An important topic in membrane biochemistry is the determination of the sidedness of the components comprising a biological membrane. Neither phospholipids nor membrane proteins are distributed symmetrically over inner and outer aspects of cellular membranes, and knowledge of the topological distribution is a prerequisite for understanding the functional properties of membranes. This chapter will deal with a recently developed, noninvasive technique to assess the distribution of phospholipids over the two leaflets of biological membranes with emphasis on the distribution of negatively charged phospholipids (1 ,2 ).
