Vibrational signatures of dynamic excess proton storage between primary amine and carboxylic acid groups

Ammonium and carboxylic moieties play a central role in proton-mediated processes of molecular recognition, charge transfer or chemical change in (bio)materials. Whereas both chemical groups constitute acid-base pairs in organic salt-bridge structures, they may as well host excess protons in acidic environments. The binding of excess protons often precedes proton transfer reactions and it is therefore of fundamental interest, though challenging from a quantum chemical perspective. As a benchmark for this process, we investigate proton storage in the amphoteric compound 5-aminovaleric acid (AV), within an intramolecular proton bond shared by its primary amine and carboxylic acid terminal groups. Infrared ion spectroscopy is combined with ab initio Molecular Dynamics (AIMD) calculations to expose and rationalize the spectral signatures of protonated AV and its deuterated isotopologues. The dynamic character of the proton bond confers a fluxional structure to the molecular framework, leading to wide-ranging bands in the vibrational spectrum. These features are reproduced with remarkable accuracy by AIMD computations, which serves to lay out microscopic insights into the excess proton binding scenario.

Chemical Characterization of the Precipitate Found in and Its Effect on Drug Release of the Scutellaria baicalensis-Coptis chinensis Extract

Precipitate generation is a challenging issue during the production of herbal decoction as it affects the stability and bioavailability of active compounds. Here we explored the composition of the natural precipitate formed from and its effect on drug release of Scutellaria baicalensis-Coptis chinensis paired extract (SCPE). Furthermore, the surface morphology of the SCPE precipitate was also investigated. Ultra-performance liquid chromatography-mass spectrometry (UPLC-MS) was used to chemical component analysis and field emission scanning electron microscope (FE-SEM) was performed to particle observation. Baicalin (BA), berberine (BBR) and starch-arginine-rich polymers were abundant in the SCPE precipitate. FE-SEM micrographs showed spheroidal shaped particles in the SCPE supernatant, while spherical and porous tissue-shaped particles in the SCPE precipitate. In vitro drug release of baicalin and berberine contained in the precipitate may increase as the polymer is removed. The presence of polymer-related interactions were confirmed by the greater increase in solubility of baicalin upon addition of arginine and polymer. This was also supported by the solubility decrease of the BA-BBR complex in polymer solution and the gelation of the BA-BBR complex in arginine solution. Our results provide a scientific basis for elucidating the pharmaceutical properties of the decoction of S. baicalensis-C. chinensis-based herbal medicine.

Insights into the binding of arginine to adenosine phosphate from mimetic complexes

The recognition of adenosine monophosphate (AMP) by arginine-rich proteins is conditioned by the competitive binding of the guanidinium side group with metal cations, as derived from vibrational spectroscopy and modelling of mimetic complexes.

Active sites of peptide from Arg-Ser-Ser protect against oxidative stress in HepG2 cells

Peptide Arg-Ser-Ser (RSS) was derived from Lactobacillus amylolyticus co-incubated with edible <i>Dendrobium aphyllum</i>. Here, we further examined the antioxidative effects of RSS in HepG2 cells subjected to 2,2-azobis(2-methylpropanimidamidine) dihydrochloride-induced oxidative stress. RSS protected cells by eliminating the level of reactive oxygen species (ROS). The protein expression of antioxidant enzymes, Nrf2 and Keap1 determined by western blot, indicated that RSS might maintain cellular homeostasis by directly scavenging free radicals instead of by enzymatic system. Furthermore, quantum chemistry calculations and a characterization of electronic-related properties showed that the highest occupied molecular orbital energy distribution was on arginine residue. Pre-treatment with RSS with the active site methylated resulted in increased ROS levels, thereby verifying that N₂-H₃ is the active site for antioxidant activity. Our findings provide valuable insights into the antioxidant activity of RSS and a basis for developing antioxidant functional foods.

Alkylammonium Cation Affinities of Nitrogenated Organobases: The Roles of Hydrogen Bonding and Proton Transfer

Alkylammonium cation affinities of 64 nitrogen-containing organobases, as well as the respective proton transfer processes from the alkylammonium cations to the base, have been computed in the gas phase by using DFT methods. The guanidine bases show the highest proton transfer values (191.9-233 kJ mol^-1 ) whereas the cis-2,2'-biimidazole presents the largest affinity towards the alkylammonium cations (>200 kJ mol^-1 ) values. The resulting data have been compared with the experimentally reported proton affinities of the studied nitrogen-containing organobases revealing that the propensity of an organobase for the proton transfer process increases linearly with its proton affinity. This work can provide a tool for designing senors for bioactive compounds containing amino groups that are protonated at physiological pH.

Strong intramolecular hydrogen bonding in confined amino acids

Intramolecular hydrogen bonding is evaluated in three different amino acids encapsulated in C₆₀ fullerene in the context of electron density analysis. While conventional intramolecular hydrogen bonding in isolated amino acids are dominated by electrostatic character, it is shown that strong intramolecular hydrogen bonding can be formed in confined amino acids so that in two cases covalent intramolecular hydrogen bonding is appeared in the confined species. Also, results show that zwitterionic amino acids are stable in confined state, where no implicit or explicit solvation is applied. Covalent character for intramolecular hydrogen bonding in amino acids have not yet been reported.

Binding Motifs in the Naked Complexes of Target Amino Acids with an Excerpt of Antitumor Active Biomolecule: An Ion Vibrational Spectroscopy Assay

The structures of proton-bound complexes of 5,7-dimethoxy-4H-chromen-4-one (1) and basic amino acids (AAs), namely, histidine (His) and lysine (Lys), have been examined by means of mass spectrometry coupled with IR ion spectroscopy and quantum chemical calculations. This selection of systems is based on the fact that 1 represents a portion of glabrescione B, a natural small molecule of promising antitumor activity, while His and Lys are protein residues lining the cavity of the alleged receptor binding site. These species are thus a model of the bioactive adduct, although clearly the isolated state of the present study bears little resemblance to the complex biological environment. A common feature of [1+AA+H]⁺ complexes is the presence of a protonated AA bound to neutral 1, in spite of the fact that the gas-phase basicity of 1 is comparable to those of Lys and His. The carbonyl group of 1 acts as a powerful hydrogen-bond acceptor. Within [1+AA+H]⁺ the side-chain substituents (imidazole group for His and terminal amino group for Lys) present comparable basic properties to those of the α-amino group, taking part to a cooperative hydrogen-bond network. Structural assignment, relying on the comparative analysis of the infrared multiple photon dissociation (IRMPD) spectrum and calculated IR spectra for the candidate geometries, derives from an examination over two frequency ranges: 900-1800 and 2900-3700 cm^-1 . Information gained from the latter one proved especially valuable, for example, pointing to the contribution of species characterized by an unperturbed carboxylic OH or imidazole NH stretching mode.

Insights into the Recognition of Phosphate Groups by Peptidic Arginine from Action Spectroscopy and Quantum Chemical Computations

The side group of the amino acid arginine is typically in its guanidinium protonated form under physiological conditions and participates in a broad range of ligand binding and charge transfer processes of proteins. The recognition of phosphate moieties by guanidinium plays a particularly key role in the interactions of proteins with ATP and nucleic acids. Moreover, it has been recently identified as the driving force for the inhibition of kinase phosphorilation activity by guanidinium derivatives devised as potential anticancer agents. We report on a fundamental investigation of the interactions and coordination arrangements formed by guanidinium with phosphoric, phosphate, and pyrophosphate groups. Action vibrational spectroscopy and ab initio quantum chemical computations are employed to characterize the conformations of benchmark positively charged complexes isolated in an ion trap. The multidentate structure of guanidinium and of the phosphate groups gives rise to a rich conformational landscape with a particular relevance of tweezer-like configurations, where phosphate is effectively trapped by two guanidinium cations. The pyrophosphate complex incorporates a Na⁺ cation, which serves to compare the interactions associated with the localized versus diffuse charge distributions of the alkali cation and guanidinium, respectively, within a common supramolecular framework.

Complexes of Crown Ether Macrocycles with Methyl Guanidinium: Insights into the Capture of Charge in Peptides

Crown ethers are well known as modulating agents of protein function and interactions. The action of crown ethers is driven by an alteration of the charged moieties of proteins through the capping of cationic amino acid side chains. This study evaluates the conformational features involved in the binding of crown ethers to the side chain of arginine. For this purpose, isolated complexes of methyl guanidinium with 12-crown-4 and 18-crown-6 are characterized with infrared action vibrational spectroscopy and quantum chemical computations. The conformational landscapes of the two complexes comprise an extensive ensemble of conformations close in energy. In the 12-crown-4 complex, the crown ether has the plane of its backbone approximately perpendicular to that of the guanidinium moiety and coordinates to two or three of its NH^δ+ bonds. In the 18-crown-6 complex, the crown ether backbone is partially folded and tilted with respect to guanidinium and fixes its position in order to facilitate up to a four-fold coordination in the complex. The access of the complexes to multiple conformations leads to broad band structures in the N-H stretching region of their vibrational spectra.

Antitumor effects of melanin from Lachnum YM226 and its derivative in H22 tumor-bearing mice

In the present study, we investigated the anti-tumor activities of the intracellular homogeneous melanin (LM) of Lachnum YM226 and its derivative (ALM) on liver cancer using murine H22 hepatocarcinoma model. The results showed that LM and ALM (50 and 200 mg kg-1) could effectively inhibit tumor growth of H22 tumour-bearing mice. The body weight, liver, spleen and thymus indices also improved in the LM and ALM treated groups. Moreover, the levels of alanine aminotransferase (ALT), aspartate aminotransaminase (AST), alkaline phosphatase (ALP), creatinine (CRE), blood urea nitrogen (BUN) and uric acid (UA) were lowered. Serum cytokines of interleukin-2 (IL-2), interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ) were increased on LM and ALM administration, while LM and ALM significantly decreased the vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) levels. The H&E staining indicated that LM and ALM exhibited antitumor activity in vivo by promoting apoptosis and inhibiting angiogenesis. The anti-tumor effect of ALM was more significant than that of LM for the same dose. In summary, the findings demonstrated that LM and ALM might be promising candidates for the prevention and treatment of HCC.