Deconvolution and assignment of different optical transitions of the blue copper protein azurin from optically detected electron paramagnetic resonance spectroscopy

Optical detection of magnetic resonance

The combination of magnetic resonance with laser spectroscopy provides some interesting options for increasing the sensitivity and information content of magnetic resonance. This review covers the basic physics behind the relevant processes, such as angular momentum conservation during absorption and emission. This can be used to enhance the polarization of the spin system by orders of magnitude compared to thermal polarization as well as for detection with sensitivities down to the level of individual spins. These fundamental principles have been used in many different fields. This review summarizes some of the examples in different physical systems, including atomic and molecular systems, dielectric solids composed of rare earth, and transition metal ions and semiconductors.This review was originally written in response to an invitation of "Progress in NMR Spectroscopy" but re-directed to Magnetic Resonance to be accessible to a wide audience. This paper has been reviewed by peers in accordance with the policy of Magnetic Resonance.

Coherent Raman detected electron spin resonance spectroscopy of metalloproteins: linking electron spin resonance and magnetic circular dichroism

The simultaneous excitation of paramagnetic molecules with optical (laser) and microwave radiation in the presence of a magnetic field can cause an amplitude, or phase, modulation of the transmitted light at the microwave frequency. The detection of this modulation indicates the presence of coupled optical and ESR transitions. The phenomenon can be viewed as a coherent Raman effect or, in most cases, as a microwave frequency modulation of the magnetic circular dichroism by the precessing magnetization. By allowing the optical and magnetic properties of a transition metal ion centre to be correlated, it becomes possible to deconvolute the overlapping optical or ESR spectra of multiple centres in a protein or of multiple chemical forms of a particular centre. The same correlation capability also allows the relative orientation of the magnetic and optical anisotropies of each species to be measured, even when the species cannot be obtained in a crystalline form. Such measurements provide constraints on electronic structure calculations. The capabilities of the method are illustrated by data from the dimeric mixed-valence CuA centre of nitrous oxide reductase (N2OR) from Paracoccus pantotrophus.

Sensitive reagentless electrochemical immunosensor based on an ormosil sol–gel membrane for human chorionic gonadotrophin

A new organically modified silicate (ormosil) material was synthesized as a matrix to encapsulate enzyme labeled antibody for preparation of immunosensors. The ormosil matrix was prepared by hydrolyzing tetraethyoxysilane and (3-aminopropyl)triethoxysilane in weak alkali solution. It possessed three-dimensional ordered nanoporous structure with high electrical conductivity and good mechanical stability. Its hydrophilicity provided a microenvironment for retaining the biological activity of the immobilized protein. Particularly, using horseradish peroxidase-labeled human chorionic gonadotrophin antibody (HRP-anti-hCG) as a model, the immobilized HRP showed direct electron transfer at about -35 mV with a rate constant of 15.8+/-3.8s(-1). By a simple one-step immunoreaction between human serum chorionic gonadotrophin (hCG) in sample solution and the immobilized HRP-anti-hCG, the differential pulse voltammetric peak current of HRP decreased linearly with an increasing hCG concentration from 0.5 to 50 mIU/ml with a relatively low limit of detection of 0.3 mIU/ml at 3sigma. Excellent analytical performance, fabrication reproducibility and operational stability of the proposed biosensor indicated its promising application in clinical diagnostics.

A detailed resonance Raman spectrum of Nickel(II)-substituted Pseudomonas aeruginosa azurin

Nickel(II) and cobalt(II) derivatives of the blue copper protein Pseudomonas aeruginosa azurin have been studied by resonance Raman (RR) spectroscopy at liquid-nitrogen temperatures. Vibrational assignments for the observed RR bands of Ni(II)-azurin have been made through a study of (62)Ni-substituted azurin. A comparison of Ni(II)-azurin RR spectra with those of the wild type (Cu-containing) protein showed Ni(II)-S(Cys) stretching vibrations, nu(Ni-S)(Cys), at substantially lower frequencies (approximately 360 versus approximately 400 cm(-1), respectively), indicating that the Ni(II)-S(Cys) bond is much weaker than the corresponding Cu(II)-S(Cys) bond. Resonance enhanced predominantly nu(Ni-N)(His) modes indicate that the metal-N(His) bond distances in the Ni(II) derivative are the same as those in native azurin. The vibrational data also confirm a tetrahedral disposition of ligands about the metal in Ni(II)-azurin found in the protein crystallographic structures. As expected, excitation profile measurements on Ni(II)-azurin show that the nu(Ni-S)(Cys) assignable modes give maxima at the 440-nm absorption band, which confirms a S(Cys) --> Ni(II) charge-transfer origin of the 440-nm electronic transition in Ni(II)-substituted azurin.

X-ray Magnetic Circular Dichroism of Pseudomonas aeruginosa Nickel(II) Azurin

We show that X-ray magnetic circular dichroism (XMCD) can be employed to probe the oxidation states and other electronic structural features of nickel active sites in proteins. As a calibration standard, we have measured XMCD and X-ray absorption (XAS) spectra for the nickel(II) derivative of Pseudomonas aeruginosa azurin (NiAz). Our analysis of these spectra confirms that the electronic ground state of NiAz is high-spin (S = 1); we also find that the L(3)-centroid energy is 853.1(1) eV, the branching ratio is 0.722(4), and the magnetic moment is 1.9(4) mu(B). Density functional theory (DFT) calculations on model NiAz structures establish that orbitals 3d(x2-y2) and 3d(z2) are the two valence holes in the high-spin Ni(II) ground state, and in accord with the experimentally determined orbital magnetic moment, the DFT results also demonstrate that both holes are highly delocalized, with 3d(x2-y2) having much greater ligand character.