Andrea Cadena-Caicedo, Beatriz Gonzalez-Cano, Rafael López-Arteaga
Dec 26, 2019
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Journal
The journal of physical chemistry. B
Abstract
β-Dihydronicotinamide adenine dinucleotide (NADH) plays a critical role in biological redox processes. Inside the cell, NADH can be in a folded or unfolded conformation depending on the chemical environment that surrounds it. It is known that selective excitation of adenine in NADH can induce energy transfer events from this nucleotide to the reduced nicotinamide chromophore. From the anticipated time scales, this process competes with adenine's ultrafast internal conversion channel, which is known to occur in the sub-picosecond time-scale for adenine. In this work we studied the dynamics of the excited states of both chromophores in NADH through the time resolution of the spontaneous emission from both chromophores. Through these experiments, we extend the knowledge about the competition between the different photophysical channels both in the folded and unfolded states. The study involved the folded and unfolded states of NADH through by experiments in water and methanol solutions. Our femtosecond fluorescence results were complemented by the first NMR magnetization transfer measurements on NADH which establish the solvent dependent folded vs unfolded states. We report measurements that determine the dynamics of the excited states by direct excitation of dihydronicotinamide at 380 nm, and adenine at 266 nm. We were able to measure for the folded state, a time constant of 90 fs for energy transfer through the time resolution of the emission from adenine in a spectral region where there is no interference from the other chromophore. Additionally, we determined that even in what is referred to as an unfolded state in methanol, non-negligible excitation transfer events do take place. Spontaneous emission anisotropy measurements were used in order to confirm the presence of a minor energy transfer channel in the methanol solutions where the unfolded state dominates.