Kresimir Rupnik

Density effects on high‐n molecular Rydberg states: CH3I and C6H6 in H2 and Ar

The absorption spectra of high‐n Rydberg states of methyl iodide and benzene perturbed by varying number densities of hydrogen or argon, range 0.9×1020–10.5×1020 cm−3 for H2 and 0.6×1020–7.5×1020 cm−3 for Ar, have been investigated. The high‐n molecular states of both absorbers were found to shift linearly with the number density of atomic Ar and molecular H2 scatterers. The Fermi formula modified by the Alekseev–Sobel’man polarization term provides an excellent fit of the shift data. The electron scattering lengths obtained are: 0.93 a0 for H2 and −1.63 a0 for Ar using the CH3I absorber; and 0.99 a0 for H2 and −1.57 a0 for Ar using the C6H6 absorber. The electron scattering lengths for H2 and Ar agree with the results of an empirical model that correlates scattering lengths and the polarizabilities α(spherical) for inert atoms and α2(nonspherical) for H2 molecule.

VTVH-MCD study of the Delta nifB Delta nifZ MoFe protein from Azotobacter vinelandii.

NifZ is a member of a series of proteins associated with the maturation of the nitrogenase MoFe protein. An MCD spectroscopic study was undertaken on the Delta nifB Delta nifZ MoFe protein generated in the absence of both NifZ and NifB (deletion of NifB generates an apo-MoFe protein lacking the FeMo cofactor). Results presented here show that, in the absence of NifZ, only one of the two P-clusters of the MoFe protein is matured to the ultimate [8Fe-7S] structure. The other P-cluster site in the protein contains a [4Fe-4S] cluster pair, representing a P-cluster precursor that is electronically identical to the analogous clusters observed in the Delta nifH MoFe protein. These results suggest that the MoFe protein is synthesized in a stepwise fashion where NifZ is specifically required for the formation of the second P-cluster.

Electron scattering in dense atomic and molecular gases : an empirical correlation of polarizability and electron scattering length

A linear correlation exists between the electron scattering length, as measured by a pressure shift method, and the polarizabilities for He, Ne, Ar, Kr, and Xe gases. The correlative algorithm has excellent predictive capability for the electron scattering lengths of mixtures of rare gases, simple molecular gases such as H2 and N2 and even complex molecular entities such as methane, CH4.

(4Fe4S) 2+ Clusters Exhibit Ground-State Paramagnetism

Two proteins involved in nitrogen fixation contain ferredoxin-type [4Fe4S] clusters that exist in paramagnetic ground state upon oxidation, a property never observed since the discovery of ferredoxins 50 years ago. This unique characteristic suggests a specific coupling in these clusters necessary for nitrogen fixation and implies an evolutionary connection between the clusters in the two proteins.

P+ state of nitrogenase p-cluster exhibits electronic structure of a [Fe4S4]+ cluster.

Mo nitrogenase consists of two component proteins: the Fe protein, which contains a [Fe(4)S(4)] cluster, and the MoFe protein, which contains two different classes of metal cluster: P-cluster ([Fe(8)S(7)]) and FeMoco ([MoFe(7)S(9)C·homocitrate]). The P-cluster is believed to mediate the electron transfer between the Fe protein and the MoFe protein via interconversions between its various oxidation states, such as the all-ferrous state (P(N)) and the one- (P(+)) and two-electron (P(2+)) oxidized states. While the structural and electronic properties of P(N) and P(2+) states have been well characterized, little is known about the electronic structure of the P(+) state. Here, a mutant strain of Azotobacter vinelandii (DJ1193) was used to facilitate the characterization of the P(+) state of P-cluster. This strain expresses a MoFe protein variant (designated ΔnifB β-188(Cys) MoFe protein) that accumulates the P(+) form of P-cluster in the resting state. Magnetic circular dichroism (MCD) spectrum of the P-cluster in the oxidized ΔnifB β-188(Cys) MoFe protein closely resembles that of the P(2+) state in the oxidized wild-type MoFe protein, except for the absence of a major charge-transfer band centered at 823 nm. Moreover, magnetization curves of ΔnifB β-188(Cys) and wild-type MoFe proteins suggest that the P(2+) species in both proteins have the same spin state. MCD spectrum of the P(+) state in the ΔnifB β-188(Cys) MoFe protein, on the other hand, is associated with a classic [Fe(4)S(4)](+) cluster, suggesting that the P-cluster could be viewed as two coupled 4Fe clusters and that it could donate either one or two electrons to FeMoco by using one or both of its 4Fe halves. Such a mode of action of P-cluster could provide energetic and kinetic advantages to nitrogenase in the complex mechanism of N(2) reduction.

Photothermal response of near-infrared-absorbing NanoGUMBOS.

The photothermal properties of several near-infrared-absorbing nanoparticles derived from group of uniform materials based on organic salts (GUMBOS) and composed of cationic dyes coupled with biocompatible anions are evaluated. These nanoparticles were synthesized using a reprecipitation method performed at various pH values: 2.0, 5.0, 7.0, 9.0, and 11.0. The cations for the nanoparticles derived from GUMBOS (nanoGUMBOS), [1048] and [1061], have absorbance maxima at wavelengths overlapping with human soft tissue absorbance minima. Near-infrared-absorbing nanoGUMBOS excited with a 1064 nm continuous laser led to heat generation, with an average temperature increase of 20.4 ± 2.7 °C. Although the [1061][Deoxycholate] nanoGUMBOS generated the highest temperature increase (23.7 ± 2.4 °C), it was the least photothermally efficient compound (13.0%) due to its relatively large energy band gap of 0.892 eV. The more photothermally efficient compound [1048][Ascorbate] (64.4%) had a smaller energy band gap of 0.861 eV and provided an average photothermal temperature increase of 21.0 ± 2.1 °C.

Automated scanning probe lithography with n-alkanethiol self assembled monolayers on Au(111): Application for teaching undergraduate laboratories

Controllers for scanning probe instruments can be programmed for automated lithography to generate desired surface arrangements of nanopatterns of organic thin films, such as n-alkanethiol self-assembled monolayers (SAMs). In this report, atomic force microscopy (AFM) methods of lithography known as nanoshaving and nano-grafting are used to write nanopatterns within organic thin films. Commercial instruments provide software to control the length, direction, speed, and applied force of the scanning motion of the tip. For nanoshaving, higher forces are applied to an AFM tip to selectively remove regions of the matrix monolayer, exposing bare areas of the gold substrate. Nanografting is accomplished by force-induced displacement of molecules of a matrix SAM, followed immediately by the surface self-assembly of n-alkanethiol molecules from solution. Advancements in AFM automation enable rapid protocols for nanolithography, which can be accomplished within the tight time restraints of undergraduate laboratories. Example experiments with scanning probe lithography will be described in this report that were accomplished by undergraduate students during laboratory course activities and research internships in the chemistry department of Louisiana State University. Students were introduced to principles of surface analysis and gained “hands-on” experience with nanoscale chemistry.

Selective digital integrator for modulated-polarization spectroscopy: An evaluation using (+)-3-methylcyclopentanone

A new device, a selective digital integrator (SDI), for the acquisition of modulated polarization spectroscopy (MPS) signals is described. Special attention is given to the accurate measurement of very small signals (ac component of interest <10−3×dc component) buried in noise. The polarization of light is altered with fast modulators (modulation frequency ∼50 kHz). A SDI connected to a single detector is used to separately and selectively measure the polarization-dependent intensities of electronic transitions. The SDI is a major improvement over the lock-in approach: it has on-the-fly time-resolved data acquisition and integration capability, and a wide gain-switching-free dynamic range (10 orders of magnitude). This design permits a variety of applications that require detection of modulated or pulsed signals. The advantages of the MPS-SDI method are demonstrated on the first Rydberg electronic transitions of (+)-3-methylcyclopentanone. The selectively measured MPS intensities are provided in 3D form (...

Pressure shifts and electron scattering lengths in atomic and molecular gases

Photoabsorption or photoionization spectra of CH3I are discussed as a function of perturber pressure for 11 different binary gas mixtures consisting of CH3I and each one of 11 different gaseous perturbers. Five of the perturbers were rare gases and six were nondipolar molecules. The energy shifts of CH3I Rydberg states become independent of n, the principal quantum number, for n≥10. The energy shifts for n≥10 vary in a linear fashion with perturber number density. The electron scattering lengths for the perturbers are extracted from the shifts using Fermi theory in which the polarization term is that of Alekseev and Sobel’man. These scattering lengths are compared with those from swarm and time‐of‐flight experiments. It is found that the uncorrected shift scattering lengths correspond to the zero energy or near‐zero energy scattering lengths obtained from extrapolated swarm and time‐of‐flight data. It is found that plots of scattering length vs polarizability α (ᾱ for molecules) define two linearities, on...

Nonenzymatic Synthesis of the P-Cluster in the Nitrogenase MoFe Protein: Evidence of the Involvement of All-Ferrous [Fe4S4]0 Intermediates

The P-cluster in the nitrogenase MoFe protein is a [Fe8S7] cluster and represents the most complex FeS cluster found in Nature. To date, the exact mechanism of the in vivo synthesis of the P-cluster remains unclear. What is known is that the precursor to the P-cluster is a pair of neighboring [Fe4S4]-like clusters found on the ΔnifH MoFe protein, a protein expressed in the absence of the nitrogenase Fe protein (NifH). Moreover, incubation of the ΔnifH MoFe protein with NifH and MgATP results in the synthesis of the MoFe protein P-clusters. To improve our understanding of the mechanism of this reaction, we conducted a magnetic circular dichroism (MCD) spectroscopic study of the [Fe4S4]-like clusters on the ΔnifH MoFe protein. Reducing the ΔnifH MoFe protein with Ti(III) citrate results in the quenching of the S = 1/2 electron paramagnetic resonance signal associated with the [Fe4S4]+ state of the clusters. MCD spectroscopy reveals this reduction results in all four 4Fe clusters being converted into the unusual, all-ferrous [Fe4S4]0 state. Subsequent increases of the redox potential generate new clusters. Most significantly, one of these newly formed clusters is the P-cluster, which represents approximately 20–25% of the converted Fe concentration. The other two clusters are an X cluster, of unknown structure, and a classic [Fe4S4] cluster, which represents approximately 30–35% of the Fe concentration. Diamagnetic FeS clusters may also have been generated but, because of their low spectral intensity, would not have been identified. These results demonstrate that the nitrogenase P-cluster can be generated in the absence of NifH and MgATP.