Development of Free Radical Products during the Greening Reaction of Caffeic Acid Esters (or Chlorogenic Acid) and a Primary Amino Compound
ESR spectra were measured directly on a marked greening reaction mixture of Et-caffeate and a primary amino compound in alkali solution under aeration. A clear hyperfine structure was commonly detected early in the greening reaction with different amino compounds. Its hyperfine spectrum split into seven peaks was analyzed and found to be due to the oxidized free radical product of the Et-caffeate using an authentic sample system. Another type of hyperfine ESR spectrum was observed later in the reaction, and was altered with different amino compounds. The hyperfine structure for n-butylamine split into 12 lines. The latter type of free radical products were assumed to be a semiquinone type radical compounds of the trihydroxy benzacridine derivative, which was identified as the principal structure of the green and yellow pigments formed by this greening reaction system. A formation mechanism of the green pigment and related products involving these free radical products is proposed.
Covalent Bonding of Chlorogenic Acid Induces Structural Modifications on Sunflower Proteins.
Proteins and phenols coexist in the confined space of plant cells leading to reactions between them, which result in new covalently bonded complex molecules. This kind of reactions has been widely observed during storage and processing of plant materials. However, the nature of the new complex molecules and their physicochemical properties are largely unknown. Therefore, we investigated the structural characteristics of covalently bonded complexes between sunflower protein isolate (SFPI, protein content 85 wt %) and the dominant phenol in the confined space of a sunflower seed cell (chlorogenic acid, CGA). It was shown that the efficiency of bond formation goes through a maximum as a function of the SFPI:CGA ratio. Moreover, the bonding of CGA with proteins resulted in changes in the secondary and tertiary structure of the protein. It was also shown that the phenol bound strongly to the protein, which resulted in new crosslinks between the polypeptide chains. As a result, secondary structures like α-helices and β-sheets diminished, which in turn resulted in more disordered domains and a subsequent modification of the tertiary structure of the proteins. These findings are relevant for establishing future protocols for extraction of high-quality proteins and phenols when utilizing plant material and offer insight into the impact of processing that these ingredients endure.
A MO theoretical study of organic dyes I. Effect of chemical softness on the electronic spectra
In order to obtain reasonable results for the π-electronic spectra of organic dyes with simple semi-empirical MO methods, a new approximation for the two-center electron repulsion integral (new gamma) is proposed which takes the chemical softness of the π-conjugated system into account. New gamma was successfully applied to PPP calculations of the electronic spectra of linear polyenes, simple cyanines, and simple merocyanines. The calculated results agree satisfactorily with the experimental data. On the contrary, a calculation based on the conventional PPP parameters gives shorter wavelengths for the absorption maxima of the electronic spectra of charge-resonance systems, such as cyanines and merocyanines. Furthermore, they give very strange electronic spectral shifts for merocyanines (see text), due to an inbalance between the one- and two-electron parts of the electronic transition energy.