Biomedical subjects
Publications and source records attributed to Stoyan K Smoukov.
Mixtures of oppositely charged nanoparticles of various sizes and charge ratios precipitate only at the point of electroneutrality. This phenomenon-specific to the nanoscale and reminiscent of threshold precipitation of ions-is a consequence of the formation of core-and-shell nanoparticle aggregates, in which the shells are composed of like-charged particles and are stabilized by efficient electrostatic screening.
Self-assembly of charged, equally sized metal nanoparticles of two types (gold and silver) leads to the formation of large, sphalerite (diamond-like) crystals, in which each nanoparticle has four oppositely charged neighbors. Formation of these non-close-packed structures is a consequence of electrostatic effects specific to the nanoscale, where the thickness of the screening layer is commensurate with the dimensions of the assembling objects. Because of electrostatic stabilization of larger crystallizing particles by smaller ones, better-quality crystals can be obtained from more polydisperse nanoparticle solutions.
Dynamic self-assembly (DySA) processes occurring outside of thermodynamic equilibrium underlie many forms of adaptive and intelligent behaviors in natural systems. Relatively little, however, is known about the principles that govern DySA and the ways in which it can be extended to artificial ensembles. This article discusses recent advances in both the theory and the practice of nonequilibrium self-assembly. It is argued that a union of ideas from thermodynamics and dynamic systems' theory can provide a general description of DySA. In parallel, heuristic design rules can be used to construct DySA systems of increasing complexities based on a variety of suitable interactions/potentials on length scales from nanoscopic to macroscopic. Applications of these rules to magnetohydrodynamic DySA are also discussed.
Electrostatic aggregation of oppositely charged silver and gold nanoparticles leads to the formation of core-shell clusters in which the shell is formed by the nanoparticles, which are in excess. Arguments based on Debye screening of interactions between like-charged particles help explain why these clusters are stable despite possessing net electric charge. The core-shell aggregates exhibit unusual optical properties with the resonance absorption of the shell particles enhanced by the particles in the core and that of the core suppressed by the shell. Experimental UV-vis absorption spectra are faithfully reproduced by Mie theory. The modeling allows for estimation of the numbers of particles forming the shell and of the shell's effective thickness. These theoretical predictions are substantiated by experiments using nanoparticles covered with different combinations of charged groups and performed at different values of pH.
Arrays of surface wrinkles of linearly increasing heights (from tens of nanometers to tens of micrometers) were prepared via a spontaneous reaction-diffusion process based on periodic precipitation. The slopes, dimensions, and positions of the precipitation bands could be controlled precisely by adjusting the concentrations of the participating chemicals as well as the material properties of patterned substrates. Additional control of periodic precipitation by localized UV irradiation allowed for the preparation of discontinuous and curvilinear structures. The nonbinary 3D surface topographies were replicated into poly(dimethylsiloxane), and the applications of replicas in microfluidics, microseparations, and cell biology have been suggested.
A reaction-diffusion process induced from a micronetwork geometry amplifies changes in the molecular structure of a thin gel film into macroscopic readout patterns. When the gel undergoes a helix-to-coil phase transition, the patterns formed by RD switch from symmetry-broken to symmetric ones. Theoretical analysis explains how the system reconfigures internally in response to mass transfer between the applied network and the probed film.
Hydrogel stamps are used to reactively micropattern various types of substrates. The method, called reactive wet stamping (r-WETS), is general in nature and overcomes several limitations of conventional soft-lithographic techniques. Illustrative applications of r-WETS in surface wettability modification, deposition of metallic microstructures, preparation of supports for electrostatic self-assembly, and multistep reactive patterning are discussed.
Key features differentiating the coordination environment of the two irons in the mixed-valent (Fe(2+),Fe(3+)) diiron site of Desulfovibrio vulgaris rubrerythrin (Rbr(mv)) were determined by continuous wave (CW) and pulsed ENDOR spectroscopy at 35GHz. (14)N ENDOR evidence indicates that a nitrogen is bound only to the Fe(2+) ion of the mixed-valent site. Assuming that this nitrogen is from His131Ndelta, the same one that furnishes an iron ligand in the crystal structure of the diferric site, the ENDOR data allow us to specify the Fe(2+) and Fe(3+) positions within the molecular reference frame. In addition, the (1,2)H ENDOR on Rbr(mv) indicates the presence of a solvent-derived aqua/hydroxo ligand bound either terminally or in a bridging mode to Fe(3+) in the mixed-valent site. The relatively large g anisotropy of Rbr(mv) and weak antiferromagnetic coupling, J approximately -8 cm(-)(1) (in the 2JS(1)*S(2) formalism), between the irons is more consistent with a bridging than terminal hydroxo ligand. gamma-Irradiation was used to cryoreduce Rbr at 77 K, thereby producing a mixed-valent diiron site [(Rbr(ox))(mv)] that retains the structure of the diferric site. The EPR spectrum of (Rbr(ox))(mv) was nearly identical to that of the as-isolated or chemically reduced samples. This near identity implies that the structure of the mixed-valent Rbr diiron site is essentially identical to that of the diferric site, except for protonation of the oxo bridge, which apparently occurred via a proton jump from hydrogen-bonded solvent at 77 K. The EPR spectrum of (Rbr(ox))(mv) thus supports the (14)N ENDOR-assigned His131 ligation to Fe(2+) and assignment of the solvent-derived ligand observed in the (1,2)H ENDOR to a hydroxo bridge between the irons of the mixed-valent diiron site.
Despite the current availability of several crystal structures of purple acid phosphatases, to date there is no direct evidence for solvent-derived ligands occupying terminal positions in the active enzyme. This is of central importance, because catalysis has been shown to proceed through the direct attack on a metal-bound phosphate ester by a metal-activated solvent-derived moiety, which has been proposed to be either (i) a hydroxide ligand terminally bound to the ferric center or (ii) a bridging hydroxide. In this work we use (2)H Q-band (35 GHz) pulsed electron-nuclear double resonance (ENDOR) spectroscopy to identify solvent molecules coordinated to the active mixed-valence (Fe(3+)Fe(2+)) form of the dimetal center of uteroferrin (Uf), as well as to its complexes with the anions MoO(4), AsO(4), and PO(4). The solvent-derived coordination of the dinuclear center of Uf as deduced from ENDOR data includes a bridging hydroxide and a terminal water/hydroxide bound to Fe(2+) but no terminal water/hydroxide bound to Fe(3+). The terminal water is lost upon anion binding while the hydroxyl bridge remains. These results are not compatible with a hydrolysis mechanism involving a terminal Fe(3+)-bound nucleophile, but they are consistent with a mechanism that relies on the bridging hydroxide as the nucleophile.
The binding of ethanol and 1,1,1-trifluoroethanol (TFE) to both the H(mv) and H(ox) forms of soluble methane monooxygenase (sMMO) in solution has been studied by Q-band (35 GHz) CW and pulsed ENDOR spectroscopy of (1)H, (2)H and (19)F nuclei of exogenous ligands. As part of this investigation we introduce (19)F, in this case from bound TFE, as a new probe for the binding of small molecules to a metalloenzyme active site. The H(mv) form was prepared in solution by chemical reduction of H(ox). For study of H(ox) itself, frozen solutions were subjected to gamma-irradiation in the frozen solution state at 77 K, which affords an EPR-visible mixed-valent diiron center, denoted (H(ox))(mv), held in the geometry of the diiron(III) state. The (19)F and (2)H ENDOR spectra of bound TFE together with (1,2)H ENDOR spectra of bound ethanol indicate that the alcohols bind close to the Fe(II) ion of the mixed-valence cluster in H(mv) and in a bridging or semi-bridging fashion to H(ox). DMSO does not affect the binding of either of the ethanols or of methanol to H(ox), nor of ethanol or methanol to H(mv). It does, however, displace TFE from the diiron site in H(mv). These results provide the first evidence that crystal structures of sMMO hydroxylase into which product alcohols were introduced by diffusion represent the structures in solution.
FosA is a manganese metalloglutathione transferase that confers resistance to the broad-spectrum antibiotic fosfomycin, (1R,2S)-epoxypropylphosphonic acid. The reaction catalyzed by FosA involves the attack by glutathione on fosfomycin to yield the product 1-(S-glutathionyl)-2-hydroxypropylphosphonic acid. The enzyme is a dimer of 16 kDa subunits, each of which harbors one mononuclear Mn(II) site. The coordination environment of the Mn(II) in the FosA x Mn(2+) complex is composed of a glutamate and two histidine ligands and three water molecules. Here we report EPR spectroscopic studies on FosA, in which EPR spectra were obtained at 35 GHz and 2 K using dispersion-detection rapid-passage techniques. This approach provides an absorption envelope line shape, in contrast to the conventional (slow-passage) derivative line shape, and is a more reliable way to collect spectra from Mn(II) centers with large zero-field splitting. We obtain excellent spectra of FosA bound with substrate, substrate analogue phosphate ion, and product, whereas these states cannot be studied by X-band, slow-passage methods. Simulation of the EPR spectra shows that binding of substrate or analogue causes a profound change in the electronic parameters of the Mn(II) ion. The axial zero-field splitting changes from [D] = 0.06 cm(-1) for substrate-free enzyme to 0.23 cm(-1) for fosfomycin-bound enzyme, 0.28 (1) cm(-1) for FosA with phosphate, and 0.27 (1) cm(-1) with product. Such a large zero-field splitting is uncommon for Mn(II). A simple ligand field analysis of this change indicates that binding of the phosphonate/phosphate group of substrate or analogue changes the electronic energy levels of the Mn(II) 3d orbitals by several thousand cm(-1), indicative of a significant change in the Mn(II) coordination sphere. Comparison with related enzymes (glyoxalase I and MnSOD) suggests that the change in the coordination environment on substrate binding may correspond to loss of the glutamate ligand.