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Optical Backbone-Sidechain Charge Transfer Transitions in Proteins Sensitive to Secondary Structure and Modifications

Abstract

The absorption of light by proteins can induce charge transfer (CT) transitions in the UV-Visible range of the electromagnetic spectrum. Metal-ligand complexes or active site prosthetic groups which absorb in the visible show prominent CT transitions. Further, the protein backbone also exhibits CT transitions in the far UV range. In this manuscript, we present a detailed computational study of new near UV-Visible CT transitions which involve amino acids with charged side chains. Specifically, using time dependent density functional theory calculations, we examine the absorption spectra of naturally charged amino acids (Lys, Glu, Arg, Asp, His) extracted from solution phase protein structures generated by classical molecular dynamics simulations and phosphorylated amino acids (Tyr, Thr, Ser) from experimentally determined protein structures. We show that amino acids with charged sidechains present a directed electronic donor-bridge-acceptor paradigm, with the lowest energy optical excitations showing peptide backbone-sidechain charge separations. The UV-Visible spectral range of the backbone-sidechain CT transitions is sensitive to the chemical nature of the donor, bridge, and acceptor groups within each amino acid, amino acid conformation, and the protein secondary structure where the amino acids are located. Photoinduced CT occurs in opposite directions for the anionic and cationic amino acids along the ground state dipole moment vector for the chromophores. We find that photoinduced charge separation is more facile for the anionic amino acids (Asp, Glu, pSer, pThr, pTyr) relative to the cationic amino acids (Lys, Arg, Hsp). Our results provide a foundation for the development of spectroscopic probes based on the recently proposed Protein Charge Transfer Spectra (ProCharTS) relevant for the study of DNA-binding or intrinsically disordered proteins which are rich in charged amino acids.

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Publication details

The article was received on 02 Oct 2017, accepted on 30 Oct 2017 and first published on 30 Oct 2017


Article type: Paper
DOI: 10.1039/C7FD00203C
Citation: Faraday Discuss., 2017, Accepted Manuscript
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    Optical Backbone-Sidechain Charge Transfer Transitions in Proteins Sensitive to Secondary Structure and Modifications

    I. Mandal, S. Paul and R. Venkatramani, Faraday Discuss., 2017, Accepted Manuscript , DOI: 10.1039/C7FD00203C

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