Specific His6-tag attachment to metal-functionalized polymersomes relies on molecular recognition.

The development of nanocarriers for drug/protein delivery is in focus today, as they can serve to both decrease dosages and improve localization to a desired biological compartment. A powerful tool to functionalize these carriers is specific affinity tagging supported by molecular recognition, a key principle in biology. However, the geometry of the binding region in a molecular recognition process, and thus its conformation and specificity, are in many cases poorly understood. Here, we demonstrate that short, model peptides, His(6)-tags, selectively recognize Cu(II)-trisnitrilotriacetic acid moieties (Cu(II)-trisNTA) when exposed at the surfaces of polymer vesicles designed to serve as nanocarriers or as surfaces for proteins binding. A mixture of poly(butadiene)-b-poly(ethylene oxide) (PB-b-PEO) and Cu(II)-trisNTA-functionalized PB-b-PEO diblock copolymers (10:1) self-assembles in aqueous solution, generating vesicles with a hydrodynamic radius of approximately 100 nm, as established by light scattering and TEM. Fluorescently labeled His(6) tags specifically bind to metal centers exposed on vesicles' surface, with a dissociation constant of 0.6 ± 0.2 µM, as determined by fluorescence correlation spectroscopy. The significant rearrangement in the geometry of the metal center upon peptide binding was characterized by a combination of CW-EPR, pulse-EPR, and DFT computations. Understanding the binding configuration around the metal center inside NTA pocket exposed at the surface of vesicles supports further development of efficient targetable nanocarriers that can be recognized selectively by molecular recognition in a biological environment and facilitates their immobilization on solid supports and their use in two-dimensional protein arrays.

Effect of chelate ring expansion on Jahn-Teller distortion and Jahn-Teller dynamics in copper(II) complexes.

The expanded ligand N,N'-dimethyl-N,N'-dipyridin-2-yl-pyridin-2,6-diamine (ddpd) coordinates to copper(II) ions in a meridional fashion giving the dicationic complex mer-[Cu(ddpd)(2)](BF(4))(2) (1). In the solid state at temperatures below 100 K the cations of 1 localize in Jahn-Teller elongated CuN(6) polyhedra with the longest Cu-N bond pointing in the molecular x or y directions while the z axis is constrained by the tridentate ddpd ligand. The elongated polyhedra are ordered in an antiferrodistortive way giving an idealized zincblende structure. At higher temperature dynamically averaged (fluxional) polyhedra in the molecular x/y directions are observed by multifrequency variable temperature electron paramagnetic resonance (EPR) and by variable temperature X-ray diffraction studies. Compared to [Cu(tpy)(2)](2+) (tpy = 2,2';6',2″-terpyridine) the Jahn-Teller splitting 4δ(1) of 1 is larger. This is very probably caused by the much more favorable orbital overlap in the Cu-N bonds in 1 which results from the larger bite angle of ddpd as compared to tpy. The "freezing-in" of the Jahn-Teller dynamics of 1 (T ≈ 100 K) occurs at higher temperature than observed for [Cu(tpy)(2)](2+) (T < 77 K) which is also probably due to the larger Jahn-Teller distortion of 1 resulting in a larger activation barrier.

EPR investigation of the role of B10 phenylalanine in neuroglobin - evidence that B10Phe mediates structural changes in the heme region upon disulfide-bridge formation.

The function of neuroglobin, a member of the vertebrate globin family, is still unknown. In human neuroglobin (NGB), the formation of a disulfide bridge between the CysCD7 and CysD5 is known to affect the heme environment and its ligand-binding kinetics. Here, we show by means of EPR that the PheB10 residue plays a key role in transmitting the structural information from the disulfide bridge to the heme-pocket region. While formation of a disulfide bridge in ferric wild-type NGB leads to a considerable change of its EPR parameters, only minor changes are observed in the case of ferric PheB10Leu NGB. Furthermore, wild-type NGB is found to be much more stable in the presence of H(2)O(2) than its PheB10Leu or its HisE7Leu mutants. While tyrosyl radicals are induced in HisE7Leu NGB by the addition of H(2)O(2), this is not the case for wild-type and PheB10Leu NGB. The results will be discussed in terms of the protein's putative functions.

Structural characterization of a highly active superoxide-dismutase mimic.

A lot of effort has been put into the synthesis of copper complexes with superoxide-dismutase (SOD) activity because of their potential pharmaceutical applications. In this work, we study a model for these so-called SOD mimics (SODm), namely a copper complex of 6-(2-hydroxy-benzaldehyde) hydrazono-as-triazine-3,5-dione, which shows an extremely high SOD-like activity in solution. X-Ray diffraction reveals that the complex adopts a di-copper structure in the solid state. However, in solution, the chloride bridges are broken, forming a mono-copper center as follows from UV/Vis absorption and electron paramagnetic resonance (EPR) experiments. Using pulsed EPR techniques in combination with DFT (density functional theory) computations, the electronic structure of the complex in solution is analyzed in detail and related to its high SOD activity. The structure-activity analysis serves to orient further synthetic efforts to obtain the optimum geometry around the metal essential for SOD-like activity.