The interaction of doxorubicin (DX) with model polynucleotides poly(dG-dC)·poly(dG-dC) (polyGC), poly(dA-dT)·poly(dA-dT) (polyAT), and calf thymus DNA has been studied by several spectroscopic techniques in phosphate buffer aqueous solutions. UV-vis, circular dichroism, and fluorescence spectroscopic data confirm that intercalation is the prevailing mode of interaction, and also reveal that the interaction with AT-rich regions leads to the transfer of excitation energy to DX not previously documented in the literature. Moreover, the DX affinity for AT sites has been found to be on the same order of magnitude as that reported for GC sites.

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The dynamics of a single qubit interacting by a sequence of pairwise collisions with an environment consisting of just two more qubits is analyzed. Each collision is modeled in terms of a random unitary operator with a uniform probability distribution described by the uniform Haar measure. We show that the purity of the system qubit as well as the bipartite and the tripartite entanglement reach time averaged equilibrium values characterized by large instantaneous fluctuations. These equilibrium values are independent of the order of collision among the qubits. The relaxation to equilibrium is analyzed also in terms of an ensemble average of random collision histories. Such average allows for a quantitative evaluation and interpretation of the decay constants. Furthermore a dependence of the transient dynamics on the initial degree of entanglement between the environment qubits is shown to exist. Finally the statistical properties of bipartite and tripartite entanglement are analyzed.Introduction. -The repeated collision model has been recently used in literature to analyze the irreversible dynamics of a qubit interacting with a reservoir consisting of a large number of environmental qubits. In particular processes like thermalization [1] and homogenization [2-5], have been analytically investigated. The same model has ben used recently also to analyze the dynamics of a qubit interacting with a very small environment consisting of just two qubits [6]. The interest for such system is due to the fact that, at variance with what happens in the case of an environment with a large number of degrees of freedom, the system dynamics cannot be described by a Markovian master equation. Indeed, due to the fact that the system qubit collides repeatedly with the same environment qubits, the dynamics is characterized by large fluctuations and only when the sequence of collision is random a time averaged equilibrium is reached. While in all the above mentioned papers the -elastic -collisions have been modeled by a partial swap unitary operator, in the present paper we will analyze the system dynamics in the case in which the two-qubit collisions are described by random

We analyze the dynamics of a system qudit of dimension \mu sequentially interacting with the \nu-dimensional\ud qudits of a chain playing the role of an environment. Each pairwise collision has been modeled as a random\ud unitary transformation. The relaxation to equilibrium of the purity of the system qudit, averaged over random\ud collisions, is analytically computed by means of a Markov chain approach. In particular, we show that the\ud steady state is the one corresponding to the steady state for random collisions with a single environment qudit\ud of effective dimension \nu e=\nu\mu. Finally, we numerically investigate aspects of the entanglement dynamics for\ud qubits (\mu =\nu=2) and show that random unitary collisions can create multipartite entanglement between the\ud system qudit and the qudits of the chain