Levitt M
Proc Natl Acad Sci U S A. 1978 Feb;75(2):640-4. doi: 10.1073/pnas.75.2.640.
Calculations on a 20-base pair segment of DNA double helix using empirical energy functions show that DNA can be bent smoothly and uniformly into a superhelix with a small enough radius (45 A) to fit the dimensions of chromatin. The variation of energy with the twist of the base pairs about the helix axis shows the straight DNA free in solution is most stable with about 10 1/2 base pairs per turn rather than 10 as observed in the solid state, whereas superhelical DNA in chromatin is most stable with about 10 base pairs per turn. This result, which has a simple physical interpretation, explains the pattern of nuclease cuts and the linkage number changes observed for DNA arranged in chromatin.
使用经验能量函数对一段20个碱基对的DNA双螺旋片段进行计算表明，DNA能够平滑且均匀地弯曲成半径足够小（45埃）的超螺旋，以适应染色质的尺寸。能量随碱基对围绕螺旋轴扭转的变化表明，溶液中自由的直链DNA每圈约有10.5个碱基对时最稳定，而不是像在固态中观察到的那样为10个碱基对，而染色质中的超螺旋DNA每圈约有10个碱基对时最稳定。这一结果具有简单的物理解释，解释了染色质中DNA的核酸酶切割模式和观察到的连接数变化。
