Mechanical measurements of a single protein molecule and human chromosomes by atomic force microscopy

The force-distance mode of atomic force microscopy has been used to mechanically unfold sandwiched protein molecules between the tip and substrate. Under favorable conditions, average spring constants in the range of several tens of pN nm−1 were obtained for the stretching event of single protein molecules. Another series of force curves was recorded to perform indentation experiments on chromosomes, by pressing down an AFM tip beyond a soft chromosomal surface. The resulting force versus depth curves were analyzed to give estimates of the Youngʹs modulus of wet chromosomes in the range 0.5 − 1.1 × 105 N m−2 at neutral and alkaline pH, but it was 10 times higher in acetate buffer at pH 4.