Cytochrome SERS spectra

The SERS spectrum of 2-PYCH at a silver electrode is shown in Figure 10 (curve A) and is basically the same as the normal Raman spectrum of a solid 2 PYCH. The adsorption of cytochrome c and 2-PYCH are competitive from the solution of (0.01 mM cytochrome c + 10 mM 2-PYCH) as shown in Figure 10 (curve B). Cytochrome c in the presence of 2-PYCH shows a voltammetric response visible at 0.08 V in cyclic voltammograms. In contrast to the results reported by Haladjian et al., similar voltammetric behavior of cytochrome c is observed at gold electrode in the presence of 2-PYCH. The role of 2-PYCH in the enhancement of the electrode reaction of cytochrome c is probably the same as that of 4-BIPY and 2-PYS. 

In addition to the effect of mutations at Phe-82 on the stability of the cytochrome c active site, the intense, negative Soret Cotton effect in the circular dichroism spectrum of ferricytochrome c is profoundly affected by the presence of non-aromatic amino acid residues at this position [115]. Recent examination of six position-82 iso-l-ferricytochrome c mutants establishes that while Tyr-82 exhibits a Soret CD spectrum closely similar to that of the wild-type protein, the intensity of the negative Soret Cotton affect varies with the identity of the residue at this position in the order Phe > Tyr > Gly > Ser = Ala > Leu > He, though the Ser, Ala, He, and Leu variants have effectively no negative Soret Cotton effect [108]. 

The electrode reaction of tetraheme cytochrome C3 adsorbed at the silver electrode surface was monitored by SERRS spectroscopy [104, 105] and compared with the results obtained by voltammetric techniques. The formal potential determined on the basis of the SERS measurement is more positive compared to the potential determined by the voltammetry, but it is in good agreement with the macroscopic formal potential of the heme-1. This is because cytochrome C3 is adsorbed on the silver electrode particularly via lysine residues surrounding heme-1, which is in fact responsible for the SERRS spectrum of this protein [105]. 

Figure 4 compares the normal Raman spectrum of a solid 4-BIPY with SERS of 4-BIPY at three different electrode potentials (+0.1, -0.2, and -0.5 V) on a silver electrode. It is quite obvious that 4-BIPY remains adsorbed on the silver electrode throughout this potential range as was shown by Cotton et al. . No shift in peak positions was observed. When the silver electrode is transferred into the cytochrome c solution, 4-BIPY molecules adsorbed on the silver electrode are partially displaced by cytochrome c molecules. That is, adsorption of cytochrome c and 4-BIPY on a silver electrode are competitive and the adsorption of 4-BIPY on a silver electrode is considered to be physical in nature rather than chemisorption. Both cytochrome c and 4-BIPY molecules are located within the range of - 0.5 nm from the electrode surface, since the SERS signals of both molecules are observable at a silver electrode. 

The SEES signals of 4-PYS are almost identical to the normal Raman spectrum of a solid 4-PYS. That is, the conformation of 4-PYS on the silver electrode is almost the same as that in the bulk. Since no SERS signals for cytochrome c are observed, as shown in Figure 6 (curve A), cytochrome c molecules are considered to be located at least more than 0.5 nm from the electrode surface. The adsorption of 4-PYS is so strong that cytochrome c pre adsorbed on the silver electrode may be completely displaced by 4-PYS and the adsorbed layer of 4-PYS is hardly displaced by cytochrome c (only small Raman peaks are observable when cytochrome c is present in the solution). This electrode exhibits a quasi-reversible voltammetric response at the potential corresponding to the formal potential of native cytochrome c. That is, cytochrome c remains immobilized on the 4-PYS modified electrode. The electron transfer reaction of... 

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