Maria De Lucia

The “Benzothiazine” Chromophore of Pheomelanins: A Reassessment†

The characteristic absorption and photochemical properties of pheomelanins are generally attributed to “benzothiazine” structural units derived biogenetically from 5‐S‐cysteinyldopa. This notion, however, conveys little or no information about the structural chromophores responsible for the photoreactivity of pheomelanins. At pH 7.4, natural and synthetic pheomelanins show a defined maximum around 305 nm, which is not affected by reductive treatment with sodium borohydride, and a monotonic decrease in the absorption in the range 350–550 nm. These features are not compatible with a significant proportion of structural units related to 2H‐1,4‐benzothiazine and 2H‐1,4‐benzothiazine‐3‐carboxylic acid, the early borohydride‐reducible pheomelanin precursors featuring absorption maxima above 340 nm. Rather, these features would better accommodate a contribution by the nonreducible 3‐oxo‐3,4‐dihydrobenzothiazine (λmax 299 nm) and benzothiazole (λmax 303 nm) structural motifs, which are generated in the later stages of pheomelanogenesis in vitro. This conclusion is supported by a detailed liquid chromatography/UV and mass spectrometry monitoring of the species formed in the oxidative conversion of 5‐S‐cysteinyldopa to pheomelanin, and would point to a critical reassessment of the commonly reported “benzothiazine” chromophore in terms of more specific and substantiated structural units, like those formed during the later stages of pheomelanin synthesis in vitro.

Plant catechols and their S-glutathionyl conjugates as antinitrosating agents: expedient synthesis and remarkable potency of 5-S-glutathionylpiceatannol.

With a view to elucidating the structural requisites for effective antinitrosating properties in plant polyphenolics and their metabolites, we have undertaken a comparative investigation of the nitrite scavenging effects of representative catechol derivatives of dietary relevance in the 2,3-diaminonaphthalene (DAN) nitrosation and tyrosine nitration assays. Compounds tested included caffeic acid (1), chlorogenic acid (2), piceatannol (3), hydroxytyrosol (4), and the corresponding S-glutathionyl conjugates 5-8, which were prepared using either tyrosinase (5 and 6) or a novel, o-iodoxybenzoic acid (IBX)-based oxygenation/ conjugation methodology (7b and 8). In the DAN nitrosation assay at pH 4.0, the rank order of inhibitory activities was found to be 5-S-glutathionylpiceatannol (7b) > 3 > 1 > 2 > 2-S-glutathionylcaffeic acid (5) > 2-S-glutathionylchlorogenic acid (6) > 4 approximately 5-S-glutathionylhydroxytyrosol (8). Quite unexpectedly, in the tyrosine nitration assay in 0.5 M HCl, 2 was the most efficient inhibitor followed by 1 > 4 > 3 > 7b approximately 5 > 8 > 6. Under the assay conditions, the glutathionyl conjugates were usually consumed at faster rates than the parent catechols (decomposition rates: 3 > 1 > 4 > 2). The 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) assay indicated that the most effective hydrogen donors were 4 > 7b > 1 approximately 3. Overall, these results indicated that catechol compounds and their glutathionyl conjugates may exhibit profoundly different inhibitory properties depending on the specific conditions of the assay, including especially pH, and that their antinitrosating properties do not correlate tout-court with their hydrogen donor capacity. The glutathionyl-piceatannol conjugate 7b was found to be one of the most potent inhibitors in the physiologically relevant DAN assay and may provide a new structural lead for the design of effective antinitrosating agents based on dietary polyphenolic compounds.

Differential Reactivity of Purified Bioactive Coffee Furans, Cafestol and Kahweol, with Acidic Nitrite: Product Characterization and Factors Controlling Nitrosation Versus Ring-Opening Pathways

Cafestol and kahweol, coffee-specific furan diterpenes, are believed to cause various physiological effects in human subjects, including an increase in cholesterol and plasma triacylglycerol levels as well as cancer chemopreventive effects. Despite the increasing interest in these compounds raised by the diverse range of biological activities, their reaction behavior and degradation pathways under physiologically relevant conditions remain uncharted. Herein, we report a detailed investigation of the structural modifications suffered by cafestol and kahweol in the presence of acidic nitrite under conditions mimicking those occurring in the stomach during digestion as well as by action of other oxidants. Prior to the chemical study, an isolation procedure for kahweol from green coffee beans was developed based on Soxhlet extraction followed by preparative HPLC. Preliminary experiments showed that kahweol is much more reactive than cafestol toward nitrite at pH 3, as evidenced by inhibition experiments with the 2,3-diaminonaphthalene assay as well as by product analysis in coffee extracts. When exposed to equimolar nitrite in phosphate buffer, pH 3, kahweol gave as a main product the ring-opened dicarbonyl derivative 1. Under more forcing conditions, cafestol reacted as well to give a main nitrogenous product identified as the 1-hydroxy-2-pyrrolinone 2. It is concluded that the conjugated double bond in kahweol is a critical structural element, increasing the susceptibility of the furan ring to protonation rather than nitrosation and favoring ring-opening routes driven by the irreversible oxidation steps. These results offer a useful background to assess the effects of coffee-specific lipids in association with abnormally high nitrite levels from the diet.

The Chemistry of Tyrosol and Hydroxytyrosol: Implications for Oxidative Stress

Publisher Summary Tyrosol (or (2-hydroxyethyl)phenol, p-hydroxyphenethyl alcohol, or 2-(4-hydroxyphenyl)ethanol) (TYR) and hydroxytyrosol (or 4-(2-hydroxyethyl)-1,2-benzenediol, or 3-hydroxytyrosol, or 2-(3,4-dihydroxyphenyl)ethanol) (HTYR) are the most representative phenolics of olive fruits and olive oil, where they occur as such, or in the form of esters of the secoiridoid elenolic acid. TYR is a colorless solid at room temperature, melting at 91–92°C, boiling at 158°C at 4 Torr, and slightly soluble in water. HTYR appears as a clear colorless liquid exhibiting a solubility in water of 5 g 100 mL–1 (25°C). The higher solubility in organic solvents of TYR with respect to HTYR is shown by the partitioning coefficients between oil and water phases determined as 0.077 and 0.010 for TYR and HTYR, respectively. From the structural viewpoint, these compounds share a phenolic functionality substituted at the para position with a hydroxyethyl chain. HTYR has an additional OH group on the benzene moiety at the position next to the other OH group and is therefore an ortho diphenol or catechol. Such structural modification results in dramatic differences in the susceptibility to oxidation and antioxidant power as well as in the potency of their chemopreventive efficacy under oxidative stress conditions. This chapter summarizes the main facts characterizing such processes associated to the onset and development of a number of diseases.

Chapter 134 – The Chemistry of Tyrosol and Hydroxytyrosol: Implications for Oxidative Stress

Publisher Summary Tyrosol (or (2-hydroxyethyl)phenol, p-hydroxyphenethyl alcohol, or 2-(4-hydroxyphenyl)ethanol) (TYR) and hydroxytyrosol (or 4-(2-hydroxyethyl)-1,2-benzenediol, or 3-hydroxytyrosol, or 2-(3,4-dihydroxyphenyl)ethanol) (HTYR) are the most representative phenolics of olive fruits and olive oil, where they occur as such, or in the form of esters of the secoiridoid elenolic acid. TYR is a colorless solid at room temperature, melting at 91–92°C, boiling at 158°C at 4 Torr, and slightly soluble in water. HTYR appears as a clear colorless liquid exhibiting a solubility in water of 5 g 100 mL–1 (25°C). The higher solubility in organic solvents of TYR with respect to HTYR is shown by the partitioning coefficients between oil and water phases determined as 0.077 and 0.010 for TYR and HTYR, respectively. From the structural viewpoint, these compounds share a phenolic functionality substituted at the para position with a hydroxyethyl chain. HTYR has an additional OH group on the benzene moiety at the position next to the other OH group and is therefore an ortho diphenol or catechol. Such structural modification results in dramatic differences in the susceptibility to oxidation and antioxidant power as well as in the potency of their chemopreventive efficacy under oxidative stress conditions. This chapter summarizes the main facts characterizing such processes associated to the onset and development of a number of diseases.

The Chemistry of Tyrosol and Hydroxytyrosol

Publisher Summary Tyrosol (or (2-hydroxyethyl)phenol, p-hydroxyphenethyl alcohol, or 2-(4-hydroxyphenyl)ethanol) (TYR) and hydroxytyrosol (or 4-(2-hydroxyethyl)-1,2-benzenediol, or 3-hydroxytyrosol, or 2-(3,4-dihydroxyphenyl)ethanol) (HTYR) are the most representative phenolics of olive fruits and olive oil, where they occur as such, or in the form of esters of the secoiridoid elenolic acid. TYR is a colorless solid at room temperature, melting at 91–92°C, boiling at 158°C at 4 Torr, and slightly soluble in water. HTYR appears as a clear colorless liquid exhibiting a solubility in water of 5 g 100 mL–1 (25°C). The higher solubility in organic solvents of TYR with respect to HTYR is shown by the partitioning coefficients between oil and water phases determined as 0.077 and 0.010 for TYR and HTYR, respectively. From the structural viewpoint, these compounds share a phenolic functionality substituted at the para position with a hydroxyethyl chain. HTYR has an additional OH group on the benzene moiety at the position next to the other OH group and is therefore an ortho diphenol or catechol. Such structural modification results in dramatic differences in the susceptibility to oxidation and antioxidant power as well as in the potency of their chemopreventive efficacy under oxidative stress conditions. This chapter summarizes the main facts characterizing such processes associated to the onset and development of a number of diseases.