# Antimicrobial and Odour Qualities of Alkylpyrazines Occurring in Chocolate and Cocoa Products
## 3 Antimicrobial Activity of Pyrazines
### Structure-Antimicrobial Activity Relationship of Alkylpyrazines  
[PERSON] [35], based on her research, hypothesized that differences in the antibacterial activity of alkylpyrazines would result from the substance's hydrophobicity. These conclusions were reached by comparing the MIC and MBC of a series of pyrazine derivatives with the corresponding structures' Log \(P\) values (partition coefficient, which is the ratio of concentrations of a compound in a mixture of two immiscible solvents at equilibrium). First, the Log \(P\) coefficient values and antibacterial activity against \(E\). \(coli\) and \(S\). \(aureus\) were ranked. Starting with the strongest antibacterial properties of 5-isobutyl-2,3-dimethylpyrazine (Log \(P\) = 2.32, MIC \(E\). \(coli\) = 3, MIC \(S\). \(aureus\) = 4), following 2-isobutyl-3-methylpyrazine (Log \(P\) = 1.96, MIC \(E\). \(coli\) = 3, MIC \(S\). \(aureus\) = 5), 2-isobutylpyrazine (Log \(P\) = 1.715, MIC \(E\). \(coli\) = 4, MIC \(S\). \(aureus\) = 8), 2-isopropyl-5-methylpyrazine (Log \(P\) = 1.626, MIC \(E\). \(coli\) = 5, MIC \(S\). \(aureus\) = 10) and ending with 2-isopropylpyrazine (Log \(P\) = 1.281, MIC \(E\). \(coli\) = 7, MIC \(S\). \(aureus\) =13) being the weakest. The more hydrophobic structures appear to achieve a limited suppression of microbial cells at lower doses, according to the correlation between those values. This theory was further supported by a comparison of 2,3-dimethylpyrazine's lower Log \(P\) and a lack of antibacterial capabilities to 2,5-dimethylpyrazine's stronger hydrophobic and antibacterial qualities. The notion that the action of pyrazines on the microbial membrane is what causes them to be germ destroying may then follow. It demonstrates that treating gram-positive \(S\). \(aureus\), which has a larger and stronger cell membrane, required higher amounts of the chemical compared with treating gram-negative \(E\). \(coli\). Wide hydrophobic surfaces are present in pyrazine compounds, and they vary depending on the substitution position in the molecule. Tetramethylpyrazine may successfully cross the brain's hematoencephal barrier, according to [PERSON] et al. [38]. Moreover, tetramethylpyrazine was transported via the transcellular pathway via a partition mechanism in an experiment by [PERSON] et al. [57], as was infiltrated buccal mucosa by a passive diffusion process. These results validate the idea that pyrazine chemicals readily dissolve in the lipid bilayers of biological membranes. [PERSON] et al. [58] compared substituents of tetramethylpyrazine, triethylpyrazine and tetraethylpyrazine and how they grow more complicated structurally and more lipophilic; these medications will partition into the lipid membrane and modify the fluidity of the membrane, which may result in a quantitative change in their biological activity, in addition to their interactions with membrane proteins. In a conducted study [36], 2-methylpyrazine, 2,5-dimethylpyrazine, 2,3-dimethylpyrazine and tetramethylpyrazine were positioned on various levels of the membrane depending on how hydrophobic they were. In the deepest area of the bilayer centre, the most hydrophobic tetramethylpyrazine was discovered. As a result of this, it may be assumed that the number and length of the substitutes of the pyrazine ring and the free volume distribution in the bilayer have an impact on pyrazine derivatives' lateral diffusion and the point of accumulation. By easily entering into the bilayer of microbial cell on different depths, due to high hydrophobicity and relatively tiny molecule dimensions, some alkylpyrazines may directly interact with membrane proteins, for example, G-protein, and cause membrane deformation through the modification of their lipid environment. Those actions may be damaging or lethal to bacteria cell [35; 36].  
Results from a literature analysis of the antimicrobial properties of alkylpyrazines from chocolate are summarized in Tables 2 and 3 below.