Optical Anisotropy of Porphyrin Nanocrystals Modified by the Electrochemical Dissolution

Reflectance anisotropy spectroscopy (RAS) coupled to an electrochemical cell represents a powerful tool to correlate changes in the surface optical anisotropy to changes in the electrochemical currents related to electrochemical reactions. The high sensitivity of RAS in the range of the absorption bands of organic systems, such as porphyrins, allows us to directly correlate the variations of the optical anisotropy signal to modifications in the solid-state aggregation of the porphyrin molecules. By combining in situ RAS to electrochemical techniques, we studied the case of vacuum-deposited porphyrin nanocrystals, which have been recently observed dissolving through electrochemical oxidation in diluted sulfuric acid. Specifically, we could identify the first stages of the morphological modifications of the nanocrystals, which we could attribute to the single-electron transfers involved in the oxidation reaction; in this sense, the simultaneous variation of the optical anisotropy with the electron transfer acts as a precursor of the dissolution process of porphyrin nanocrystals.

Anion replacement at Au(110)/electrolyte interfaces

A characteristic reflection anisotropy spectrum (RAS) is observed from a Au(110) surface in a wide range of electrolytes and combinations of pH and applied potentials. It is suggested that this common RAS profile arises from an interaction between the potential applied to the Au(110) electrode and the dipole moments of oxidized species that locates the Fermi level at a common position with respect to the electronic band structure of Au. Rapid changes in this RAS profile are observed for Au(110)/H2SO4 as the potential is switched between 0.3 V and 0.6 V, a potential range in which the surface is not reconstructed and below the potential range of surface oxidation. The spectral changes are completed in less than 10 ms, are reversible and are attributed to the replacement of adsorbed anions by an oxygenated species.

The influence of the structure of the Au(110) surface on the ordering of a monolayer of cytochrome P450 reductase at the Au(110)/phosphate buffer interface

The reflection anisotropy spectra (RAS) observed initially from Au(110)/phosphate buffer interfaces at applied potentials of -0.652 and 0.056 V are very similar to the spectra observed from ordered Au(110) (1 × 3) and anion induced (1 × 1) surface structures respectively. These RAS profiles transform to a common profile after cycling the potential between these two values over 72 h indicating the formation of a less ordered surface. The RAS of a monolayer of a P499C variant of the human flavoprotein cytochrome P450 reductase adsorbed at 0.056 V at an ordered Au(110)/phosphate buffer interface is shown to arise from an ordered layer in which the optical dipole transitions are in a plane that is orientated roughly normal to the surface and parallel to either the [11̄0] or [001] axes of the Au(110) surface. The same result was found previously for adsorption of P499C on an ordered interface at -0.652 V. The adsorption of P499C at the disordered surface does not result in the formation of an ordered monolayer confirming that the molecular ordering is strongly influenced by both the local structure and the long range macroscopic order of the Au(110) surface.

Conformational change induced by electron transfer in a monolayer of cytochrome P450 reductase adsorbed at the Au(110)-phosphate buffer interface

The reflection anisotropy spectroscopy profiles of a variant of cytochrome P450 reductase adsorbed at the Au(110)-phosphate buffer interface depend on the sequence of potentials applied to the Au(110) electrode. It is suggested that this dependence arises from changes in the orientation of the isoalloxazine ring structures in the protein with respect to the Au(110) surface. This offers a method of monitoring conformational change in this protein by measuring variations in the reflection anisotropy spectrum arising from changes in the redox potential.

Controlling the formation of a monolayer of cytochrome P450 reductase onto Au surfaces

The conditions necessary for the formation of a monolayer and a bilayer of a mutated form (P499C) of human cytochrome P450 reductase on a Au(110)/electrolyte interface have been determined using a quartz crystal microbalance with dissipation, atomic force microscopy, and reflection anisotropy spectroscopy (RAS). The molecules adsorb through a Au-S linkage and, for the monolayer, adopt an ordered structure on the Au(110) substrate in which the optical axes of the dipoles contributing to the RAS signal are aligned roughly along the optical axes of the Au(110) substrate. Differences between the absorption spectrum of the molecules in a solution and the RAS profile of the adsorbed monolayer are attributed to surface order in the orientation of dipoles that contribute in the low energy region of the spectrum, a roughly vertical orientation on the surface of the long axes of the isoalloxazine rings and the lack of any preferred orientation in the molecular structure of the dipoles in the aromatic amino acids. Our studies establish an important proof of principle for immobilizing large biological macromolecules to gold surfaces. This opens up detailed studies of the dynamics of biological macromolecules by RAS, which have general applications in studies of biological redox chemistry that are coupled to protein dynamics.

Spectral signatures of the surface reconstructions of Au(110)/electrolyte interfaces

It is demonstrated that the (1 × 1) structure and the (1 × 2) and (1 × 3) surface reconstructions that occur at Au(110)/electrolyte interfaces have unique optical fingerprints. The optical fingerprints are potential, pH and anion dependent and have potential for use in monitoring dynamic changes at this interface. We also observe a specific reflection anisotropy spectroscopy signature that may arise from anions adsorbed on the (1 × 1) structure of Au(110).

Adsorption of calf thymus DNA on Au(110) studied by reflection anisotropy spectroscopy

Reflection anisotropy spectroscopy (RAS) was used to investigate the adsorption of single-stranded (ss-) and double-stranded (ds-) calf thymus DNA on Au(110) in an electrochemical cell. Both types of DNA form ordered structures for electrode potentials in the range from +0.6 to -0.4 V. Both types of DNA desorb at -0.6 V and may start desorbing at lower negative potentials. When adsorbed at +0.6 V, both forms give rise to a similar RAS signal and adsorb through the phosphate groups. As the potential is reduced, the RAS intensity observed from ss-DNA increases to roughly twice that observed from ds-DNA, a result that is interpreted as due to a change in the adsorption of the ss-DNA from sites involving the phosphate groups to sites involving the bases.

Reflection anisotropy spectroscopy study of the adsorption of sulfur-containing amino acids at the Au(110)/electrolyte interface

Protein interactions with surfaces are key to understanding the behavior of implantable medical devices. The optical technique of reflection anisotropy spectroscopy (RAS) has considerable potential for the study of interactions between important biological molecules and surfaces. This study used RAS to investigate the adsorption of S amino acids onto Au(110) in a liquid environment under different conditions of potential and pH. Certain spectral features can be associated with the Au(110), as reported previously, while other features are assigned to bonds between the amino acids and the Au surface. The RA spectra are shown to be influenced by the structure of the amino acid, the solution pH, and the applied electrode potential. This work has assigned the negative feature at 2.5 eV to the Au-thiolate, bond while the positive feature at 2.5 eV is assigned to the disulfide bond. The broad spectral feature at 3.5 eV is attributed to the Au-amino interaction, while the sharper feature at slightly higher energy is associated with the Au-carboxylate interaction. Sulfur-containing amino acids are frequently found on the outside of protein molecules and could be used to anchor the protein to the surface.

Monitoring the transitions of the charge-induced reconstruction of Aau(110) by reflectance anisotropy spectroscopy

Missing-row reconstructions on Au(110) immersed in electrolytes have been studied by in situ reflectance anisotropy spectroscopy. Transitions between the 1 x 3, 1 x 2, and 1 x 1 surface structures were monitored as a function of the applied potential. A kinetic model allowed us to reproduce the data satisfactorily. These results confirm the theoretical predictions showing that the surface charge determines the surface reconstruction. The transition potentials and the activation barriers were determined.