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The observation of D - spacing distribution by self - assembly of type I collagen alone is significant as it eliminates the necessity to invoke other preassembly or postassembly hypotheses , such as variation in the content of collagen types , enzymatic cross - linking , or other post - translational modifications , as mechanistic origins of D - spacing distribution .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
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