Sentence:  They were then centrifuged at 59780 rpm for 35 to 80 min at 20 ` C in a Spinco model E ultracentrifuge at a protein <WSD>concentration</WSD> of 1.00 to 1.25 % . 
Meaning of the word marked with <WSD> tags:  the strength of a solution; number of molecules of a substance in a given volume