Complex sentence: The problem is that fluorescent labelled ligands have to bear a bulky fluorophore that may cause it to hinder the ligand binding, therefore the fluorophore used, length of the linker and its position must be carefully selected.
Split into simpler sentences: The problem is that fluorescent-labeled ligands have to bear a bulky fluorophore that may cause it to hinder the ligand binding. Therefore, the fluorophore used, length of the linker, and its position must be carefully selected.