Bis-ANS is superior to 1-8-ANS as a probe for nonpolar cavities in proteins, often binding with an affinity that is orders-of-magnitude higher. Anilinonaphthalene sulfonates (ANS) are essentially nonfluorescent in water, only becoming appreciably fluorescent when bound to membranes. This property makes it a sensitive indicator of protein folding, conformational changes and other processes that modify the exposure of the probe to water.