Study On DNA Condensed Conformation And Euler Critical Force Of DNA Molecule

For a long time,much attention has been paid to polyelectrolyte condensation for the complex behavior of counterion,namely how to balance the charge of electrolyte.The study of polyelectrolyte has been focused on the DNA condensation,and the single molecule experiments provide some meaningful results,such as the discontinuity of DNA condensation,theoretical work should be employed to reveal its physical mechanism.Based on the strong correlation of condensed counterion,a strong correlation model was established.And Monte Carlo simulation is used to study the properties of DNA condensation.During the condensation process,gyration radii of condensed conformation appear step-wise manner indicating the discontinuousness of process of DNA condensation.Autocorrelation of the segment reflects the number of loop structure in the condensed conformation,the condensed phase diagram shows the complexity of the condensed conformation,the majority of condensed conformations take on the complex flower structure,in comparison with flower structure,and the occurrence rate of the toroidal structure is small.Recently,the elastic force of 9.18-nm-length DNA detected by the fluorescence single molecule experiment is about 6pN.When the experimental error ± 5pN is considered,the force ranges from 1 to 11 pN,but its theoretical value is 25.4pN.The discrepancy between experimental values and theoretical ones will be resolved by using Monte Carlo simulation method as following,(1)The DNA micro-mechanical spring model has been established to study the Euler critical force of DNA molecule.The influence of temperature and kink structure on the Euler critical force of DNA molecule have been considered as the same time.Our simulation results show that temperature and kink structure will affect the Euler critical force: temperature can make DNA "soft",that is,the bending stiffness becomes smaller,so that the Euler critical force becomes small,at the same time,the formation of kink structure leads to the reduction of its force,The lower the excitation energy is,easier to form flexible hinge structure.(2)Based on the DNA micro mechanical model established above,considering the transition at joint point between the double-stranded DNA and a single stranded DNA that the double-stranded DNA is opened and transited to a single stranded,so that the length of single stranded DNA becomes longer,resulting in the decrease of the force.In this paper,we try to identify whether the difficulty of single molecule experiment originate from the transition at joint point.Our simulation results show that length of single strand DNA increases,resulting in the strand tension decreases,but the connecting points of double strand breaks this mechanism can make the measured tension in the range of 1~11pN,so the mechanism not the cause of single molecule fluorescence experiment theory value may be inconsistent with the experimental value of the mechanism.