Sentence:  If tyrosine and a system generating hydrogen peroxide are added to a cell-free homogenate of the thyroid , large quantities of <WSD>free</WSD> mono-iodotyrosine can be formed ( Alexander , 1959 ) . 
Meaning of the word marked with <WSD> tags:  unconstrained or not chemically bound in a molecule or not fixed and capable of relatively unrestricted motion