## 1 Introduction  
Zinc, as an essential micronutrient, is critical to human health and involved in several biological processes, such as enzyme catalysis, DNA replication, RNA transcription, cellular signal transduction, and carbohydrate metabolism [1]. Zinc also plays significant roles in regulating cell growth, gene expression, and immune responses [2]. Non-digestible plant components as diverse as phytate and fiber can bind with zinc to form insoluble complexes, which can restrain zinc absorption in an organism, and calcium ions, high concentrations of ferrous ions, and folic acid are also perceived to inhibit zinc absorption during gastrointestinal tract digestion, thereby leading to zinc deficiency [1; 2; 3]. Symptoms of zinc deficiency, including growth defects, gonadal dysfunction, dermal and immune alterations, and neurological dysfunctions [4; 5] are widespread in developing counties. Because of its low bioavaiability and its ability to cause gastrointestinal tract irritation after long-term consumption [6], the use of zinc salt as a general source of zinc supplement has decreased. Accordingly, it is particularly important to determine alternative methods to enhance the absorption and the bioaccessibility of zinc.  
Research on food-derived nutritional supplements has significantly risen in recent years. Compared with zinc salt and zinc-protein complexes, food-derived chelating peptides possess metal ligands such as His, Cys, Asp, Glu, and Ser, which can bind with zinc to form soluble complexes with higher dietary zinc absorption and bioaccessibility [2,7,8]. For example, the solubility of the oyster protein hydrolysate-zinc complex is markedly increased compared with that of ZnSO4 in various buffer solutions and simulated gastrointestinal digestion conditions, which indicates that this protein hydrolysate-zinc complex can enhance zinc bioaccessibility [9]. Whey-derived peptides with high-magnitude net negative surface charge have strong zinc chelation capacity with high gastric stability, which can enhance zinc delivery. However, their zinc bioaccessibility is reduced during gastrointestinal (GI) digestion. The stability of the peptide-zinc complex affects the release of zinc during gastric digestion, and zinc ion could combine with phytic acid upon entering the duodenum, thereby inhibiting zinc absorption and bioaccessibility [10].  
The structural properties of peptides, such as their molecular weight and amino acid residues, are related to their metal-chelating activity, which can affect the bioaccessibility of the metal. A previous study confirmed that the contents of His, Glu, Cys, and Asp in peptides contribute to the metal-chelating activity [11]. The molecular weight of the peptide also influences the metal-chelating activity. However, many studies have reported that both low molecular weight peptides and high molecular weight peptides exhibit higher metal-chelating activity [12,13]. These inconsistent results are still not completely understood.  
One of the largest freshwater cultured fish in China is tilapia. The processing utilization rate is only 30-40%, and a massive amount of non-edible by-product, including fish skin, a potential source of collagen, is produced. The use of tilapia skin-derived collagen poses no risk of disease transmission and faces no religious barriers, and this high-yield product can serve as an excellent substrate for the production of peptides [14]. Many studies have focused on the purification of metal-chelating collagen peptides and the preparation and analysis of peptide-zinc complexes [9,11,15]. However, there is little information on the relationship between the molecular weight and zinc-chelating activity of collagen peptide fractions. Therefore, this study aimed to investigate the role of the molecular weight of collagen peptide fractions in their zinc chelation capacity and the zinc solubility. Furthermore, the stability and zinc bioaccessibility of their zinc complexes in vitro were also assessed.