Microscopic Mechanism of the Denaturation of Proteins and Nucleic Acids of Pseudomonas Aeruginosa Resonantly Absorbing 6-μm Femtosecond Laser Pulses

The samples of Pseudomonas aeruginosa (P. aeruginosa) plankton on an infrared-transparent silicon wafer have been studied for the first time by the method of dynamic transmission spectroscopy of 6-μm femtosecond laser pulses with a varying peak intensity within a range of (3−5) × 10² GW/cm², which is above the threshold of ≈ 3 × 10² GW/cm² for nondestructive inactivation of these bacteria. With an increase in the laser radiation intensity, the resonance harmonic absorption band of the C=O bond of the amide group of proteins and nucleic acids of the bacteria (at a wavenumber of 1650 cm^–1) is saturated, the blueshift of the band by \( \approx \)200 cm^–1 is then observed, and the subsequent anharmonic redshift of this band by up to 350–400 cm^–1 occurs. The quantum-mechanical analysis of vibrations of structurally similar molecules in the framework of vibrational second-order perturbation theory has shown that the position of the C=O band in the case of the blueshift corresponds to vibrations in the absence of hydrogen bonds, while its significant redshift is caused by anharmonicity up to 30 cm^–1 for the first vibrational quantum. Based on these data, we have suggested the molecular mechanism of the ultrafast destruction of the secondary structure of proteins and nucleic acids in the bacterium P. aeruginosa exposed to 6-μm femtosecond laser pulses through an instantaneous break of the hydrogen bond at the C=O mode during the absorption of the first infrared photon and permanent fixation of the break through high-level vibrational excitation at the threshold intensity of radiation.