The [Ru(CN)(5)(pyS)](4-) complex, an efficient self-assembled monolayer for the cytochrome c heterogeneous electron transfer studies

The organothiol 4-mercaptopyridine (pyS) has been used extensively as facil itator for the assessment of heterogeneous electron transfer reaction of cy tochrome c (cyt c). Its efficiency, however, is strongly affected by the in stability of the adlayer due to the C-S bond cleavage. The K-4[Ru(CN)(5)(py S)]. 3H(2)O complex was synthesized and characterized aiming its utilizatio n as an inorganic self-assembled monolayer (SAM) that would enhance the gol d adlayer stability. The SAM formed by this complex onto gold (RupySAu) was characterized by spectroscopic (FTIRRAS and SERS) and electrochemical (LSV ) techniques. The ex situ vibrational SERS and FTIRRAS spectra data of this SAM formed onto gold suggest a sigma interaction between the gold and sulf ur atoms of the complex, inducing a perpendicular arrangement in relation t o the surface normal. Additionally, SERS and FTIRRAS spectra performed for freshly prepared RupySAu adlayer and for large immersion times in the precu rsor solution have not shown any significant change that would reflect the degradation of the adlayer. The LSV desorption curves of this SAM indicate an enhancement in the C-S bond strength of the pyS ligand when coordinated to the [Ru(CN)(5)](3-) moiety. Comparatively to the data obtained for the d esorption process of the pyS monolayer, the reductive desorption potential, E-rd, of the RupySAu presents a shift of -17 mV. This bond strength intens ification leads to an increase in the stability of the monolayer. The volta metric curves of cyt c carried out with the RupySAu electrode showed electr ochemical parameters consistent with those reported for the native protein, as well as the maintenance of the electrochemical kinetic data after repet itive cycles. The results all together suggest that the pi back-bonding eff ect from the [Ru(CN)(5)](3-) metal center plays an important role in the st ability of the RupySAu adlayer, improving the assessment of the cyt c heter ogeneous electron transfer reaction.