In Situ Surface-Enhanced Raman Scattering Studies of the Nitrosyl Adduct of Hemin Adsorbed on Roughened Silver Surfaces in Aqueous Electrolytes

Abstract

Modifications in the electrochemical and spectral properties of hemin (Hm) adsorbed on roughened Ag electrodes in an aqueous electrolyte (phosphate buffer, pH 3) induced by brief exposure to mildly acidic solutions containing NO have been examined by in situ surface-enhanced Raman scattering (SERS) using Q band excitation (λexc = 532 nm). Two lines of evidence support the formation of the adsorbed nitrosyl adduct of Hm (NO−Hm) under the conditions employed for these experiments: the complete disappearance of the characteristic voltammetric redox peaks of Hm centered at ca. −0.25 V versus SCE, and a significant drop in the intensity, I, of the ring-based ν30 mode at 1165 cm-1 in the in situ SERS spectrum compared to that of adsorbed Hm under the same conditions. Such a decrease in the intensity of the I(ν30) mode was also found in the solution phase Raman spectrum of Hm in THF and DMF upon NO exposure. Notably absent from the SERS spectrum were features attributable to vibrational modes due to NO and Fe−NO, which are clearly discernible for NO iron macrocyclic adducts in solution phase using Soret band excitation (λexc = 413 and 406 nm). Scanning to sufficiently negative potentials led to the reemergence of the Hm redox peaks and restored the I(ν30) mode in the SERS spectrum recorded at potentials that were positive enough for the reduced form of Hm to undergo full oxidation. These observations clearly indicate that, to the level of sensitivity of these measurements, the formation and subsequent reduction of the adduct can be effected without compromising the integrity of Hm. Careful inspection of the data showed that the SERS spectrum of the NO−Hm adduct resembles that of adsorbed (axially uncoordinated) Hm, rather than its reduced counterpart, measured under otherwise identical conditions, suggesting that the electronic environment within the ring in the NO−hemin adduct and the (NO-free) oxidized Hm are similar. This behavior is in agreement with information derived from Mossbauer effect spectra for NO−Fe macrocyclic adducts.