Amino Acid Side Groups Research Articles

Fabrication of Hierarchical Nanostructures featuring Amplified Asymmetry through Co‐assembly of Liquid Crystalline Block Copolymer and Chiral Amphiphiles

The widespread presence of hierarchical asymmetric structures in nature has sparked considerable interest because of their unique functionalities. These ingenious structures across multiple scales often emerge from the transfer and amplification of asymmetry from chiral molecules under various synergistic effects. However, constructing artificial chiral asymmetric structures, particularly in developing hierarchical multicomponent structures analogous to those formed in nature through synergistic non‐covalent interactions, still presents tremendous challenges. Herein, we propose a co‐assembly strategy to fabricate hierarchical chiral mesostructures by combining a liquid crystalline block copolymer (LC‐BCP) with a small molecular amphiphile containing chiral alanine or phenylalanine as a linker. Through a classic solvent‐exchange process, chiral amphiphiles embedded within LC‐BCP finely regulate the LC ordering effect and facilitate transfer and amplification of asymmetry. Consequently, various co‐assembled structures with significant hierarchical chirality features are obtained through synergetic effects. Remarkably, subtle alterations to the side groups of amino acids in the amphiphiles effectively adjust the hierarchical morphology transition. Moreover, the covalent bonding sequence of amino acids in the amphiphiles emerges as a critical factor governing the formation of hierarchical nanofibers and multilayered vesicles exhibiting a superhelical sense.

Fabrication of Hierarchical Nanostructures featuring Amplified Asymmetry through Co-assembly of Liquid Crystalline Block Copolymer and Chiral Amphiphiles.

The widespread presence of hierarchical asymmetric structures in nature has sparked considerable interest because of their unique functionalities. These ingenious structures across multiple scales often emerge from the transfer and amplification of asymmetry from chiral molecules under various synergistic effects. However, constructing artificial chiral asymmetric structures, particularly in developing hierarchical multicomponent structures analogous to those formed in nature through synergistic non-covalent interactions, still presents tremendous challenges. Herein, we propose a co-assembly strategy to fabricate hierarchical chiral mesostructures by combining a liquid crystalline block copolymer (LC-BCP) with a small molecular amphiphile containing chiral alanine or phenylalanine as a linker. Through a classic solvent-exchange process, chiral amphiphiles embedded within LC-BCP finely regulate the LC ordering effect and facilitate transfer and amplification of asymmetry. Consequently, various co-assembled structures with significant hierarchical chirality features are obtained through synergetic effects. Remarkably, subtle alterations to the side groups of amino acids in the amphiphiles effectively adjust the hierarchical morphology transition. Moreover, the covalent bonding sequence of amino acids in the amphiphiles emerges as a critical factor governing the formation of hierarchical nanofibers and multilayered vesicles exhibiting a superhelical sense.

A novel two-domain laccase with a middle redox potential: physicochemical and structural properties

The gene for a previously unexplored two-domain laccase was identified in the genome of the actinobacterium Streptomyces carpinensis VKM Ac-1300. A two-domain laccase, named ScaSL, was produced in a heterologous expression system (Escherichia coli strain M15 [pREP4]). The enzyme was purified to homogeneity using affinity chromatography. ScaSL laccase, like most two-domain laccases, exhibited activity in the homotrimer form. However, unlike most two-domain laccases, it was also active in multimeric forms. The enzyme exhibited maximum activity at 80°C and was thermally stable. The half-inactivation time of ScaSL at 80°C was 40 min. Laccase oxidized the non-phenolic organic compound ABTS at a maximum rate at pH 4.7; oxidized the phenolic compound 2,6-dimethoxyphenol at a maximum rate at pH 7.5. Laccase stability was observed in the pH range 9-11. At pH 7.5, laccase was weakly inhibited by sodium azide, sodium fluoride, and sodium chloride; at pH 4.5, laccase was completely inhibited by 100 mM sodium azide. Km and kcat of the enzyme for ABTS are 0.1 mM and 20 s-1, respectively. Km and kcat for 2,6-dimethoxyphenol are 0.84 mM and 0.36 s-1, respectively. ScaSL catalyzed the polymerization of humic acids and lignin. The redox potential of laccase was 0.472 ± 0.007 V. Thus, ScaSL laccase is the first characterized two-domain laccase with a middle redox potential. The crystal structure of ScaSL was determined with a resolution of 2.35 Å. A comparative analysis of the structures of ScaSL and other two-domain laccases suggested that the middle potential of ScaSL may be associated with conformational differences in the position of the side groups of amino acids at position 230 (in ScaSL numbering), which belong to the second coordination sphere of the copper atom of the T1 center.

A Novel Two-Domain Laccase with Middle Redox Potential: Physicochemical and Structural Properties.

The gene for a previously unexplored two-domain laccase was identified in the genome of actinobacterium Streptomyces carpinensis VKM Ac-1300. The two-domain laccase, named ScaSL, was produced in a heterologous expression system (Escherichia coli strain M15 [pREP4]). The enzyme was purified to homogeneity using affinity chromatography. ScaSL laccase, like most two-domain laccases, exhibited activity in the homotrimer form. However, unlike the most two-domain laccases, it was also active in multimeric forms. The enzyme exhibited maximum activity at 80°C and was thermally stable. Half-inactivation time of ScaSL at 80°C was 40 min. The laccase was able to oxidize a non-phenolic organic compound ABTS at a maximum rate at pH 4.7, and to oxidized a phenolic compound 2,6-dimethoxyphenol at a maximum rate at pH 7.5. The laccase stability was observed in the pH range 9-11. At pH 7.5, laccase was slightly inhibited by sodium azide, sodium fluoride, and sodium chloride; at pH 4.5, the laccase was completely inhibited by 100 mM sodium azide. The determined Km and kcat of the enzyme for ABTS were 0.1 mM and 20 s-1, respectively. The Km and kcat for 2,6-dimethoxyphenol were 0.84 mM and 0.36 s-1, respectively. ScaSL catalyzed polymerization of humic acids and lignin. Redox potential of the laccase was 0.472 ± 0.007 V. Thus, the ScaSL laccase is the first characterized two-domain laccase with a middle redox potential. Crystal structure of ScaSL was determined with 2.35 Å resolution. Comparative analysis of the structures of ScaSL and other two-domain laccases suggested that the middle potential of ScaSL may be associated with conformational differences in the position of the side groups of amino acids at position 230 (in ScaSL numbering), which belong to the second coordination sphere of the copper atom of the T1 center.

Amino acid decorated xanthan gum coatings: Molecular arrangement and cell adhesion

In this work, citric acid molecules mediated simultaneously the crosslinking of xanthan gum (XG) chains and the grafting of glutamic acid (Glu), cysteine (Cys), histidine (His) or tryptophan (Trp) to XG chains, as evidenced by Fourier-transform infrared spectroscopy, elemental and thermogravimetric analyses, and high-resolution X-ray photoelectron spectroscopy. XG coatings (∼60 nm thick) presented total surface energy (γS) of 65 mJ/m². The attachment of amino acids to the XG chains decreased γS values to ∼ 48 mJ/m² because the surface energy polar (γpS) component decreased. Circular dichroism (CD) spectra revealed that the helix conformation of XG chains was retained in XG and XG-Glu-coatings, but it was lost in the XG-His, XG-Cys-and XG-Trp-coatings due to intermolecular interactions between the amino acid side groups (thiol, imidazole, and indole). XG-based coatings presented no cytotoxicity. After 3 h of incubation, the adhesion of SH-5YSY neural cells in comparison to the control (100%) was XG-His (18%), XG-Cys (21.4%), XG (29%), XG-Trp (28.6%) and XG-Glu (46.4%). The decrease of% cell adhesion tended to be favored by the decrease of γpS, except for XG-Glu-and XG-Trp, and by the loss of helix molecular conformation of chains in the coatings.

Synthesis and properties of helical polystyrene derivatives with amino acid side groups

A series of polystyrene derivatives with chiral amide groups with a controlled molecular weight and narrow molecular weight distribution were synthesized by reversible addition–fragmentation chain transfer (RAFT) radical polymerization.

Single molecular insight into steric effect on C-terminal amino acids with various hydrogen bonding sites

Amino acids are basic units to construct a protein with the assistance of various interactions. During this building process, steric hindrance derived from amino acid side groups or side chains is a factor that could not be ignored. In this contribution, adsorption behaviors of C-terminal amino acid derivatives with amino acid residues fused in 3,4,9,10-perylenetetracarboxylic dianhydride were investigated by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations at various liquid/solid interfaces. STM results at 1-phenyloctane/HOPG interface show that N,N'-3,4,9,10-perylenedicarboximide (GP) and N,N'-methyl-3,4,9,10-perylenedicarboximide (AP) formed linear and herringbone structures, respectively. The driving force could be attributed to different H-bonding sites induced by steric hindrance at side groups. N,N'-Benzyl-3,4,9,10-perylenedicarboximide (PP) generates both linear and herringbone structures because steric hindrance changes the H-bonding sites between PP molecules, whereas N,N'-isopropyl-3,4,9,10-perylenedicarboximide (LP) failed to be imaged because of strong steric hindrance coming from larger side group. To further investigate the impact of steric hindrance, we utilized octanoic acid (OA) as solvent to capture the adsorption details of LP and PP. We found that OA molecules drag PP and LP molecules in a different direction to generate linear structure, impeding the molecular rotation. The structure–solvent relationship shows that the steric hindrance is brought by the large side group, which makes it easier to recognize OA molecules at the interface. These results demonstrate that steric effect plays a significant role in altering interaction sites of the compounds during the adsorption process at the liquid/solid interface.

Controlled synthesis of dicalcium phosphate dihydrate (DCPD) from metastable solutions: insights into pathogenic calcification

Calcium phosphate (CaP) compounds may occur in the body as abnormal pathogenic phases in addition to their normal occurrence as bones and teeth. Dicalcium phosphate dihydrate (DCPD; CaPO4·2H2O), along with other significant CaP phases, have been observed in pathogenic calcifications such as dental calculi, kidney stones and urinary stones. While other studies have shown that polar amino acids can inhibit the growth of CaPs, these studies have mainly focused on hydroxyapatite (HAp; Ca10(PO4)6(OH)2) formation from highly supersaturated solutions, while their effects on DCPD nucleation and growth from metastable solutions have been less thoroughly explored. By further elucidating the mechanisms of DCPD formation and the influence of amino acids on those mechanisms, insights may be gained into ways that amino acids could be used in treatment and prevention of unwanted calcifications. The current study involved seeded growth of DCPD from metastable solutions at constant pH in the presence of neutral, acidic and phosphorylated amino acid side chains. As a comparison, solutions were also seeded with calcium pyrophosphate (CPP; Ca2P2O7), a known calcium phosphate inhibitor. The results show that polar amino acids inhibit DCPD growth; this likely occurs due to electrostatic interactions between amino acid side groups and charged DCPD surfaces. Phosphoserine had the greatest inhibitory ability of the amino acids tested, with an effect equal to that of CPP. Clustering of DCPD crystals giving rise to a “chrysanthemum-like” morphology was noted with glutamic acid. This study concludes that molecules containing an increased number of polar side groups will enhance the inhibition of DCPD seeded growth from metastable solutions.

Photoprintable Gelatin-graft-Poly(trimethylene carbonate) by Stereolithography for Tissue Engineering Applications.

The stereolithography process is a powerful additive manufacturing technology to fabricate scaffolds for regenerative medicine. Nevertheless, the quest for versatile inks allowing one to produce scaffolds with controlled properties is still unsatisfied. In this original article, we tackle this bottleneck by synthesizing a panel of photoprocessable hybrid copolymers composed of gelatin-graft-poly(trimethylene carbonate)s (Gel-g-PTMCn). We demonstrated that by changing the length of PTMC blocks grafted from gelatin, it is possible to tailor the final properties of the photofabricated objects. We reported here on the synthesis of Gel-g-PTMCn with various lengths of PTMC blocks grafted from gelatin using hydroxy and amino side groups of the constitutive amino acids. Then, the characterization of the resulting hybrid copolymers was fully investigated by quantitative NMR spectroscopy before rendering them photosensitive by methacrylation of the PTMC terminal groups. Homogeneous composition of the photocrosslinked hybrid polymers was demonstrated by EDX spectroscopy and electronic microscopy. To unravel the individual contribution of the PTMC moiety on the hybrid copolymer behavior, water absorption, contact angle measurements, and degradation studies were undertaken. Interestingly, the photocrosslinked materials immersed in water were examined using tensile experiments and displayed a large panel of behavior from hydrogel to elastomer-like depending on the PTMC/gel ratio. Moreover, the absence of cytotoxicity was conducted following the ISO 10993 assay. As a proof of concept, 3D porous objects were successfully fabricated using stereolithography. Those results validate the great potential of this panel of inks for tissue engineering and regenerative medicine.

The chemistry of Amino Acid and Peptides via Solution-Phase-Peptide Synthesis

In this review, we have presented the different chemical structures of proteinogenic amino acids. Also, we performed a precise reference scan of the proton balance of amino acids. It is worth noting that, the two most important parts in this review are the principles of chemical peptide synthesis in solution and the protection of the amino acid side group, so we focused the reference survey on them, and we reached many old and new references in these two important parts in the field of peptide synthesis. On the other hand, we also performed an important reference survey of peptide-coupling methodologies. Finally, in this review we presented an important part, the use of peptide coupling reagents, in this part; we focused on the most important and most popular methods used in peptide chemistry reactions.

Hydrogen bonding and protonation effects in amino acids' anthraquinone derivatives - Spectroscopic and electrochemical studies.

Six novel amino acid chromophores were synthesized and their spectroscopic, acid-base, and electrochemical properties are discussed in this work. In studied compounds, selected amino acid residues (l-Aspartic acid, l-Glutamic acid, l-Glutamine, l-Histidine, l-Lysine, l-Arginine) are attached to the 1-(piperazine) 9,10-anthraquinone skeleton via the amide bond between the carboxyl group of amino acid and nitrogen atom of the piperazine ring. All derivatives have been characterized using a variety of spectroscopic techniques (mass spectrometry, 1HNMR, UV-Vis, IR spectroscopy), acid-base (electrochemical and UV-Vis) titrations, and cyclic voltammetry methods. Basing on observed experimental effects, supported by quantum chemical simulations, the structure-properties links were established. They are indicative of the specific interactions within and/or in-between amino acid side groups, which are prone to form both, intra- and intermolecular hydrogen bonds as well as electrostatic interactions with the anthraquinone system.

Interactions between acid and proteins under in vitro gastric condition - a theoretical and experimental quantification.

The gastric digestion of proteins is influenced by the pH and the gastric pH fluctuates after food consumption. However, the dynamics of gastric pH still need to be quantitatively understood. Proteins in food strongly influences the gastric pH. Therefore, we studied the interaction between acid and proteins, including the buffer reaction and the acid diffusion in protein gels. The buffer capacity of proteins stems from its content of ionizable amino acid side groups. Based on this, we set up a model and method to parameterize the buffer capacity of proteins. Moreover, the liberated carboxyl and amino groups during enzymatic hydrolysis of protein can also contribute to the buffer capacity. While we expected protons to diffuse faster than pepsin, we found that the penetration distance of acid is comparable to that of pepsin. The buffer reaction caused the acid to concentrate tenfold in the gel compared to the bulk acid concentration. Therefore, we postulated that the buffer reaction reduces acid diffusivity in gels.

Controlling Self-Assembling Peptide Hydrogel Properties through Network Topology

Self-assembling peptide-based hydrogels have encountered increasing interest in the recent years as scaffolds for 3D cell culture or for controlled drug delivery. One of the main challenges is the fine control of the mechanical properties of these materials. The bulk properties of hydrogels not only depend on the intrinsic properties of the fibers but also on the network topology formed. In this work we show how fiber-fiber interactions can be manipulated by design to control the final hydrogel network topology and therefore control the final properties of the material. This was achieved by exploiting the design features of β-sheet forming peptides based on hydrophobic and hydrophilic residue alternation and exploiting the ability of the arginine's guanidine side group to interact with itself and with other amino acid side groups. By designing octa-peptides based on phenylalanine, glutamic acid, lysine, and arginine, we have investigated how fiber association and bundling affect the dynamic shear modulus of hydrogels and how it can be controlled by design. This work opens the possibility to fine-tune by design the bulk properties of peptide hydrogels.

One-Pot Synthesis of Amino Acid-Based Polyelectrolytes and Nanoparticle Synthesis.

In this manuscript, a biscyclic monomer with an epoxide and a thiolactone ring connected by a urethane bond is used for the synthesis of amino acid-functional polyelectrolytes. In a first step, lithium salts of amino acids react selectively with the thiolactone ring by ring-opening, formation of an amide bond, and a thiol group. In a second step and in the presence of a base a polymeric building block is formed by polyaddition of the thiolate to the epoxide ring. The reaction occurs at room temperature in water as solvent. The resulting polymeric building block has a poly(thioether urethane) backbone, with hydroxyl- and amino acid side groups; the connection of the amino acid to the backbone occurs by an amide bond. As proof of concept, a selected series of amino acids and derivatives of such is used: glycine, alanine, tyrosine, glutamic acid, ε-amino caproic acid (as a lysine surrogate), BOC-lysine-O-methyl ester, BOC-lysine, and the dipeptide carnosine. The resulting polymer building blocks with molecular weights of Mn = 1830-9590 g/mol are entirely based on both bio-based and biodegradable components. Exemplarily, using the lithium salts of glycine and lysine methyl ester, anionic and cationic polyelectrolyte building blocks are obtained. A mixture of the two polyelectrolyte solutions results in the formation of polyelectrolyte complexes (PECs). With decreasing concentration of the polyelectrolyte solutions, the radii of PECs decrease.

Single-crystal Au microflakes modulated by amino acids and their sensing and catalytic properties

Single-crystal Au microflakes with the planar area over 103μm2 (i.e. being accessible to the human eye resolution) were synthesized in an environment-friendly route by directing two-dimensional growth of Au nanocrystals into macroscopic scales with amino acids as both reducing agents and capping agents. Side groups of amino acids were found to be a determinant parameter to tune the dimension and size of Au single crystals. The successful synthesis of Au microflakes provides an unprecedented opportunity to bridge nanotechnology and macroscopic devices, and hereby to start a new scenario of exploring their unique properties and applications in optoelectronic devices and bio-sensing fields across multiple length scales. For example, Au microflakes respond to air humidity upon depositing on films of chitin nanofibrils, and sense various physiological molecules as electrode materials of biosensors.

Insights into Solid-State Electron Transport through Proteins from Inelastic Tunneling Spectroscopy: The Case of Azurin.

Surprisingly efficient solid-state electron transport has recently been demonstrated through "dry" proteins (with only structural, tightly bound H2O left), suggesting proteins as promising candidates for molecular (bio)electronics. Using inelastic electron tunneling spectroscopy (IETS), we explored electron-phonon interaction in metal/protein/metal junctions, to help understand solid-state electronic transport across the redox protein azurin. To that end an oriented azurin monolayer on Au is contacted by soft Au electrodes. Characteristic vibrational modes of amide and amino acid side groups as well as of the azurin-electrode contact were observed, revealing the azurin native conformation in the junction and the critical role of side groups in the charge transport. The lack of abrupt changes in the conductance and the line shape of IETS point to far off-resonance tunneling as the dominant transport mechanism across azurin, in line with previously reported (and herein confirmed) azurin junctions. The inelastic current and hence electron-phonon interaction appear to be rather weak and comparable in magnitude with the inelastic fraction of tunneling current via alkyl chains, which may reflect the known structural rigidity of azurin.

Modeling the Histidine-Phenylalanine Interaction: The NH···π Hydrogen Bond of Imidazole·Benzene.

NH···π hydrogen bonds occur frequently between the amino acid side groups in proteins and peptides. Data-mining studies of protein crystals find that ∼80% of the T-shaped histidine···aromatic contacts are CH···π, and only ∼20% are NH···π interactions. We investigated the infrared (IR) and ultraviolet (UV) spectra of the supersonic-jet-cooled imidazole·benzene (Im·Bz) complex as a model for the NH···π interaction between histidine and phenylalanine. Ground- and excited-state dispersion-corrected density functional calculations and correlated methods (SCS-MP2 and SCS-CC2) predict that Im·Bz has a Cs-symmetric T-shaped minimum-energy structure with an NH···π hydrogen bond to the Bz ring; the NH bond is tilted 12° away from the Bz C6 axis. IR depletion spectra support the T-shaped geometry: The NH stretch vibrational fundamental is red shifted by -73 cm(-1) relative to that of bare imidazole at 3518 cm(-1), indicating a moderately strong NH···π interaction. While the S0(A1g) → S1(B2u) origin of benzene at 38 086 cm(–1) is forbidden in the gas phase, Im·Bz exhibits a moderately intense S0 → S1 origin, which appears via the D(6h) → Cs symmetry lowering of Bz by its interaction with imidazole. The NH···π ground-state hydrogen bond is strong, De=22.7 kJ/mol (1899 cm–1). The combination of gas-phase UV and IR spectra confirms the theoretical predictions that the optimum Im·Bz geometry is T shaped and NH···π hydrogen bonded. We find no experimental evidence for a CH···π hydrogen-bonded ground-state isomer of Im·Bz. The optimum NH···π geometry of the Im·Bz complex is very different from the majority of the histidine·aromatic contact geometries found in protein database analyses, implying that the CH···π contacts observed in these searches do not arise from favorable binding interactions but merely from protein side-chain folding and crystal-packing constraints. The UV and IR spectra of the imidazole·(benzene)2 cluster are observed via fragmentation into the Im·Bz+ mass channel. The spectra of Im·Bz and Im·Bz2 are cleanly separable by IR hole burning. The UV spectrum of Im·Bz2 exhibits two 000 bands corresponding to the S0 → S1 excitations of the two inequivalent benzenes, which are symmetrically shifted by -86/+88 cm(-1) relative to the 000 band of benzene

Analysis of the Contribution of Chromophores in Side Groups of Amino Acids to the Absorption Spectrum of Hemoglobin

Based on spectral analysis of solutions of aromatic, heterocyclic, and sulfur-containing amino acids, we propose an additive model and assess the roles of the studied types of amino acid residues in formation of the overall absorption spectrum of hemoglobin. We have established that the identifi ed absorption maxima (transitions) at 243.4, 248.4, 253.2, 258.8, 261.6, 264.8, and 268.4 nm belong to phenylalanine amino acid residues. Probably the latter also form the unassigned transition at 241.0 nm. The transitions at 272.8, 274.6, 280.0, and 284.4 nm are a superposition of the absorption by the side groups of tyrosine and tryptophan; the transition at 278.2 nm is associated with tyrosine, masked by adjacent transitions of tryptophan, and the transition at 291.2 nm belongs to tryptophan. We consider the possibility of estimating the changes in the spectral properties of proteins under the infl uence of various physical and chemical factors using data from additive spectra.

Comparison of the Synthesis and Bioerodible Properties of N-Linked Versus O-Linked Amino Acid Substituted Polyphosphazenes

Phosphazene polymers with N-linked and O-linked amino acid side groups are of biomedical interest, especially for their ability to bioerode under physiological conditions. Polyphosphazenes containing serine and threonine substituents, which contain two different functional sites for attachment to a polyphosphazene backbone (N- and O-terminus), were synthesized using an improved technique, and their hydrolysis behavior was investigated. In aqueous media the solid polymers yield hydrolysis products phosphate, ammonia, the amino acid, and ethanol and have the potential to be used in several different biomedical applications ultimately determined by their hydrolysis behavior. A hydrolysis study in deionized water revealed that all the polymers are hydrolytically sensitive, regardless of the type of linkage to the polyphosphazene backbone, although the hydrolysis rates may be different. Polymers with amino acid ester side groups linked through the N-terminus underwent solid phase hydrolysis between 16 and 60 % within a 6-week period. This is the fastest reported solid state hydrolysis of any amino acid ester substituted polyphosphazene. The mechanism of hydrolysis is by bulk erosion as monitored by environmental scanning electron microscopy. Polymers with the amino acid units linked through the O-terminus are soluble in water; thus their solid state erosion profile in aqueous media could not be determined. However, 31P NMR spectroscopy confirmed their hydrolytic sensitivity in aqueous solution and the formation of phosphorus-containing oligomeric species, the concentrations of which increased during the 6-week hydrolysis period. Complete hydrolysis did not occur within 6 weeks. The O-linked species are possible starting points for bioerodible hydrogel formation.

The influence of hydrophobic amino acid side groups on the acidity of the aromatic imidazole ring of histidine: A theoretical study

In this study we have calculated the acidity constant (pKa) of imidazole ring in Histidine-Hydrophobic amino acid dipeptides using the quantum chemistry and continuum solvation methods. Density functional theory calculations with the large basis sets are used to determine the Gibbs free energy of deprotonate in the gas and liquid phases. Based on our results ΔGS values are located between −69.38 and −18.82 kcal mol−1 which are related to His+–Gly and His forms, respectively. pKa of the dipeptides in the aqueous phase was obtained from the calculated gas-phase and solvation free energies through a thermodynamic cycle and the solvation model chemistry of Martin Karplus et al. Solvation effects are treated using a self-consistent reaction field formalism involving polarized continuum models. According to our calculations pKa values are between 5.50 and 8.19 that are belong to His+–ILe and His+–Ala forms, respectively. Natural bond orbital analysis of dipeptides reveals that the electron delocalization in imidazole ring is the most effective factor in determination of acidity order for these compounds. Structural analysis confirmed that the orientation of carbonyl group with respect to imidazole ring is an effective factor in imidazole ring stability. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011