Possibility of tracking imino–amino tautomerism of cytosine by ultra-short laser pulses using high-order harmonic generation

Extracting the dynamic structural information of molecules at the time resolution of tens to subtens of femtoseconds is extremely important for understanding many processes of chemistry and biology. In this paper, by theoretical calculations and simulation methods, we show the possibility of tracking the tautomerism of cytosine by ultra-short laser pulses using high-order harmonic generation. First, by analyzing the calculated high-order harmonic spectra emitted while a cytosine molecule in gaseous phase interacts with ultra-short laser pulses, we show that the dependence on molecular alignment is quite different for cases when cytosine is in the imino, the amino, or the transition states. This may serve to distinguish the imino–amino states of cytosine by ultra-short laser pulses. Second, by calculating high-order harmonic spectra emitted by cytosine while being along the chemical reaction path of the tautomerism, we obtain surprising and very interesting results. The high-order harmonic spectra have intensity peaks near the stable states (imino and amino) as well as the transition state. This finding may be a useful tool for tracking the DNA tautomerism, which is one of the reasons for gene mutation.