Pasquale Bianco, Lorenzo Bongini, Luca Melli, Mario Dolfi and Vincenzo Lombardi
We study the kinetics of the overstretching transition in λ-phage double-stranded (ds) DNA from the basic conformation (B state) to the 1.7-times longer and partially unwound conformation (S state), using the dual-laser optical tweezers under force-clamp conditions at 25°C. The unprecedented resolution of our piezo servo-system, which can impose millisecond force steps of 0.5–2 pN, reveals the exponential character of the elongation kinetics and allows us to test the two-state nature of the B-S transition mechanism. By analyzing the load-dependence of the rate constant of the elongation, we find that the elementary elongation step is 5.85 nm, indicating a cooperativity of
25 basepairs. This mechanism increases the free energy for the elementary reaction to 94 kBT, accounting for the stability of the basic conformation of DNA, and explains why ds-DNA can remain in equilibrium as it overstretches.
DOI
We study the kinetics of the overstretching transition in λ-phage double-stranded (ds) DNA from the basic conformation (B state) to the 1.7-times longer and partially unwound conformation (S state), using the dual-laser optical tweezers under force-clamp conditions at 25°C. The unprecedented resolution of our piezo servo-system, which can impose millisecond force steps of 0.5–2 pN, reveals the exponential character of the elongation kinetics and allows us to test the two-state nature of the B-S transition mechanism. By analyzing the load-dependence of the rate constant of the elongation, we find that the elementary elongation step is 5.85 nm, indicating a cooperativity of
25 basepairs. This mechanism increases the free energy for the elementary reaction to 94 kBT, accounting for the stability of the basic conformation of DNA, and explains why ds-DNA can remain in equilibrium as it overstretches.DOI
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