Heteronuclear and homonuclear direct (D) and indirect (J) spin-spin interactions are important sources of structural information about nucleic acids (NAs). The Hamiltonians for the D and J interactions have the same functional form; thus, the experimentally measured apparent spin-spin coupling constant corresponds to a sum of J and D. In biomolecular NMR studies, it is commonly presumed that the dipolar contributions to Js are effectively canceled due to random molecular tumbling. However, in strong magnetic fields, such as those employed for NMR analysis, the tumbling of NA fragments is anisotropic because the inherent magnetic susceptibility of NAs causes an interaction with the external magnetic field. This motional anisotropy is responsible for non-zero D contributions to Js. Here, we calculated the field-induced D contributions to 33 structurally relevant scalar coupling constants as a function of magnetic field strength, temperature and NA fragment size. We identified two classes of Js, namely (1)JCH and (3)JHH couplings, whose quantitative interpretation is notably biased by NA motional anisotropy. For these couplings, the magnetic field-induced dipolar contributions were found to exceed the typical experimental error in J-coupling determinations by a factor of two or more and to produce considerable over- or under-estimations of the J coupling-related torsion angles, especially at magnetic field strengths >12 T and for NA fragments longer than 12 bp. We show that if the non-zero D contributions to J are not properly accounted for, they might cause structural artifacts/bias in NA studies that use solution NMR spectroscopy.

• MeSH
• kvantová teorie MeSH
• magnetické pole * MeSH
• nukleární magnetická rezonance biomolekulární metody   MeSH
• nukleové kyseliny chemie   MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH

The l-alanyl-l-alanine (AA) molecule behaves differently in acidic, neutral, and basic environments. Because of its molecular flexibility and strong interaction with the aqueous environment, its behavior has to be deduced from the NMR spectra indirectly, using statistical methods and comparison with ab initio predictions of geometric and spectral parameters. In this study, chemical shifts and indirect spin-spin coupling constants of the AA cation, anion, and zwitterion were measured and compared to values obtained by density functional computations for various conformers of the dipeptide. The accuracy and sensitivity of the quantum methods to the molecular charge was also tested on the (mono)-alanine molecule. Probable AA conformers could be identified at two-dimensional potential energy surfaces and verified by the comparison of the computed parameters with measured NMR data. The results indicate that, whereas the main-chain peptide conformations of the cationic (AA+) and zwitterionic (AAZW) forms are similar, the anion (AA-) adopts also another, approximately equally populated conformer in the aqueous solution. Additionally, the NH2 group can rotate in the two main chain conformations of the anionic form AA-. According to a vibrational quantum analysis of the two-dimensional energy surfaces, higher-energy conformers might exist for all three charged AA forms but cannot be detected directly by NMR spectroscopy because of their small populations and short lifetimes. In accord with previous studies, the NMR parameters, particularly the indirect nuclear spin-spin coupling constants, often provided an excellent probe of a local conformation. Generalization to peptides and proteins, however, has to take into account the environment, molecular charge, and flexibility of the peptide chain.

• MeSH
• dipeptidy chemie   MeSH
• elektrochemie MeSH
• financování organizované MeSH
• koncentrace vodíkových iontů MeSH
• konformace proteinů MeSH
• magnetická rezonanční spektroskopie MeSH
• molekulární modely MeSH
• rozpouštědla MeSH

Cytotoxic properties of radiosensitizers are due to the fact that, in the metabolic pathway, these compounds undergo one-electron reduction to generate radical anions. In this study we focused our interest on the electrochemical transfer of the first electron on radiosensitizer Etanidazole (ETN) and, consequently, on the ETN radical-anion formation in the buffered aqueous media. ETN was electrochemically treated in the broad pH range at various scan rates. Three reduction peaks and one oxidation peak were found. At strong alkaline pH the four-electron reduction peak was separated into one-electron and three-electron reductions. Under these conditions the standard rate constant k(0) for the redox couple ETN-NO(2)+e(-) <--> ETN-NO(2)(*-) was calculated. Moreover, the value of a so called E(7)(1) potential that accounts for the energy necessary to transfer the first electron to an electroactive group at pH=7 in aqueous medium to form a radical anion was also determined. The obtained value of E(7)(1) indicates that lower energy compared to the other possible chemical radiosensitizers is necessary for the system to transfer the first electron to ETN. On the other hand, the necessity of the strong alkaline pH may decrease the ability of ETN to act as hypoxic radiosensitizer in the human body.

• MeSH
• anionty chemie   MeSH
• elektrická vodivost MeSH
• elektrochemie MeSH
• etanidazol chemie   MeSH
• hypoxie buňky MeSH
• kinetika MeSH
• koncentrace vodíkových iontů MeSH
• radiosenzibilizující látky chemie   MeSH
• transport elektronů MeSH
• voda chemie   MeSH
• volné radikály chemie   MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH

The stability of α-bromophenylacetic acid (BPAA) in 50% aqueous methanol solution has been tested. CE in different running buffers was used to separate BPAA from the decomposition reaction products α-hydroxyphenylacetic (mandelic) acid and α-methoxyphenylacetic acid. Suitable CE separation of all three compounds and other product, bromide, was achieved in 60 mmol/L formate buffer (pH 3.0) at -30 kV in 50 μm (i.d.) poly(vinyl alcohol)-coated fused silica capillary (30 cm/24.5 cm) with UV detection at 200 nm. The CE method was applied to determine the reaction order of the decomposition of BPAA (0.47 mmol/L) via nucleophilic substitution in 50% aqueous methanol. The first-order reaction kinetics was confirmed by linear and non-linear regression, giving the rate constants 1.52 × 10-4 ± 2.76 × 10-5 s-1 and 7.89 × 10-5 ± 5.02 × 10-6 s-1, respectively. Additionally, the degradation products were identified by CE coupled to mass spectrometric (MS) detection. The CE-MS experiments carried out in 60 mmol/L formate buffer (pH 3.0) and in 60 mmol/L acetate buffer (pH 5.0) confirmed the results obtained by CE-UV. Furthermore, the stability of BPAA in polar solvents was tested by 1H NMR experiments. Our results provide strong evidence of the instability and fast degradation of BPAA in 50% aqueous methanol indicating that BPAA is not suitable as the model analyte for chiral separations.

• MeSH
• chemické modely * MeSH
• elektroforéza kapilární metody   normy   MeSH
• fenylacetáty chemie   izolace a purifikace   MeSH
• hmotnostní spektrometrie metody   MeSH
• reprodukovatelnost výsledků MeSH
• stabilita léku MeSH
• stereoizomerie MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH

• MeSH
• imunita MeSH
• imunitní systém MeSH
• imunoterapie MeSH
• Publikační typ
• monografie MeSH

• Konspekt
• Patologie. Klinická medicína
• NLK Obory
• alergologie a imunologie
• biologie

• MeSH
• imunita MeSH
• imunitní systém fyziologie   patofyziologie   MeSH
• imunoterapie MeSH

• Konspekt
• Patologie. Klinická medicína
• NLK Obory
• alergologie a imunologie
• biologie
• fyziologie
• NLK Publikační typ
• kolektivní monografie