Biomedical subjects

AK Gaigalas

Publications and source records attributed to AK Gaigalas.

3 recordsLinked to original sources

PubMed1998

Stark Spectroscopy of Tryptamine Immobilized on a Gold Electrode.

Electroreflectance (ER), ellipsometry, and surface-enhanced Raman spectroscopy (SERS) measurements were performed for immobilized tryptamine on polycrystalline gold electrodes. SERS study indicates that the indole ring does not interact directly with the gold surface. Ellipsometric measurements yielded a thickness of the tryptamine layer of (1.98 +/- 0.05) nm and an inferred surface concentration of 3.3 x 10(-6) mol/m2. ER measurements were performed in the wavelength region 200-300 nm using the NIST synchrotron ultraviolet radiation facility (SURF). The ER response was interpreted as a Stark shift in the tryptamine absorption line at 218 nm. Assuming that the tryptamine layer consists of two stacked tryptamine molecules associated at the indole rings, a unified analysis of ER and ellipsometric data gave the difference between the static dipole moments of the solvated ground and excited states of 0.65 x 10(-30) C m in low ionic strength buffer (0.01 M PBS) and 0.25 x 10(-30) C m in high ionic strength buffer (0.01 M PBS + 1.0 M NaClO4). The results point to the importance of the composition of the solvation shell. Semiempirical quantum chemistry calculations of tryptamine molecules gave a qualitative explanation of the ER response in the low ionic buffer. The large ER response suggests that the method could be applied to the study of the tryptophan environment in adsorbed proteins. Copyright 1998 Academic Press.

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PubMed1997

Measurement of Electron Transfer Rates between Adsorbed Azurin and a Gold Electrode Modified with a Hexanethiol Layer

In this work we report the application of electroreflectance (ER) spectroscopy for the study of the redox reaction of the nonheme protein azurin. Azurin was irreversibly adsorbed on a polycrystalline gold surface modified with a hexanethiol layer. Surface-enhanced resonant Raman spectroscopy (SERRS) demonstrated that the immobilized azurin's copper site remained intact upon adsorption and that it underwent reversible reduction-oxidation. Cyclic voltammetry (CV) data showed that the peak current of the oxidation (or reduction) wave depended linearly on the sweep rate as appropriate for an immobilized species. The formal potential of the adsorbed azurin was 95 ± 8 mV vs Ag/AgCl electrode, nearly identical to the earlier observed value of the native azurin in solution. All measurements were analyzed in the context of Marcus theory of electron transfer. The separation of the reduction-oxidation peaks in the CV data gave an estimate of the electron transfer (ET) rate in the range from 4 to 12 s-1. ER, with light of wavelength of 640 nm, yielded a strong signal at the same potential as the midpotential determined in CV. The frequency dependence of the ER response was consistent with an ET rate of 150 to 200 s-1. Electrochemical impedance measurements indicated an ET rate of the order of 300 s-1. The disparity between the ET rates measured with CV and ER suggests that ET may not be a single-step process. The results also point to the importance of hydrophobic interactions in adsorption and redox transformations of azurin. Copyright 1997Academic Press

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PubMed1997

Observation of Electron Transfer between a Silver Electrode and Putidaredoxin Using Electromodulated Reflectance Spectroscopy

This paper reports measurements of the redox reaction rate for the {2Fe-2S} protein puditaredoxin (Pdx) on bare and bekanamycin-modified silver electrodes. The electron turnover rates were determined using electroreflectance (ER) and cyclic voltammetry (CV). The measurements were analyzed with two kinetic models: (1) strongly adsorbed protein layer and (2) freely diffusing protein. For the adsorbed layer model, the rates obtained with ER were almost two orders of magnitude greater than the rates obtained with CV. For the freely diffusing model, the rates determined by the two methods were similar, suggesting that this kinetic model is more appropriate. Modification of the Ag electrode with bekanamycin resulted in a negative 0.1-V shift of the Pdx formal potential, but showed only a small increase in the observed redox reaction rates. The measured rate constants were compared with the predictions of the Marcus theory of electron transfer.

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