Effects of Conformational Diversity and Structural Dynamics on the Optical Properties of Diazoluminomelanin
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ABSTRACT Diazoluminomelanin (DALM) is a luminescent polymer belonging to the broader class of conjugated polyphenylene materials, which has shown significant optical activity in response to perturbing fields. In this paper we use semiempirical electronic structure calculations and molecular dynamics simulations to investigate the molecular structure and absorption characteristics of model phenolic oligomers. Molecular dynamics simulations show the polymer backbone can be extremely flexible depending upon the ionization state of the phenolic hydroxyl groups. The interring torsion angle is a critical variable as it relates charge localization effects and electronic excitation energies. Comparison with experimental data demonstrates the need for a multistate basis to describe the absorption properties of this system. INTRODUCTION Diazoluminomelanin (DALM) is an electroluminscent polymer formed biosynthetically in bacteria and mammalian cells or in the laboratory through a diazonium reaction with nitrotyrosine [1-3]. The sensitivity of the DALM to microwave radiation and its ability to be formed within cells, suggest that when the mechanisms of its luminescence are better understood, it may potentially be useful as a cellular dosimeter in measuring microwave absorption and as a molecular temperature probe [4,5]. Recent work by Wright [6] has suggested that the backbone of DALM has a poly(m-phenylene) structure, more specifically, a poly-tyrosine as shown in Figure 1-left. More recent electronic structure modeling has supported this proposed structure, and has found that the polyphenolic system can be a suitable model for many of the electronic properties of this material (Figure 1-right). CO O H CH -NH 2 CH 2
Figure 1 – Phenolic oligomers as models for the polytyrosine backbone of DALM.
OH
OH
OH
OH CH 2 CH -NH CO OH
2
OH
OH CH 2 CH -N H
2
CO O H
This material has a number of distinguishing characteristics to differentiate it from more conventional conjugated materials. The interring twist angle between adjacent subunits along the polymer backbone effectively limits the extent of electronic conjugation in these materials, and localizes the effects of electronic excitations [7].
However, these limited interring interactions also make the rotational energy barrier to interring motions low, making the DALM backbone capable of large fluxional motions [8]. In the polymeric material, side group interactions (hydrogen bonding by hydroxyl groups) of both intra and interchain natures would modulate these effects. Further, these side groups are potentially capable of acid ionization, which would create an anionic material. In this paper, we have investigated the electronic properties of the DALM system and the potential dynamical and chemical influences using electronic structure and molecular dynamics methods. Given the correlation between conjugative effects and optical properties, we have examined electronic transitions, polymer backbone flexibility, and the relative Bronsted acidity of the sidechain hydroxyl grou
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