## 1 Introduction  
In recent years, the utilization of fruits, especially berries, with a high amount of phytochemicals, such as polyphenols, anthocyanins and proanthocyanidins, has increased significantly [1, 2]. The black chokeberry _Aroria melanocarpa_ as well as _Aroria \(\times\) prunifolia_, a natural hybrid of _Aroria arbutifoliaia \(\times\) Aronia melanocarpa_, belong to this group of plants, the so-called superfoods [3]. Chokeberries have a high content of polyphenols; therefore, they are claimed to have health-promoting properties such as antioxidant, antimicrobial and antiproliferative effects [4, 5, 6]. This is why chokeberry extracts are also being used as a dietary supplement [7, 8]. The global market for dietary supplements has grown strongly in recent years, particularly in North America, Asia and Europe, which is why this sector has become the focus of increasing economic and scientific attention [9]. The polyphenol content of chokeberries is composed of flavonoids, branchols, phenolic acids, anthocyanins and, in particular, proanthocyanidins [10]. Proanthocyanidins are oligomers and polymers of polyhydroxylated flavan-3-ols linked by interflavan carbon-carbon bonds located at C4 \(\rightarrow\) C8 or at C4 \(\rightarrow\) C6 (B-type linkage) with a further possible ether bond between C2 and C7 (A-type linkage) (Figure 1B).  
The major group of OPCs are the procyanidins, which are formed of (epi)catechin monomeric units (Figure 1A). Less abundant are the prodelphinidins, which consist of (epi)gallocatechin units and the propelargonidins, which are composed of (epi)afzelechin monomers [12]. Due to the different monomers from which the OPCs can be composed and the two possible types of linkage between the subunits, OPCs have a very large structural diversity. Another factor for the huge diversity of this substance class is the different DP.  
Among the dark berries, chokeberries contain by far the highest amount of proanthocyanidins with up to 664 mg/100 g fresh weight, where mainly OPCs with a DP of 4-10 and >10 are represented. In comparison, the total proanthocyanidin content of blueberries and blackberries is only 27-148 mg/100 g fresh weight [13,14]. Based on the high content of OPCs, chokeberries are attributed strong antioxidant potential. Since oxidative damage, caused by reactive oxygen species (ROS), plays a role in aging and various associated degenerative diseases such as heart disease, cognitive disorders and cancer, antioxidant mechanisms are important to combat these diseases [15]. Eliminating these damaging ROS is possible with two approaches. Firstly, through endogenous antioxidants, which the human body produces itself, and secondly, through exogenous antioxidants, which must be obtained from the diet [16]. Endogenous antioxidants are enzymes such as glutathione peroxidase, superoxide dismutase and catalase which transform the ROS into more stable and usually less reactive species [17]. Exogenous antioxidants often include low-molecular-weight compounds such as vitamin C or E, carotenoids, glutathione or flavonoids. The antioxidant effect of proanthocyanidins is suggested due to the presence of vicinal hydroxy groups on the B-ring of the respective monomers, which are able to perform an H-atom transfer to the radicals and thus modify them [18]. According to this, it can be assumed that OPCs have a stronger antioxidant effect than monomers, as a larger number of hydroxy groups per mol are present.  
The aim of this study was to determine whether the highly mentioned antioxidant effects of OPC-rich extracts are related to the DP of OPCs present in _Arontia \(\times\) prunifolia 'Nero'_ using cell-free and cell-based test systems. OPCs were previously isolated and characterized via high-resolution mass spectrometry.