination polymer, Ru−(dqpCOO)−Zn−(OOCH)2. Through use of X‐ray crystallography, advanced microscopy techniques as well as photophysical studies, we have extensively characterised the coordination polymer. The optical properties of the ruthenium complex and corresponding coordination polymer show that the material experiences a dramatic increase in photostability compared to the free parent metalloligand, in solution. Electrochemical measurements of the coordination polymer also confirm the RuII/RuIII redox couple is maintained in the polymeric network. The development of this material gives a new strategy in the design of novel photoactive materials as multimetallic building blocks for their use in light‐based applications.

Photo- and Radiation-Induced One-Electron Oxidation of Methionine in Various Structural Environments Studied by Time-Resolved Techniques.

Oxidation of methionine (Met) is an important reaction that plays a key role in protein modifications during oxidative stress and aging. The first steps of Met oxidation involve the creation of very reactive and short-lived transients. Application of complementary time-resolved radiation and photochemical techniques (pulse radiolysis and laser flash photolysis together with time-resolved CIDNP and ESR techniques) allowed comparing in detail the one-electron oxidation mechanisms initiated either by ●OH radicals and other one-electron oxidants or the excited triplet state of the sensitizers e.g., 4-,3-carboxybenzophenones. The main purpose of this review is to present various factors that influence the character of the forming intermediates. They are divided into two parts: those inextricably related to the structures of molecules containing Met and those related to external factors. The former include (i) the protection of terminal amine and carboxyl groups, (ii) the location of Met in the peptide molecule, (iii) the character of neighboring amino acid other than Met, (iv) the character of the peptide chain (open vs cyclic), (v) the number of Met residues in peptide and protein, and (vi) the optical isomerism of Met residues. External factors include the type of the oxidant, pH, and concentration of Met-containing compounds in the reaction environment. Particular attention is given to the neighboring group participation, which is an essential parameter controlling one-electron oxidation of Met. Mechanistic aspects of oxidation processes by various one-electron oxidants in various structural and pH environments are summarized and discussed. The importance of these studies for understanding oxidation of Met in real biological systems is also addressed.

Protein-Protein Interaction Inhibitors Targeting the Eph-Ephrin System with a Focus on Amino Acid Conjugates of Bile Acids.

The role of the Eph-ephrin system in the etiology of pathological conditions has been consolidated throughout the years. In this context, approaches directed against this signaling system, intended to modulate its activity, can be strategic therapeutic opportunities. Currently, the most promising class of compounds able to interfere with the Eph receptor-ephrin protein interaction is composed of synthetic derivatives of bile acids. In the present review, we summarize the progresses achieved, in terms of chemical expansions and structure-activity relationships, both in the steroidal core and the terminal carboxylic acid group, along with the pharmacological characterization for the most promising Eph-ephrin antagonists in in vivo settings.

Structure, preparation and properties of liquid fluoroelastomers with different end groups.

In order to design and prepare liquid fluoroelastomers with different end groups, and reveal the relationship between the molecular chain structure and properties, we studied on the oxidation degradation method and functional group conversion method to prepare carboxyl-terminated and hydroxyl-terminated liquid fluoroelastomers, respectively. The reaction mechanisms were also deduced. Furthermore, the curing system was created for liquid fluoroelastomers, and systematically analyzed their properties. The sequence type and content of the –C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C– and oxygen-containing groups in the samples were measured and characterized by attenuated total reflectance/Fourier transform infrared (ATR-FTIR) spectroscopy, 1H nuclear magnetic resonance (1H-NMR), 19F-NMR spectroscopy and chemical titration, the molecular weights of liquid fluoroelastomers were measured by gel permeation chromatography (GPC). Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to examine the thermal properties, while a viscometer was used to measure the dynamic viscosity of the liquid fluoroelastomers. Then the mechanical and surface properties of the cured samples were examined by universal testing machine and contact angle measurement instrument, respectively. The results show that carboxyl-terminated liquid fluoroelastomer with 2.71 wt% carboxyl terminal groups can be prepared by oxidation degradation method. When lithium aluminium hydride (LiAlH4) was used as the reducing agent, it can efficiently convert carboxyl group to hydroxyl group with a conversion rate of more than 95%. In addition, it can be seen that the dynamic viscosity of the liquid fluoroelastomers were all decreased with the increase of temperature, and it is similar to about 10 Pa s at 70 °C. Compared with carboxyl-terminated liquid fluoroelastomers, hydroxyl-terminated liquid fluoroelastomers has higher curing reactivity, higher glass transition temperature (Tg) and thermal decomposition temperature (Td), and better mechanical properties of cured samples. The two types of liquid fluoroelastomers with distinct end groups presented distinct hydrophilicity.

Carboxylic Acid End-Capped Brushes on Titanium via Interface-Mediated RAFT Polymerization and Cell–Surface Interactions

Polymer brushes with tailored surface functionalities are important materials to manipulate the interactions between cells and surfaces in a biomedical context. To enhance the control of the conjugation of cell-adhesive peptides to the polymer brush terminus, poly(di(ethylene glycol)methyl ether methacrylate) (PDEGMA) brushes were fabricated via interface-mediated reversible addition-fragmentation chain transfer (RAFT) polymerization on titanium substrates. Due to the immobilization of 4-((((2-carboxyethyl)thio)carbonothioyl)thio)-4-cyanopentanoic acid as a chain transfer agent (CTA) on the titanium substrates before the subsequent RAFT polymerization, terminal carboxylic acid groups were introduced in the brushes. The brushes obtained were characterized by ellipsometry, static water contact angle measurements, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FT-IR) spectroscopy. PDEGMA brushes allow the facile conjugation of peptides containing the arginine-glycine-aspartic acid (GRGDS) sequence, which afforded the peptide-specific attachment of NIH 3T3 fibroblasts. By contrast, the inhibition of cell attachment was observed on PDEGMA brushes with 7 nm dry thickness conjugated with arginine-alanine-aspartic acid (GRADS). The strategy of surface functional group modification with controllable antifouling or cell-adhesive properties will allow a versatile bio-functionalization approach independent of the underlying surface condition as polymeric biomaterials, among others, in titanium-based medical implant devices.

Modified MXene-doped conductive organosilicon elastomer with high-stretchable, toughness, and self-healable for strain sensors

Polymers/nanomaterials conductive elastomers based on reversible interactions are of great significance in the application of resistive sensors because they repair themselves after mechanical injury, just like living things. The electronic devices developed with such materials will be more stable and reliable. Herein, we innovatively used ring opening polymerization (ROP) of sulfur-containing five membered rings to endow organosilicon supermolecule and MXene with hyperbranched structure which had terminal carboxyl groups. Subsequently, organosilicon conductive elastomer (PDMS@MXene) crosslinked by hydrogen bonds and metal–ligand bonds was prepared. The fully disconnected PDMS@MXene could recover 91.7% of its original mechanical properties after spontaneous repair. The well-organized composite had high stretchability and excellent tensile strength, which enhanced its application prospects in soft robots, motion monitoring and other fields. This novel design idea and simple preparation process will provide a reference for the development of self-healable conductive elastomer in the future.

Density functional theory study of adsorption and diffusion of potassium atoms on zigzag graphene nanoribbons with different terminal groups

Despite the extensive use of graphene-based materials in K-ion batteries, the effects of various edge morphologies of graphene on K atom adsorption and diffusion are unclear. In this study, the effects of K atom adsorption and diffusion on zigzag graphene nanoribbons (ZGNRs) with hydrogen (−H), ketone (=O), hydroxyl (−OH), and carboxyl (−COOH) terminal groups were investigated by density functional theory calculations. ZGNRs terminating with −H, =O and −COOH promote K atom adsorption, whereas those terminating with −OH suppress it. The −H, =O, −OH and −COOH terminations have a negligible effect on K atom diffusion in the inner region of ZGNRs. In the edge region, the diffusion barriers are nearly unchanged for −H and −OH terminations; however, they are increased for =O and −COOH terminations in the edge region compared to those in the inner region. All the terminal groups hinder K atom diffusion from the edge region toward the inner region. Our results suggest that −H termination enhances K atom adsorption and has a negligible effect on the diffusion barrier of K atom in the edge region. Therefore, the ZGNR with −H termination could be a promising candidate for K-ion batteries.

Premade Nanoparticle Films for the Synthesis of Vertically Aligned Carbon Nanotubes

Carbon nanotubes (CNTs) offer unique properties that have the potential to address multiple issues in industry and material sciences. Although many synthesis methods have been developed, it remains difficult to control CNT characteristics. Here, with the goal of achieving such control, we report a bottom-up process for CNT synthesis in which monolayers of premade aluminum oxide (Al2O3) and iron oxide (Fe3O4) nanoparticles were anchored on a flat silicon oxide (SiO2) substrate. The nanoparticle dispersion and monolayer assembly of the oleic-acid-stabilized Al2O3 nanoparticles were achieved using 11-phosphonoundecanoic acid as a bifunctional linker, with the phosphonate group binding to the SiO2 substrate and the terminal carboxylate group binding to the nanoparticles. Subsequently, an Fe3O4 monolayer was formed over the Al2O3 layer using the same approach. The assembled Al2O3 and Fe3O4 nanoparticle monolayers acted as a catalyst support and catalyst, respectively, for the growth of vertically aligned CNTs. The CNTs were successfully synthesized using a conventional atmospheric pressure-chemical vapor deposition method with acetylene as the carbon precursor. Thus, these nanoparticle films provide a facile and inexpensive approach for producing homogenous CNTs.

Dual decomposition pathways for L-aspartic acid on Ni(100)

The thermal decomposition of aspartic acid (Asp, HO2CCH(NH2)CH2CO2H) on Ni surfaces occurs by a more complex pathway than on Cu surfaces where it decomposes via an aspartate species to yield CO2, H2, and CH3C≡N stoichiometrically. In addition to these three products observed on Cu, the decomposition mechanism on Ni yields CO, H2O and HC≡N. Herein, we probe the Asp decomposition mechanism on Ni(100) by X-ray photoelectron spectroscopy and by temperature programmed reaction spectra of a variety of adsorbed aspartic acid isotopomers (L-Asp, 1–13C-L-Asp, 4–13C-L-Asp, 1,4–13C2-L-Asp, 3,4–13C2-L-Asp, 1,2,3,4–13C4–15N-L-Asp, 15N-L-Asp, and D7-L-Asp). These isotope labeling experiments provide insight into the regiospecific origin within adsorbed aspartic acid of the carbon and nitrogen atoms found in the desorbing products. At low coverage, the adsorbed aspartic acid primarily undergoes inter- and intra-molecular condensation/dehydration to ketone and/or anhydride intermediates that ultimately yield CO. At higher coverages, the decomposition process is similar to that observed on Cu surfaces yielding mostly CO2 from the terminal carboxyl groups and CH3C≡N from the interior of the adsorbed aspartic acid. The continued presence of CO and H2O products at high coverages suggests the persistence of the condensation/dehydration reactions across all coverages.

Thermodynamics of semi-specific ligand recognition: the binding of dipeptides to the E.coli dipeptide binding protein DppA

This investigation of the temperature dependence of DppA interactions with a subset of three dipeptides (AA. AF and FA) by isothermal titration calorimetry has revealed the negative heat capacity (Delta {C}_{p}^{o}) that is a characteristic of hydrophobic interactions. The observation of enthalpy–entropy compensation is interpreted in terms of the increased structuring of water molecules trapped in a hydrophobic environment, the enthalpic energy gain from which is automatically countered by the entropy decrease associated with consequent loss of water structure flexibility. Specificity for dipeptides stems from appropriate spacing of designated DppA aspartate and arginine residues for electrostatic interaction with the terminal amino and carboxyl groups of a dipeptide, after which the binding pocket closes to become completely isolated from the aqueous environment. Any differences in chemical reactivity of the dipeptide sidechains are thereby modulated by their occurrence in a hydrophobic environment where changes in the structural state of entrapped water molecules give rise to the phenomenon of enthalpy–entropy compensation. The consequent minimization of differences in the value of ΔG0 for all DppA–dipeptide interactions thus provides thermodynamic insight into the biological role of DppA as a transporter of all dipeptides across the periplasmic membrane.