4. What is the valency of cation in K2504 m
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Answers
Answer:
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Explanation:
It has been established that, upon the addition of multivalent cations, long DNA chains in an aqueous solution exhibit a remarkable discrete transition from a coil state to a compact state at the level of a single chain. In this study, we investigated the polyelectrolyte nature of DNA with the experimental methodology of single-DNA observation, and provide a theoretical interpretation. We examined the effects of co-ions with different valencies (Cl−, SO42−, PO43−) on DNA compaction. As a result, we found that co-ions with a greater valency induce the coil state rather than the compact state. Based on a simple model with mean-field approximation that considered ion pairing, we show how the increase in entropy of small ions contributes to the stability of the compact state, by overcoming entropic penalties such as elastic confinement of the chain and a decrease in the translational freedom of counterions accompanied by charge neutralization.
Introduction
In a many-body Coulomb system, the coexistence of asymmetric charges with different sizes and valencies is a fascinating subject. A typical example is a living cellular system, in which negatively charged DNA molecules are confined into a small space that contains various small ions to counter the negative charge of DNA. Therefore, it is necessary to gain insight into the structure of DNA in terms of a multiple-component asymmetric Coulomb system. In addition, our understanding of the folding transition of a polyelectrolyte is still primitive, compared to what we know of the manner of dispersion/aggregation of charged colloids (1,2).
Through in vitro experiments, it has been shown that the addition of multivalent cations induces a remarkable compacting transition of DNA accompanied by a change in volume on the order of 104 (3,4). The minimum system for observing such a transition consists of DNA, water, multivalent counterions, and co-ions. Usually, monovalent counterions, such as potassium and sodium ions, are also added to mimic the environment of an intracellular solution.
Moreover, based on monomolecular observations of DNA by fluorescent microscopy, it has been confirmed that a single DNA chain undergoes a large discrete compacting transition (5). This transition is characterized as a first-order phase transition under the criterion of Landau's symmetry argument, which reflects the semiflexible nature of a giant DNA molecule. Such a characterization is also supported by experimental observations that compact DNA takes an ordered state, while coiled DNA takes a disordered state (6,7). The elastic entropy of the chain in the disordered coil state is clearly greater than that in the ordered compact state. At first glance, this seems to give a situation in which a compact phase appears at low temperature in a phase diagram. However, in DNA compaction induced by multivalent cations, the compact state is more stable than the coil state when the temperature is increased (8–10), which means that the ordered compact state shows greater entropy than the elongated coil state. This discrepancy indicates that some other essential factors influence this phenomenon.