Effect on Redox Potential

Increased reducing power, i.e., increased antioxidant activity or more negative redox potential, is also one of the symptoms of the Maillard reaction (see Chapter 1). This has considerable significance, because, as far as foods are concerned, one of the main ways in which they deteriorate chemically is through oxidation, particularly of unsaturated fats and oils, leading to oxidative rancidity. The topic has considerable physiological significance as well (see Chapter 8). 

In the food field, attention has been paid to this aspect of the Maillard reaction for over 50 years, as can be seen from Table 9.1. However, it must not be assumed that the Maillard reaction is the only factor involved in producing the effects listed. 

Overall, there is no doubt that the Maillard reaction produces substances with an antioxidative effect, but it is by no means clear as yet how and to what extent that effect is exerted. Melanoidins not only have increased reducing power, but are able to complex metals and, in this way, lower their ability to act as catalysts in lipid and other oxidations. 

Determining antioxidative activity is one of the complicating factors. There are several methods and each deals with an aspect more or less different from those addressed by the others. Most of the methods that have been applied to Maillard products are briefly described in Table 9.2. 

Addition of 1.5% glucose plus glycine or corresponding melanoidin Addition of 5% glucose to dough to replace an equal amount of sucrose Addition of amino acids or protein hydrolysates to dough 

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Consequently, it is also apparent that the solvent effect can be described on the basis of mathematical relationships between parameters which fall within the relationships defined as free energy correlations. In fact, the more parameters that are included in the mathematical treatment (multi-parameter equations), the better the description of the solvent effect that results. However, we will consider here only those parameters which take into account the solvent effect on redox potentials. 

Rytdmaa, M., and Kinnunen, P.K.J., 1995, ReversibUity of the binding of cytochrome c to Uposomes. Implications for lipid-protein interactions./. B/oZ. Chem., 270 3197-3202 Salamon, Z., and ToUin, G., 1996, Surface plasmon resonance studies of complex formation between cytochrome c and bovine cytochrome c oxidase incorporated into a supported planar Upid bUayer. II. Binding of cytochrome c to oxidase-containing cardiohpin /phosphatidylcholine membranes. Biophys. J., 71 858-867 Salamon, Z., and ToUin, G., 1997, Interaction ofhorse heart cytochrome c with Upid bilayer membranes effects on redox potentials. J. Bioenerg. Biomembr. 29 211-221 Scarlett, J.L., and Murphy, M.P., 1997, Release of apoptogenic proteins from the... 

Substituent effects on redox potentials can be rationalized by free energy relationships in many cases Although 1 can be related to aromatic compounds, a linear correlation with Hammett s a-constants fails. As can be seen from the substituents -N(CH3)2 Ik) and -OCH3 11) the electron attracting effect is the dominating one and not the resonance effect. Therefore Taft s a "-constants describe the substituent effects more correctly. According to (1)... 

The association of sulfur and iron into simple to more complex molecular assemblies allows a great flexibility of electron transfer relays and catalysis in metalloproteins. Indeed, the array of different structures, the interactions with amino-acid residues and solvent and their effect on redox potential and spectroscopic signatures is both inspiring for chemists and electrochemists, and of paramount importance for the study of these centers in native conditions. Most of the simpler natural clusters have been synthesized and studied in the laboratory. Particularly, the multiple redox and spin states can be studied on pure synthetic samples with electrochemical and spectroscopic techniques such as EPR or Fe Mossbauer spectroscopy. More complex assembhes still resist structural... 

This chapter deals with the fundamental aspects of redox reactions in non-aque-ous solutions. In Section 4.1, we discuss solvent effects on the potentials of various types of redox couples and on reaction mechanisms. Solvent effects on redox potentials are important in connection with the electrochemical studies of such basic problems as ion solvation and electronic properties of chemical species. We then consider solvent effects on reaction kinetics, paying attention to the role of dynamical solvent properties in electron transfer processes. In Section 4.2, we deal with the potential windows in various solvents, in order to show the advantages of non-aqueous solvents as media for redox reactions. In Section 4.3, we describe some examples of practical redox titrations in non-aqueous solvents. Because many of the redox reactions are realized as electrode reactions, the subjects covered in this chapter will also appear in Part II in connection with electrochemical measurements. 

Solvent Effects on Redox Potentials and Redox Reaction Mechanisms... 

An approach to quantifying the interaction between solute and solvent and hence to solvent effects on redox potentials is that developed by Gutmann.41 Interactions between solvent and solute are treated as donor-acceptor interactions, with each solvent being characterized by two independent parameters which attempt to quantify the electron pair donor properties (donor number)... 

Formation of the MMOH-MMOR complex with or without MMOB has the opposite effect on redox potential from that described for the binding of MMOB. In this case, a stoichiometric concentration of MMOR... 

The solvatochromic method developed by Taft and coworkers [94] has also been used to explain the solvent effect on redox potentials. Lay [95] considered specific hydrogen bonding contributions to the thermodynamics and kinetics of electron transfer. 

Shimomura, M., Utsugi, K., Horikoshi, J., Okuyama, K., Hatozaki, O., and Okuyama, N. Two-Dimensional Ordering of Viologen Polymers Fixed on Chai d Surface of Bilayer Membranes A Peculiar Odd-Even Effect on Redox Potential and Adsorption Spectrum. Langmuir 7, 760 (1991). 

It seems apparent then, that the final form of iron in a food system should be directly influenced by the reduction potential of that system and anything which affects the reduction potential might affect the bioavailability of iron in the system. The enhancement of bioavailability of iron by the reducing compounds ascorbic acid and fructose is well known and has been discussed. However, it should be reemphasized that the addition of these, and other reducing agents, may in part increase iron bioavailability by their effect on redox potential as well as by their ability... 

At conditions where the thermodynamic activities of all substances are not unity (i.e., nonstandard conditions), such concentration effects on redox potential are expressed by the Nernst equation as follows ... 

Commonly, light enhances the rate of many reactions, probably because of the increased population of radicals and/or other favorable changes in the structure of melanin molecules. Thus, photoinduced changes have been reported in redox equilibria established between melanin and nitroxides (Section IV) (579) these may result from a photochemical effect on redox potentials of either one or both com-... 

F Xu, RM Berka, JA Wahleithner, BA Nelson, JR Shuster, SH Brown, AE Palmer, El Solomon. Site-directed mutations in fungal laccase Effect on redox potential, activity and pH profile. Biochem. J. 334(Pt. l) 63-70, 1998. 

Acenes, definition in terms of graphite translational periodicity axes, 369 Aggregation, effect on redox potentials, 16-21... 

Several different approaches have been developed for calculations of electrostatic energies in proteins. We have used the "Protein Dipoles, Langevin Dipoles" (PDLD) method (19,20), which has been applied to other related problems including electrostatic effects on redox potentials in cytochromes (21) and calculations of the energies of charge-transfer transitions in crystalline methylbacteriopheophorbide (22). The change in free energy associated with an electon-transfer reaction is writen as... 

In agreement with calculations, cyclovoltammetric data on selected thiophene-5,5-dioxides and -5-oxides showed that the addition of oxygen atoms to sulfur has a dramatic effect on redox potentials and hence on the HOMO and LUMO position and energy gap [12], Table 4.1 illustrates this effect, comparing the redox potentials of 2,5-bis(trimethylsilyl)thiophene with that of the corresponding 5-oxide and 5,5-dioxide. 

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