Unzipping of a double-stranded block copolymer DNA by a periodic force*

Using Monte Carlo simulations,

we study the hysteresis in unzipping of a double stranded block copolymer

DNA with −A_nB_n− repeat units. We define hetero-polymer DNA as a block

copolymer DNA of type (A_nB_n)_M , where A and B are two different types of

base pairs having two- and three-bonds, respectively, 2n is the total number

of base pairs in a block unit, also be called as block length, and M = N/2n

represents the total number of blocks in the DNA of length N . The end of

the DNA are subjected to a time-dependent periodic force with frequency

(ω) and amplitude (g₀ ) keeping the other end fixed. We find that the equi-

librium force-temperature phase diagram for the static force is independent

of the DNA sequence. The separation between the end monomers of the two

strands, x(t), changes under the influence of the applied external force g(t),

is monitored as a function of time t. For a periodic force case, the results

are found to be dependent on the block copolymer DNA sequence and also

on the base pair type on which the periodic force is acting. We observe hys-

teresis loops of various shapes and sizes and obtain the scaling of loop area

both at low- and high-frequency regimes.