# Antioxidant Activity of Sulfate Metabolites of Chlorogenic Acid
## 1 Introduction  
Recently, a vast number of studies on free radicals and oxidative stress have been conducted due to their roles in many noncommunicable diseases, i.e., cardiovascular diseases, Alzheimer's disease, cancer and skin aging [1; 2; 3; 4]. Studies have indicated that a diet rich in polyphenols has health-promoting properties and acts as a defensive shield in organisms fighting against degenerative processes in the human body. Phenolic acids are a group of polyphenols mainly occurring in honey, plants, food and beverages of plants origin [5; 6; 7]. The total intake of various polyphenols can be up to 1 g per diem, which is associated with a diet rich in fruits and vegetables [8; 9]. The esters formed between quinic acid and cinnamic acids such as caffeic and ferulic acids are called chlorogenic acids (CGAs). These compounds are among the most common plant polyphenols found in the human diet [10]. Therefore, in recent years, their metabolism has become a popular subject of research. Both in vivo and in vitro studies proved that CGAs are extensively metabolized in the human body [11; 12; 13; 14; 15]. The metabolism of CGAs includes hydrolysis to quinic acid and different hydroxycinnamates, mainly caffeic and ferulic acids, making them available for further absorption [16; 17; 18]. Additionally, various conjugated metabolites, such as glucuronides and/or sulfates of caffeic acids, have been detected in urine and plasma samples after CGA administration, while the precursor CGAs were found only in trace amounts [12; 13; 19; 20]. Moreover, previous in vitro and in vivo data highlight that the main metabolites excreted in urine after CGA administration were free hydrogenated analogues of caffeic and ferulic acids and their sulfate conjugates [12; 15; 20; 21; 22; 23]. For this reason, studies on biological responses cannot be carried out without taking into consideration that the metabolic processes of phenolic compounds may change their positive capacities. O-sulfation and O-glucuronidation reactions may significantly reduce or even inhibit antioxidant activity due to the blocking of hydroxyl groups, which are responsible for this property, in the phenolic structure [24; 25; 26; 27]. [PERSON] et al. [28] demonstrated that someglucuronwide and sulfate derivatives of quercetin retain antioxidant properties, although the magnitude of these properties is about half that measured for unconjugated quercetin. In addition, it was proven that the antioxidant activity of the purified CGA dimer was significantly enhanced after simulated digestion in vitro [29]. [PERSON] et al. [30] proved that \(O\)-sulfation of caffeic and ferulic acids leads to the inhibition of their antioxidant properties. In parallel, changes in hydrophobicity could significantly influence the reducing capacity of these compounds. Information on the antioxidant activity of phenolic metabolites seems to be key to understanding their potential effects on human health.  
In this paper, we aim at estimating the antioxidant properties of sulfate conjugates of caffeic and ferulic acids and, for the first time, the antioxidant properties of sulfate conjugates of dihydrocaffeic and dihydroferulic with regard to their precursor compounds. To our knowledge, there has never been a comprehensive screening study examining the antioxidant capacity of the major CGAs metabolites. As these compounds are not available as standards commercially, we synthesized them and purified the sulfate conjugates of the main metabolites of CGAs: ferulic, caffeic, dihydroferulic and dihydrocaffeic acids (Figure 1).