instruction:
In this task, you are given a sentence from the research paper and your task is to classify the given sentence into the following categories: Background (Why is this problem important? What relevant works have been created before? What is still missing in the previous works? What are the high-level research questions? How might this help other research or researchers?), Purpose (What specific things do the researchers want to do? What specific knowledge do the researchers want to gain? What specific hypothesis do the researchers want to test?), Method (How did the researchers do the work or find what they sought? What are the procedures and steps of the research?), or Finding (What did the researchers find out? Did the proposed methods work? Did the thing behave as the researchers expected?). 
question:
crystallization , enzymatic characterization , and structure determination.
answer:
purpose


question:
a positive reaction could be obtained when an infected gill tissue sample that contained 430 viral copies/μg was diluted up to five orders of magnitude.
answer:
finding


question:
Optimization of each key step is extensively discussed and simplified protocols for library panning and Fab production are also described.
answer:

purpose