Periodicity, Electronic Structures, and Bonding of Gold Tetrahalides [AuX4]- (X = F, Cl, Br, I, At, Uus).

Systematic theoretical and experimental investigations have been performed to understand the periodicity, electronic structures, and bonding of gold halides using tetrahalide [AuX4](-) anions (X = F, Cl, Br, I, At, Uus). The [AuX4](-) (X = Cl, Br, I) anions were experimentally produced in the gas phase, and their negative-ion photoelectron spectra were obtained, exhibiting rich and well-resolved spectral peaks. As expected, Au-X bonds in such series contain generally increasing covalency when halogen ligands become heavier. We calculated the adiabatic electron detachment energies as well as vertical electron detachment energies using density functional theory methods with scalar relativistic and spin-orbit coupling effects. The computationally simulated photoelectron spectra are in good agreement with the experimental ones. Our results show that the trivalent Au(III) oxidation state becomes progressively less stable while Au(I) tends to be preferred when the halides become heavier along the Periodic Table. This series of molecules provides an example for manipulating the oxidation state of metals in complexes through ligand design.

Influence of the charge state on the structures and interactions of vancomycin antibiotics with cell-wall analogue peptides: experimental and theoretical studies.

Charge matters! The charge state significantly influences the conformation and the binding energy between vancomycin antibiotic and bacterial cell-wall analogue peptides (see figure). Surface-induced dissociation (SID) studies provide a quantitative comparison between the stabilities of different charge states of the complex.In this study we examined the effect of the charge state on the energetics and dynamics of dissociation of the noncovalent complex between the vancomycin and the cell-wall peptide analogue N(alpha),N(epsilon)-diacetyl-L-Lys-D-Ala-D-Ala (V-Ac(2)LKdAdA). The binding energies between the vancomycin and the peptide were obtained from the RRKM (Rice, Ramsperger, Kassel, Marcus) modeling of the time- and energy-resolved surface-induced dissociation (SID) experiments. Our results demonstrate that the stability of the complex towards fragmentation increases in the order: doubly protonated

Experimental and theoretical studies of the structures and interactions of vancomycin antibiotics with cell wall analogues.

Surface-induced dissociation (SID) of the singly protonated complex of vancomycin antibiotic with cell wall peptide analogue (N(alpha),N(epsilon)-diacetyl-L-Lys-D-Ala-D-Ala) was studied using a 6 T Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (FT-ICR MS) specially configured for SID experiments. The binding energy between the vancomycin and the peptide was obtained from the RRKM modeling of the time- and energy-resolved fragmentation efficiency curves (TFECs) of the precursor ion and its fragments. Molecular dynamics simulations of the vancomycin, peptide, and vancomycin-peptide complex were carried out to explore the low energy conformations. Density functional theory (DFT) calculations of the geometries, proton affinities, and binding energies were performed for several model systems including vancomycin (V), vancomycin aglycon (VA), N(alpha),N(epsilon)-diacetyl-L-Lys-D-Ala-D-Ala, and noncovalent complexes of VA with N-acetyl-D-Ala-D-Ala and V with N(alpha),N(epsilon)-diacetyl-L-Lys-D-Ala-D-Ala. Comparison between the experimental and computational results suggests that the most probable structure of the complex observed in our experiments corresponds to the neutral peptide bound to the vancomycin protonated at the primary amine of the disaccharide group. The experimental binding energy of 30.9 +/- 1.8 kcal/mol is in good agreement with the binding energy of 36.3-42.0 kcal/mol calculated for the model system representing the preferred structure of the complex.

Computer simulation of uranyl uptake by the rough lipopolysaccharide membrane of Pseudomonas aeruginosa.

Heavy metal environmental contaminants cannot be destroyed but require containment, preferably in concentrated form, in a solid or immobile form for recycling or final disposal. Microorganisms are able to take up and deposit high levels of contaminant metals, including radioactive metals such as uranium and plutonium, into their cell wall. Consequently, these microbial systems are of great interest as the basis for potential environmental bioremediation technologies. The outer membranes of Gram-negative microbes are highly nonsymmetric and exhibit a significant electrostatic potential gradient across the membrane. This gradient has a significant effect on the uptake and transport of charged and dipolar compounds. However, the effectiveness of microbial systems for environmental remediation will depend strongly on specific properties that determine the uptake of targeted contaminants by a particular cell wall. To aid in the design of microbial remediation technologies, knowledge of the factors that determine the affinity of a particular bacterial outer membrane for the most common ionic species found in contaminated soils and groundwater is of great importance. Using our previously developed model for the lipopolysaccharide (LPS) membrane of Pseudomonas aeruginosa, this work presents the potentials of mean force as the estimate of the free energy profile for uptake of sodium, calcium, chloride, uranyl ions, and a water molecule by the bacterial LPS membrane. A compatible classical parameter set for uranyl has been developed and validated. Results show that the uptake of uranyl is energetically a favorable process relative to the other ions studied. At neutral pH, this nuclide is shown to be retained on the surface of the LPS membrane through chelation with the carboxyl and hydroxyl groups located in the outer core.

Computational investigation and hydrogen/deuterium exchange of the fixed charge derivative tris(2,4,6-trimethoxyphenyl) phosphonium: implications for the aspartic acid cleavage mechanism.

Aspartic acid (Asp)-containing peptides with the fixed charge derivative tris(2,4,6trimethoxyphenyl) phosphonium (tTMP-P+) were explored computationally and experimentally by hydrogen/deuterium (H/D) exchange and by fragmentation studies to probe the phenomenon of selective cleavage C-terminal to Asp in the absence of a "mobile" proton. Ab initio modeling of the tTMP-P+ electrostatic potential shows that the positive charge is distributed on the phosphonium group and therefore is not initiating or directing fragmentation as would a "mobile" proton. Geometry optimizations and vibrational analyses of different Asp conformations show that the Asp structure with a hydrogen bond between the side-chain hydroxy and backbone carbonyl lies 2.8 kcal/mol above the lowest energy conformer. In reactions with D2O, the phosphonium-derived doubly charged peptide (H+)P+LDIFSDF rapidly exchanges all 12 of its exchangeable hydrogens for deuterium and also displays a nonexchanging population. With no added proton, P+LDIFSDF exchanges a maximum of 4 of 11 exchangeable hydrogens for deuterium. No exchange is observed when all acidic groups are converted to the corresponding methyl esters. Together, these H/D exchange results indicate that the acidic hydrogens are "mobile locally" because they are able to participate in exchange even in the absence of an added proton. Fragmentation of two distinct (H+)P+LDIFSDF ion populations shows that the nonexchanging population displays selective cleavage, whereas the exchanging population fragments more evenly across the peptide backbone. This result indicates that H/D exchange can sometimes distinguish between and provide a means of separation of different protonation motifs and that these protonation motifs can have an effect on the fragmentation.

Facile Synthesis and Nonlinear Optical Properties of Push-Pull 5,15-Diphenylporphyrins.

A series of 5,15-diphenylporphyrinatonickel(II) derivatives containing electron-releasing (Me(2)NC(6)H(4)C&tbd1;C-) and -withdrawing groups [-CHO, -CH=C(CN)(2), -CH=C(CO(2)Et)(2), -CH=CHCHO] in opposite meso positions has been synthesized through the classical formylation, halogenation, Knoevenagel condensation, and palladium-catalyzed cross-coupling reaction of porphyrins. The molecular first hyperpolarizabilities (beta) of two of these push-pull porphyrins, namely 5-(2',2'-dicyanoethenyl)-15-[[4"-(N,N-dimethylamino)phenyl]ethynyl]-10,20-diphenylporphyrinatonickel(II) (7) and 5-formyl-15-[[4'-(N,N-dimethylamino)phenyl]ethynyl]-10,20-diphenylporphyrinatonickel(II) (8) have been determined experimentally by electric-field-induced second-harmonic generation (EFISH) at 1907 nm and computationally by semiempirical methods (ZINDO sum-over-states). The values of beta(&mgr;) (124 and 66 x 10(-)(30) cm(5) esu(-)(1), for 7 and 8, respectively) are moderate because the acceptor group is significantly tilted with respect to the porphyrin core in these complexes, which decreases the electronic coupling across the system. This nonplanar geometry has been confirmed by X-ray structure analyses and molecular modeling studies.

Incorporation of hydroxypyridinone ligands into self-assembled monolayers on mesoporous supports for selective actinide sequestration.

In this study, three isomers of hydroxypyridinones (1,2-HOPO, 3,2-HOPO, and 3,4-HOPO) were attached to self-assembled monolayers on mesoporous silica (SAMMS). The HOPO-SAMMS materials have superior solid adsorbents properties: they do not suffer from solvent swelling; their rigid, open pore structure allows rapid sorption kinetics; their extremely high surface area enables the installation of high functional density; and being silica-based, they are compatible with vitrification into a final vitreous waste form. Kinetics, equilibrium, and selectivity of the adsorptions of actinide on the HOPO-SAMMS at various pH values and in the presence of other metal cations, anions, and competing ligands are reported. Rapid sequestration of U(VI), Np(V), and Pu(IV) was observed. Very little competition from transition metal cations and common species was observed.

Direct experimental observation of the low ionization potentials of guanine in free oligonucleotides by using photoelectron spectroscopy.

Photodetachment photoelectron spectroscopy is used to probe the electronic structure of mono-, di-, and trinucleotide anions in the gas phase. A weak and well defined threshold band was observed in the photoelectron spectrum of 2'-deoxyguanosine 5'-monophosphate at a much lower ionization energy than the other three mononucleotides. Density function theory calculations revealed that this unique spectral feature is caused by electron-detachment from a pi orbital of the guanine base on 2'-deoxyguanosine 5'-monophosphate, whereas the lowest ionization channel for the other three mononucleotides takes place from the phosphate group. This low-energy feature was shown to be a "fingerprint" in all the spectra of dinucleotides and trinucleotides that contain the guanine base. The current experiment provides direct spectroscopic evidence that the guanine base is the site with the lowest ionization potential in oligonucleotides and DNA and is consistent with the fact that guanine is most susceptible to oxidation to give the guanine cation in DNA damage.

Characterization of electronic structure and properties of a Bis(histidine) heme model complex.

Ferric and ferrous hemes, such as those present in electron transfer proteins, often have low-lying spin states that are very close in energy. To explore the relationship between spin state, geometry, and cytochrome electron transfer, we investigate, using density functional theory, the relative energies, electronic structure, and optimized geometries for a high- and low-spin ferric and ferrous heme model complex. Our model consists of an iron-porphyrin axially ligated by two imidazoles, which model the interaction of a heme with histidine residues. Using the B3LYP hybrid functional, we found that, in the ferric model heme complex, the doublet is lower in energy than the sextet by 8.4 kcal/mol and the singlet ferrous heme is 6.7 kcal/mol more stable than the quintet. The difference between the high-spin ferric and ferrous model heme energies yields an adiabatic electron affinity (AEA) of 5.24 eV, and the low-spin AEA is 5.17 eV. Both values are large enough to ensure electron trapping, and electronic structure analysis indicates that the iron d(pi) orbital is involved in the electron transfer between hemes. Mössbauer parameters calculated to verify the B3LYP electronic structure correlate very well with experimental values. Isotropic hyperfine coupling constants for the ligand nitrogen atoms were also evaluated. The optimized geometries of the ferric and ferrous hemes are consistent with structures from X-ray crystallography and reveal that the iron-imidazole distances are significantly longer in the high-spin hemes, which suggests that the protein environment, modeled here by the imidazoles, plays an important role in regulating the spin state. Iron-imidazole dissociation energies, force constants, and harmonic frequencies were calculated for the ferric and ferrous low-spin and high-spin hemes. In both the ferric and the ferrous cases, a single imidazole ligand is more easily dissociated from the high-spin hemes.