Spectroscopic properties of tyrosyl radicals in dipeptides

Redox-active tyrosine residues play important roles in long-distance electron reactions in enzymes, including prostaglandin H synthase, galactose oxidase, ribonucleotide reductase, and photosystem II. Magnetic resonance and vibrational spectroscopy provide methods with which to study the structures of redox-active amino acids in proteins. In this report, ultraviolet photolysis was used to generate tyrosyl radicals from polycrystalline tyrosinate or dipeptides, and the structure of the radical was investigated with EPR and reaction-induced FT-IR spectroscopy at 77 K. Photolysis at 77 K is expected to generate a neutral tyrosyl radical through oxidation of the aromatic ring. EPR and FT-IR results obtained from (13)C-labeled tyrosine were consistent with that expectation. Surprisingly, labeling of the tyrosyl amino group with (15)N also resulted in isotope-shifted bands in the photolysis spectrum. The force constant of a NH deformation mode increased when the tyrosyl radical was generated. These data suggest an interaction between the pi system of the tyrosyl radical and the amino group. In spectra acquired from the dipeptides, evidence for a sequence-dependent interaction between the tyrosyl radical and the amide bond of the dipeptide was also obtained. We postulate that perturbation of the amino or the amide/imide groups may occur through a spin polarization mechanism, which is indirectly detected as a change in NH force constant. This conclusion is supported by density functional calculations, which suggest a conformationally sensitive delocalization of spin density onto the amino and carboxylate groups of the tyrosyl radical. These experiments provide a step toward a detailed spectral interpretation for protein-based tyrosyl radicals.     

Enantioselective Construction of Vicinal Stereogenic Quaternary Centers by Dialkylation: Practical Total Syntheses of (+)- and meso-Chimonanthine

All three stereoisomers of the hexacyclic 3a,3a'-bispyrrolidino[2,3-b]indoline moiety found in complex indole alkaloids can be prepared, as illustrated by total syntheses of meso-chimonanthine (1) and (+)-chimonanthine (2). A rare example of high diastereoselectivity arising from the combination of a prostereogenic enolate with a chiral electrophile containing a sp(3) carbon atom is the key feature of the asymmetric synthesis of the C(2) stereoisomer.     

Substitution and condensation reactions with poly(anilineboronic acid): reactivity and characterization of thin films

Poly(anilineboronic acid) thin films are treated under various conditions to achieve substitution or condensation reactions involving the boronic acid moiety. These reactions are studied with polarization modulated infrared reflection absorption spectroscopy, cyclic voltammetry, and UV-vis spectroscopy. The results suggest the single-step formation of substituted polyanilines, such as poly(hydroxyaniline), halogenated polyanilines, and mercury chloride-substituted polyaniline. A condensation reaction of poly(anilineboronic acid) with cis-diol compounds in aqueous solution, as well as with phenylenebisboronic acid and salycilamide in THF, indicates the formation of boronic esters. The latter reactions appear to be a good entry point for the formation of complex or supramolecular polymer structures.     

Mechanistic characterization of aerobic alcohol oxidation catalyzed by Pd(OAc)(2)/pyridine including identification of the catalyst resting state and the origin of nonlinear [catalyst] dependence

The Pd(OAc)(2)/pyridine catalyst system is one of the most convenient and versatile catalyst systems for selective aerobic oxidation of organic substrates. This report describes the catalytic mechanism of Pd(OAc)(2)/pyridine-mediated oxidation of benzyl alcohol, which has been studied by gas-uptake kinetic methods and (1)H NMR spectroscopy. The data reveal that turnover-limiting substrate oxidation by palladium(II) proceeds by a four-step pathway involving (1) formation of an adduct between the alcohol substrate and the square-planar palladium(II) complex, (2) proton-coupled ligand substitution to generate a palladium-alkoxide species, (3) reversible dissociation of pyridine from palladium(II) to create a three-coordinate intermediate, and (4) irreversible beta-hydride elimination to produce benzaldehyde. The catalyst resting state, characterized by (1)H NMR spectroscopy, consists of an equilibrium mixture of (py)(2)Pd(OAc)(2), 1, and the alcohol adduct of this complex, 1xRCH(2)OH. These in situ spectroscopic data provide direct support for the mechanism proposed from kinetic studies. The catalyst displays higher turnover frequency at lower catalyst loading, as revealed by a nonlinear dependence of the rate on [catalyst]. This phenomenon arises from a competition between forward and reverse reaction steps that exhibit unimolecular and bimolecular dependences on [catalyst]. Finally, overoxidation of benzyl alcohol to benzoic acid, even at low levels, contributes to catalyst deactivation by formation of a less active palladium benzoate complex.     

Optimization of auxiliary basis sets for the LEDO expansion and a projection technique for LEDO-DFT

We present a systematic procedure for the optimization of the expansion basis for the limited expansion of diatomic overlap density functional theory (LEDO-DFT) and report on optimized auxiliary orbitals for the Ahlrichs split valence plus polarization basis set (SVP) for the elements H, Li--F, and Na--Cl. A new method to deal with near-linear dependences in the LEDO expansion basis is introduced, which greatly reduces the computational effort of LEDO-DFT calculations. Numerical results for a test set of small molecules demonstrate the accuracy of electronic energies, structural parameters, dipole moments, and harmonic frequencies. For larger molecular systems the numerical errors introduced by the LEDO approximation can lead to an uncontrollable behavior of the self-consistent field (SCF) process. A projection technique suggested by L?wdin is presented in the framework of LEDO-DFT, which guarantees for SCF convergence. Numerical results on some critical test molecules suggest the general applicability of the auxiliary orbitals presented in combination with this projection technique. Timing results indicate that LEDO-DFT is competitive with conventional density fitting methods.     

Enantioselective uptake of amino acid with overoxidized polypyrrole colloid templated with L-lactate

An overoxidized polypyrrole colloid, which can recognise enantiomers of amino acids has been prepared by a newly developed molecular imprinting technique. A polypyrrole colloid, which had been polymerised from a mixture of pyrrole (monomer), polyvinylpyrrolidone (steric stabiliser), peroxodisulfate (oxidant) and L-lactate (dopant), was overoxidized to create a dopant-complementary cavity. The enantioselectivity of the overoxidized colloid was evaluated by comparing the uptake of L-alanine and L-cysteine with that of the respective D-enantiomers. The L/D uptake ratios for these amino acids were about 2, while phenylalanine showed suppressed uptake for both the enantiomers. The absence of phenylalanine uptake can be explained in terms of the molecular size, which is too large to be accommodated by the cavity created by L-lactate. In contrast, a colloid templated with L-phenyllactate took up L-phenylalanine with a higher enantioselectivity of about 7. A colloid templated with L-lactate was applied to surface chirality analysis through enantioselective adsorption on cysteine-modified gold surfaces. Quartz microbalance experiments and scanning electron microscope observation of the gold surface revealed that the colloidal particle has higher affinity to a surface modified with L-cysteine than to one modified with D-cysteine.     

Kinetic modeling of the adsorption rate of a gaseous adsorbate on a granular adsorbent by ultra-rapid-scanning Fourier transform infrared spectrometry

A novel method for determining the rates of adsorption of gaseous adsorbates on granular adsorbents in the millisecond time regime is described. Mixtures of the adsorbate and nitrogen are passed through a solenoid valve and 100 mg of the adsorbent for periods of (typically) 100 ms. The concentration of the adsorbate passing into a low-volume, long path length gas cell is measured with an ultra-rapid-scanning Fourier transform infrared spectrometer capable of gathering 200 mid-infrared spectra per second with 6-cm(-1) resolution. The pressure of the gas entering the cell is measured simultaneously with a capacitance manometer. A dynamic mathematical model was developed to analyze and describe the results in terms of a Langmuir isotherm. The success of this approach is demonstrated by the estimation of the rate of adsorption of vapor-phase acetaldehyde on aminopropylsilylated granular silica gel.     

Solution redox chemistry of carbon nanotubes

UV/vis/NIR absorbance spectra were used to monitor electron transfer between small-molecule redox reagents and carbon nanotubes (CNTs). The oxidation of (6, 5)-enriched nanotubes in water with K(2)Ir(Cl)(6) reveals a valence electron density of 0.2-0.4 e(-)/100 carbon atoms and a reduction potential of approximately 800 mV versus NHE. The reduction potential of CNTs is found to increase with increasing band gap and to decrease with the introduction of an anionic dispersant. In light of this newly revealed redox chemistry of CNTs, we propose that the previously observed bleaching of the CNT absorbance spectrum at low pH is most likely a consequence of the oxidation of the nanotubes by oxygen. These results demonstrate facile oxidation and reduction of CNTs, provide a way to quantify the population of valence electrons, and point to possible applications of CNT in the catalysis of redox reactions.