NIR‐Raman spectrum and DFT calculations of okadaic acid DSP marine biotoxin microprobe

Commercially available diarrhetic shellfish poisoning (DSP) marine toxins are limited to micrograms samples invisible to the naked eye or in small amounts of micromole concentration solutions that are not suitable for normal Raman spectroscopy. As the Raman-derived techniques are increasingly employed in various detection schemes of harmful substances, Raman spectra of the target compounds are essential for molecular recognition, detection and sensing reasons. Using a new μ-RIM™ stainless steel hydrophobic substrate, we recorded near-infrared micro-Raman spectrum of okadaic acid (OA), a DSP marine biotoxin from 75 µg recrystallized toxin after drop coating deposition. Excitation with the 785-nm line allowed the recovery and assembling of the Raman spectrum over the 100–3200 cm−1 spectral range on several OA microparticles, while the 532-nm line excited the fluorescence emission that hampered the Raman signal. Density functional theory calculations were conducted on the isolated species both in gas phase and in ethanol solution to accurately assess and interpret the experimentally observed Raman bands. A good correlation between the experimental and theoretical Raman bands allowed for a reliable vibrational Raman assignment. Owing to the molecular geometry with intramolecular hydrogen bonds, the CC conjugated systems together with the methyl groups exhibited dominant OA Raman bands. Unlike domoic acid, an amnesic shellfish poisoning toxin whose carboxyl group showed the most intense Raman band, OA Raman characteristic band was not assigned to carboxyl group. Copyright © 2016 John Wiley & Sons, Ltd.