On the formation of radical dications of protonated amino acids in a "microsolution" of water or acetonitrile and their reactivity towards the solvent

In high-energy collisions (50 keV) between O2 and protonated amino acids AH+, radical dications AH2+* are formed for A = Phe, His, Met, Tyr, and Trp. When solvated by water or acetonitrile (S), AH2+*(S)1,2 are formed for A = Arg, His, Met, Tyr, and Trp. The stability of the hydrogen-deficient AH2+* in the "microsolution" depends on the energetics of the electron transfer reaction AH2+* +S --> AH++S+*, the hydrogen abstraction reaction AH2+*+S --> AH2(2+)+[S-H]*, and the proton transfer reaction AH2+* + S --> A+*+SH+. Using B3LYP/ 6-311+G(2d,p)//B3LYP/6-31+G(d) model chemistry, we describe these three reactions in detail for A=Tyr and find that the first two reactions are unfavorable whereas the third one is favorable. However, energy is required for the formation of Tyr+* and SH+ from TyrH2+*(S) to overcome the Coulomb barrier, which renders the complex observable with a life-time larger than 5 micros. The ionization energy, IE, of TyrH+ is calculated to be 11.1 eV in agreement with an experimental measurement of 10.1+/-2.1 eV ([IE(CH3CN)+IE(Tyr)]/ 2); hydration further lowers the IE by 0.3 eV [IE(TyrH+(H2O) = 10.8 eV, calculated]. We estimate the ionization energies of TrpH+, HisH+, and MetH+ to be 10.1+/-2.1 eV, 12.4+/-0.2 eV, and 12.4+/-0.2 eV, and that of PheH+ to be larger than 12.6 eV.     

SCFMO calculations of heteroatomic systems with the variableβ approximation

The SCFMO method in the variable approximation has been extended to heteroatomic systems. The transition energies and bond lengths of a group of nitrogen- and oxygen-containing molecules have been calculated and the singlet transition energies are in good agreement with experiment.

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Zusammenfassung Die SCF MO Methode in der Näherung der variablen wurde auf Systeme mit Heteroatomen ausgedehnt. Die Übergangsenergien und Bindungslängen einer Gruppe von Molekülen mit Stickstoff- und Sauerstoffatomen wurden berechnet. Die Singulettübergaugsenergien befinden sich in guter Übereinstimmung mit dem Experiment.

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Résumé Nous avons étendu la méthode SCF MO dans l'approximation des variables aux systèmes hétéroatomiques et calculé les énergies de transition et les longueurs de liaison d'un groupe de molécules avec nitrogène et oxygène. Les énergies des transitions singulets s'accordent bien à l'expérience.

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Supported by the U. S. Atomic Energy Commission and National Science Foundation.     

Precision measurement of the electric quadrupole interaction in zinc metal

To decide on the validity of various values for the quadrupole coupling constant of Zn metal available in the literature, we have used the frequency modulation technique to the 93.3 keV Mössbauer resonance of⁶⁷Zn. We obtained for a⁶⁷Zn metal absorber: e²qQ/h =v _O = 12.34 ± 0.03 MHz. The asymmetry parameter was found to be =0.00 _–0 ^+0.06 . These precise values can be used as a calibration standard for⁶⁷Zn Doppler drives.This work has been supported by the Bundesministerium für Forschung und Technologie and by the Kernforschungszentrum Karlsruhe.     

The photophysics of a luminescent ruthenium polypyridyl complex with pendant β-cylodextrin; pH modulation of lifetime and photoinduced electron transfer

At room temperature, [Ru(bpy)₂(phen-CD)][PF₆]₂, (phen-CD is 6 ^A -(5-amino-1, 10-phenanthroline)-6 ^A -deoxy- β-Cyclodextrin and bpy is 2,2'-bipyridine) exhibits an intense metal ligand charge transfer (MLCT) transition at 452 nm and a long lived luminescence, centred at 618 nm. We demonstrate, for the first time, that the luminescence quantum yield and lifetime of the Ru (II) polypyridyl centre depends markedly on the solution pH. The pH sensitive range extends from pH 3.9 to pH 13.2 and the luminescence quantum yield changes by more than 60% over this range. This pH sensitivity is attributed to protonation/deprotonation of the secondary amine group bridge between the phenanthroline unit and CD. The complex exhibits strong host-guest binding to anthraquinone and anthraquinone-2-carboxylic acid, with concomitant quenching of the [Ru(bpy)₂(phen-CD)]²⁺ excited state. This quenching arises from efficient intramolecular electron transfer. The sensitivity of this photoinduced process to the protonation state of the bridge is discussed.     

SCF calculations of aromatic hydrocarbons the variable β approximation

The transition energies and intensities of naphthalene, anthracene, phenanthrene, pyrene, and azulene are calculated with a variable modification of the Pariser-Parr-Pople method. In this procedure each is determined from the bond order after every iteration. The dependence of on bond order is given by = –0.51 p + A ₀ eV where A ₀ is –1.90 eV (naphthalene and azulene), –1.84 eV (anthracene and phenanthrene), and –1.82 eV (pyrene). Precise knowledge of the molecular geometry is not required and the results are in good agreement with experiment.

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Zusammenfassung Die elektronischen Anregungsenergien sowie die zugehörigen Intensitäten von Naphthalin, Anthrazen, Phenantren, Pyren und Azulen wurden mit der Modifikation der variablen der Pariser-Parr-Pople Methode berechnet, bei welcher die -Werte nach jedem Iterationsschritt als Funktion der Bindungsordnung neu berechnet werden. Ihre Abhängigkeit ist durch = –0,51 p + A ₀ eV gegeben, wobei A ₀ –1,90 (Naphthalin und Azulen), –1,84 (Anthrazen und Phenantren) bzw. –1,82 (Pyren) ist. Für das Verfahren ist die genaue Kenntnis der Geometrie des Moleküls nicht vonnöten und die Resultate befinden sich in guter Übereinstimmung mit dem Experiment.

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Résumé Nous avons calculé les énergies et intensités des transitions électroniques de naphthalène, Anthracène, phénanthrène, pyrène et azulène par une méthode PPP modifiée, où les sont déterminés des indices de liaison p après chaque itération: = –0,51 p + A ₀ eV, où A ₀ = –1,90 (pour naphthalène et azulène), –1,84 (anthracène et phénanthrène) et –1,82 (pyrène), respectivement. On n'a pas besoin des géometries exactes des molécules. Les résultats s'accordent bien à l'expérience.

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Supported by the U.S. Atomic Energy Commission and National Science Foundation.     

Comparison of compuational methods for simulating nuclear Overhauser effects in NMR spectroscopy

Various algorithms for solving the Solomon equations describing nuclear Overhauser effects (nOes) in NMR spectroscopy have been compared. The applicability of the eigenvalue/eigenvector and the numerical integration approaches have been investigated. The eigenvalue/eigenvector approach is not a computationally efficient means of simulating nOe experiments in which a saturating radiofrequency field is applied during the time course. For experiments in which nOes develop in the absence of an RF field, this approach should only be used in simulating a full NOESY spectrum. Integration schemes have been found to be more efficient at simulating nOe experiments in which the nOe evolves in the presence of a saturating field, at simulating a partial set of initial perturbation experiments and at simulating a few rows or columns in a NOESY spectrum. Various integration schemes were applied to a two-spin system for which an analytic solution is available and to a model B-DNA oligonucleotide hexamer. The previously unused Taylor series algorithm was found to be superior to the Euler, midpoint, and fourth-order Runge–Kutta methods with regard to integration accuracy/computation time. An adaptive step size control routine for the Taylor series integration scheme was developed. Integration schemes can be speeded up in a simple fashion by introducing a distance cutoff for the dipolar interaction. Using a cutoff of 8 Å the Taylor series algorithm was able to compute the NOESY spectrum more rapidly than the eigenvalue/eigenvector algorithm for large spin systems at short mixing times. At longer mixing times the eigenvalue/eigenvector approach becomes the more efficient scheme.