New raman spectroscopy technique unveils subtle structural changes in porphyrin molecules

Recently, the research team led by Dr. HUANG Qing from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences, developed a new method using low-frequency Raman spectroscopy to detect very subtle changes in the structure of Porphyrin molecules - changes that are usually too small to be detected by traditional tools.

The relevant results were published in Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy.

The tiny, out-of-plane deformations of metalloporphyrin molecules are important for the biological functions of many enzymes. However, these deformations are so small and vary slightly between molecules, making them hard to detect with traditional methods. Unlike techniques like X-ray diffraction or nuclear magnetic resonance, resonance Raman spectroscopy is better at identifying structural features in these molecules. However, the relationship between specific deformations and the shifts in Raman peak frequencies has been unclear due to the complexity of the molecular interactions and the difficulty of measuring low-frequency Raman signals.

In this study, the team first employed nickel porphyrin as a model molecule and conducted density functional theory calculations to explore the correlation between two biologically relevant deformations (ruffling and saddling) and Raman spectral shifts. They then tested their approach on three different types of nickel porphyrins and successfully captured Raman spectra with extremely low frequencies (around 10 cm⁻¹), revealing clear patterns.

They found that all three porphyrins showed similar ruffling deformations, with only small differences in their saddling deformations. Most importantly, they discovered that the shift in a particular Raman peak (the γ18 peak) could be directly linked to the degree of saddling deformation in these molecules. This relationship means that researchers can now use the γ18 peak as a reliable indicator for detecting subtle structural changes in porphyrins.

This study not only shows how low-frequency Raman shifts can detect OOP deformations in porphyrin molecules caused by environmental changes, but also provides a useful tool to understand how these deformations affect their biological functions.

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