# Picosecond time-resolved infra-red spectroscopic study of a water-soluble cationic copper-porphyrin with nucleic acids

## TL;DR

This study uses advanced spectroscopy to explore how a copper-based compound interacts with DNA, revealing how it binds to specific parts of nucleic acids.

## Contribution

The paper introduces new insights into the binding mechanisms of a copper-porphyrin with nucleic acids using picosecond time-resolved techniques.

## Key findings

- TRIR spectra show distinct features when CuTMPyP4 forms a five-coordinate exciplex with thymine-containing nucleic acids.
- The compound binds to the C2O carbonyl group of thymine in the oligonucleotide {d(CGCAAATTTGCG)}2.
- The study differentiates between the formation of a five-coordinate transient species and a four-coordinate triplet excited state.

## Abstract

The greater abundance and lower cost of copper make its porphyrin derivatives an attractive alternative to precious-metal based photosensitisers. The Cu(ii) complex of 5,10,15,20-meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (CuTMPyP4) is a useful photophysical probe for biomolecules. It is non-luminescent in aqueous solution, as it forms a metastable five-coordinate water complex after photo-excitation. CuTMPyP4 is known to bind to DNA both through the grooves and by intercalation. In this paper picosecond transient absorption (TA) and time-resolved infra-red (TRIR) spectra of CuTMPyP4 in D2O and in the presence of polydeoxythymidylic acid (poly(dT) and the double-stranded oligonucleotides {d(GC)5}2 and {d(CGCAAATTTGCG)}2 are reported. These spectra show characteristic features depending on whether a five-coordinate transient species or the four-coordinate triplet excited state is formed. Notably, in the case of thymine-containing nucleic acids the TRIR method unambiguously demonstrates the binding of the porphyrin to the C2

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O carbonyl group of that nucleobase.

TRIR spectra show distinctive features when CuTMPyP4 forms a five-coordinate ‘exciplex’ species, e.g. in D2O or with thymine in poly(dT), or the triplet excited state in {d(GC)5}2. They show binding to the C2O of thymine in {d(CGCAAATTTGCG)}2.