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PubMed2002

Synthesis, structure, electronic and vibrational spectra of 9-(Diethylamino)-benzo(a)phenoxazin-7-ium-5-N-methacrylamide.

The electronic and vibrational spectra of 9-(Diethylamino)-benzo(a)phenoxazin-7-ium-5-N-methacrylamide (Nile Blue-5-N-methacrylamide) are measured, and the results are compared with the theoretical values obtained by quantum chemical calculations. The geometry, electronic transitions, charge distribution, and the IR normal modes of this new dye and of its precursor Nile Blue have been computed by using Density Functional Theory (DFT) method with the functional B3LYP and the 6-31G(d) Gaussian basis set. The molecular properties of the two dyes, predicted and observed, are very similar in the electronic ground state. In the excited state, however, the longer lifetime and larger fluorescence quantum yield of the Nile Blue-5-methacrylamide is ascribed to an inhibition of the twisted intramolecular charge transfer (TICT) process, when the NH2 is substituted by the methacrylamide in the 5-position of the aromatic extended ring of the dye. The change in charge density of the N atom in 5-position, as well as the difference in dipole moment and ionization potential of the two dyes molecules, explain the attenuation of TICT process. The vibration spectra of both dyes are simulated properly by using the DFT method.

Lasers↗

PubMed1986

NMR study of the molecular and electronic structure of the heme cavity of Aplysia metmyoglobin. Resonance assignments based on isotope labeling and proton nuclear Overhauser effect measurements.

The 1H NMR characteristics of the high-spin metmyoglobin from the mollusc Aplysia limacina have been investigated and compared with those of the myoglobin (Mb) from sperm whale. Aplysia metMb exhibits a normal acid----alkaline transition with pK approximately 7.8. In the acidic form, the heme methyl and meso proton resonances have been assigned by 1H NMR using samples reconstituted with selectively deuterated hemins and in the latter case by 2H NMR as well. On the basis of the methyl peak intensities and shift pattern, heme rotational disorder could be established in Aplysia Mb; approximately 20% of the protein exhibits a reversed heme orientation compared to that found in single crystals. Three meso proton resonances have been detected in the upfield region between -16 and -35 ppm, showing that the chemical shift of such protons can serve as a diagnostic probe for a pentacoordinated active site in hemoproteins, as previously shown to be the case in model compounds. The temperature dependence of the chemical shift of the meso proton signals deviates strongly from the T-1 Curie behavior, reflecting the presence of a thermally accessible Kramers doublet with significant S = 3/2 character. Nuclear Overhauser effect, NOE, measurements on Aplysia metMb have provided the assignment of individual heme alpha-propionate resonances and were used to infer spatial proximity among heme side chains. The hyperfine shift values for assigned resonances, the NOE connectivities, and the NOE magnitudes were combined to reach a qualitative picture of the rotational mobility and the orientation of the vinyl and propionate side chains of Aplysia metMb relative to sperm whale MbH2O.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1989

Proton nuclear magnetic resonance study of the molecular and electronic structure of the heme cavity in Aplysia cyanometmyoglobin.

The 1H NMR spectrum of the low-spin, cyanide-ligated ferric complex of the myoglobin from the mollusc Aplysia limacina has been investigated. All of the resolved resonances from both the hemin and the proximal histidine have been assigned by a combination of isotope labeling, spin decoupling, analysis of differential paramagnetic relaxation, and nuclear Overhauser (NOE) experiments. The pattern of the heme contact shifts is unprecedented for low-spin ferric hemoproteins in exhibiting minimal rhombic asymmetry. This low in-plane asymmetry is correlated with the X-ray-determined orientation of the proximal histidyl imidazole plane relative to the heme and provides an important test case for the interpretation of hyperfine shifts of low-spin ferric hemoproteins. The bonding of the proximal histidine is shown to be similar to that in sperm whale myoglobin and is largely unperturbed by conformational transitions down to pH approximately 4. The two observed conformational transitions appear to be linked to the titration of the two heme propionate groups, which are suggested to exist in various orientations as a function of both pH and temperature. Heme orientational disorder in the ratio 5:1 was demonstrated by both isotope labeling and NOE experiments. The exchange rate with bulk water of the proximal histidyl labile ring proton is faster in Aplysia than in sperm whale myoglobin, consistent with a greater tendency for local unfolding of the heme pocket in the former protein. A similar increased heme pocket lability in Aplysia myoglobin has been noted in the rate of heme reorientation [Bellelli, A., Foon, R., Ascoli, F., & Brunori, M. (1987) Biochem. J. 246, 787-789].

Animals↗

PubMed1979

Conformations and electronic structures of oxidized and reduced isoalloxazine.

The conformations of oxidized and reduced isoalloxazine have been examined by a molecular orbital method, PRDDO (partial retention of diatomic differential overlap). The angle theta of fold about the N...N line of the central ring is zero for the planar oxidized form, but a bend of theta = 10 degrees requires only 2 kcal/mol. On the other hand, the reduced form is nonplanar (theta approximately 15 degrees), and the barrier for reversal of this bend is 4 kcal/mol, comparable with that in simple amines. Molecular properties and reactivity are interpreted in terms of charge and orbital distributions, and localized molecular orbitals have been derived by the method of Boys.

Flavins↗

PubMedDate not supplied

Description of the electronic structure of organic chemicals using semiempirical and ab initio methods for development of toxicological QSARs.

The quality of quantitative structure-activity relationship (QSAR) models depends on the quality of their constitutive elements including the biological activity, statistical procedure applied, and the physicochemical and structural descriptors. The aim of this study was to assess the comparative use of ab initio and semiempirical quantum chemical calculations for the development of toxicological QSARs applied to a large and chemically diverse data set. A heterogeneous collection of 568 organic compounds with 96 h acute toxicity measured to the fish fathead minnow (Pimephales promelas) was utilized. A total of 162 descriptors were calculated using the semiempirical AM1 Hamiltonian, and 121 descriptors were compiled using an ab initio (B3LYP/6-31G**) method. The QSARs were derived using multiple linear regression (MLR) and partial least squares (PLS) analyses. Statistically similar models were obtained using AM1 and B3LYP calculated descriptors supported by the use of the logarithm of the octanol-water partition coefficient (log K(ow)). The main difference between the models derived by both MLR and PLS with the two sets of quantum chemical descriptors was concentrated on the type of descriptors selected. It was concluded that for large-scale predictions, irrespective of the mechanism of toxic action, the use of precise but time-consuming ab initio methods does not offer considerable advantage compared to the semiempirical calculations and could be avoided.

Animals↗

PubMed2004