We describe a biophysical approach to follow changes in DNA structure during nucleosome formation in in vitro reconstitution with select DNA sequences. Our rapid photochemical technique (PhAST) provides snapshots' in time of changes in DNA configuration during nucleosome formation. Changes in DNA photochemical reactivity upon protein binding are correlated with alterations in individual base pair roll angles. This shows the importance of the role of an initial H3/H4 histone tetramer-DNA interaction and modulation of this early event by the DNA sequence. H3/H4 binding precedes and dictates subsequent H2A/H2B tetramer-DNA interactions, leading to the final octameric nucleosome. We provide a novel,
exciting way to investigate those biophysical properties of DNA that constitute a crucial component in
nucleosome formation and stabilization.