# The Characteristics of Intrinsic Fluorescence of Type I Collagen Influenced by Collagen I
## 1 Introduction  
Fluorescence spectroscopy is considered an effective method of studying the reactions of biomolecules. Owing to its simple operation, cheap instrumentation, high sensitivity and selection, it has been widely used in medicine, food, nutrition and many other fields [1]. [PERSON] [2] investigated the interaction between type I collagen and Hypocrellin B using fluorescence spectroscopy. The bioelectric property of collagen was explored, which resulted from the ideal molecular arrangement and potential intramolecular and intermolecular interactions. Furthermore, it was found the intrinsic fluorescence of type I collagen could be excited through hydrogen bonding and electrostatic forces [3]. [PERSON] proposed an alternative fluorescence method to measure the thermal stability of collagen mimic peptides [4]. Additionally, acid-soluble collagen, which was extracted from walleye pollock skin, was analyzed by the external fluorescence probe of pyrene, and the aggregation behavior of collagen was studied [5]. The aggregation concentration of collagen was revealed through the transient fluorescence decay study. There is potential for developing fluorescence spectroscopy technology for collagen mechanism research in different areas, including medicine, nutrition, food and agriculture.  
It is well known that the intrinsic fluorescence of proteins comes from aromatic amino acids, such as phenylalanine, tyrosine and tryptophan residues; this is a valuable probe to explore protein-proteininteractions apart from other external influences [6]. Due to a lack of tryptophan [7] and phenylalanine residues [8] in type I collagen, its intrinsic fluorescence may be a result of tyrosine. Up to now, type I collagen plays an essential role in embryonic development, tissue repair, wound healing and organ formation, owing to its unique biochemical characteristics [9; 10]. In recent years, the anomaly of type I collagen in connective tissue results in arterial stiffness, joint damage, reduced osteogenesis, decreased lung expansion and even neuropathy [11; 12]. Therefore, investigating collagen reaction mechanisms is significant and important. As is known, collagen structure can be changed by collagenase cleavage, which may influence the intrinsic fluorescence of collagen, especially in degradation pathways, observed by fluorescence spectroscopy. [PERSON] presented that collagenase I could hydrolyze type I collagen by reacting with diverse sites in type I collagen [13]. Thus, how the reaction between type I collagen and collagenase influences the intrinsic fluorescence of collagen deserves to be studied. It is significant and essential to characterize the behavior of not only collagenase but also other collagenolytic proteases.  
This paper studied the influence of collagenase I on collagen intrinsic fluorescence, and the mechanism is explained. Type I collagen was reacted with collagenase I; the intrinsic fluorescence intensity was evaluated and the interaction mechanism was further analyzed. The changes of collagen intrinsic fluorescence, which was influenced by the aggregation status of collagen molecules, was observed and analyzed. This method would provide the basis of researching the collagen reaction mechanism using fluorescence spectroscopy, and it is expected to guide the application of collagenase I in artificial skin, bone repair, collagen casing, animal food and cosmetic products.