Tris-cacodylate

Fig. 5. Effect of urea on extent of binding of methyl orange, at pH 7 and 25°C, by polyethylenimine with 8.4% of residues acvlated by lauroyl groups. (1) Tris-cacodylate buffer. 0.1 M. (2) Buffer and 6.0 M urea. (3) Buffer and 9.0 M urea.

Fig. 13. Schematic presentation of the courses of the concentration of Tris-cacodylate and their respective fluxes and of the resulting pH gradient. Microcomputer calculated flux values are also plotted, assuming for Tris (dashed lines) a mobility of 2.42 cm A -sec and a pfC of 8.3, and for cacodylate (solid lines) a mobility of 2.31 cm /V— secandapKof6.21. (From Bier etal., 1984. Reproduced with permission of the publisher.)...

Figure 15. Differential pulse polarograms of cytochrome c at various concentrations in 0.10 M Tris-cacodylate buffer, pH 6.05, and 0.10 M sodium perchlorate. Adapted from Reference (102) with permission.

Figure 16. Plot of the halt-wave potential of cytochrome c as a function of log l/[cytochrome c (fiM)] for different media (O) Tris-perchlorate, (A), Tris-acetate, (A), Tris-cacodylate (0.01 M, pH 7.6), ( +) [0.10 M Tris-cadocylate + 0.10 Msodium perchlorate], and ( ) Tris-chloride buffer (pH 6.05). Adapted from Reference (102) with permission.

Figure 17. Effect of pH variation on (A) the differential pulse polarograms and (B) on the 695 nm absorbance band of 114/xM cytochrome c. Solution contained mixed 0.01 M Tris-cacodylate and 0.10 M sodium perchlorate buffer/electrolyte. For a series of solution pH values, 6.4, 7.0, 7.6, 8.2, and 9.5, the arrows indicate the direction of change in the responses with increasing pH. Adapted from Reference (102) with permission.

Roller and Hawkridge used a tin-doped indium-oxide electrode and made measurements over the temperature range 5 to 75 °C at pH 7 using phosphate (I = 0.20 M) or Tris/cacodylate (I = 0.20 M) buffer media [86]. In further work they extended the pH range to 5.3 and 8.0 [87]. 

Figure 4. Linear sweep voltammograms (solid lines) at a PySSPy modified gold microelectrode array for the reduction of 44 pM cytochrome c in 0.2 M tris/cacodylic acid buffer at the scan rate of (a) 5 mVs , (b) 10 mVs i, (c) 20 mVs-, (d) 50 mVs i. cyclic voltammogram (dashed lines) at a PySSPy modified gold electrode with a large size (1.4 mm in diameter) was measured in the same solution for comparison.

The half maximum potentials of linear sweep voltammograms using this microelectrode array are constant at scan rates lower than 50 mVs . In this work all measurements were carried out with a scan rate of 10 mVs . The half maximum potential, Em/2, measured with a microelectrode is not equal to E<> or the mid-point potential, (Epa-Epc)/2, measured with a large electrode as shown in Figure 4. Using Equation (9) at a scan rate of 10 mVs i, with a disk microelectrode of 5.6 pm in radius and a diffusion coefficient of 1.1 x 10 cm s , gives an Es value of 6 mV. This is the same as the experimental value, the difference between 256 mV measured at a large electrode and 262 mV measured at a microelectrode array in 0.2 M tris/cacodylic acid buffer. 

Figure 7. Linear sweep voltammogram of cytochrome c at (a) PySSPy modified gold microelectrode array and (b) Tin doped In203 band microelectrode in 0.2 M ionic strength of tris/cacodylic acid buffer at scan rate of 10 mVs-. ...

The temperature dependence of the formal potential was studied at different concentrations of electrolytes. A linear relation was obtained at pH = 7.0 in the temperature range of 3-55 C. Figure 8 shows the temperature dependence of the half maximum potentials of cytochrome c in tris/cacodylic acid buffer at ionic strengths of 200, 20 and 5 mM. Since a nonisothermal cell was used, the temperature of the reference electrode was held constant. In this case, the entropy change (A Src ) for the reduction of ferricytochrome c can be given by the difference in entropy between ferro- and ferricytochrome c... 

Figure 8. Observed half maximum potential of cytochrome c as a function of temperature at the ionic strengths of tris/cacodylic acid buffer (a) 200 mM, (b) 20 mM and (c) 5 mM.

The following procedures are similar to those previously described . Prior to adsorption and voltammetric measurements, the tin oxide electrodes were cleaned by sequential ten-minute sonications in Alconox and isopropanol followed by two ten-minute sonications in Milli-Q water. The electrodes then were heated in an evacuated Vycor tube for two hours at 525 C. After cooling, the electrodes were transferred to a pH 7, 10 mM ionic strength tris/cacodylic buffer and allowed to equilibrate for at least 12 hours. 

Effect of Cytochrome c Lyophilization on Electron Transfer Kinetics in Tris/Cacodylate Media... 

In a prior publication, initial measurements of ket were reported for tris/cacodylate buffers of pH 6-8.5 and ionic strengths of 1-100 mM. The cytochrome c samples in those experiments had been chromatographically purified according to established procedures and then lyophilized for subsequent storage at -18 C. Solutions were then prepared directly from the lyophilized material. Since lyophilization has been shown to have very deleterious effects on the silver electrochemistry of cytochrome c and also results in the appearance of new chromatographic bands, we have repeated our earlier experiments using purified, non-lyophilized samples. 

Figure 1. Cyclic voltammograms in tris/cacodylate media of non-lyophilized cytochrome c adsorbed on tin oxide. See Experimental Section for adsorption details. Solution conditions tris/cacodylate, pH 8, 100 mM ionic strength. Scan rates (mV/s) (a) 50, (b) 20, (c) 10.

Tris/cacodylate buffers were used initially because their components do not specifically bind to cytochrome Phosphate, on the other... 

Figure 2. Plot of ket vs v for non-lyophilized cytochrome c at various tris/cacodylate solution conditions. See Experimental Section for adsorption conditions. Symbols represent an average of measurements taken at two separate electrodes under the same solution conditions. Relative standard deviations ranged from 0.31 to 0.02. Solution conditions (A) pH 6, 1.0 mM (A) pH 6,...

Unimolecular electron transfer kinetics for adsorbed cytochrome c in various phosphate media are reported in Table 1. Several points can be made. First, at constant pH, these rate constants are, without exception, larger than those obtained in the corresponding tris/cacodylate non-binding... 

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