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A particularly common and important form of this self - assembly consists of type I collagen fibrils , which exhibit a nanoscopic signature , D - periodic gap / overlap spacing , with a distribution of values centered at approximately 67 nm .
 None
Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
 Entities:  
Answer:
 None