Studies on external electric field effects on absorption and fluorescence spectra of NADH

Two-Photon Excited Fluorescence of NADH-Alcohol Dehydrogenase Complex in a Mixture with Bacterial Enzymes

Thorough study of composition and fluorescence properties of a commercial reagent of active equine NAD-dependent alcohol dehydrogenase expressed and purified from E. coli has been carried out. Several experimental methods: spectral- and time-resolved two-photon excited fluorescence, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, fast protein liquid chromatography, and mass spectrometry were used for analysis. The reagent under study was found to contain also a number of natural fluorophores: free NAD(P)H, NADH-alcohol dehydrogenase, NADPH-isocitrate dehydrogenase, and pyridoxal 5-phosphate-serine hydroxymethyltransferase complexes. The results obtained demonstrated the potential and limitations of popular optical methods as FLIM for separation of fluorescence signals from free and protein-bound forms of NADH, NADPH, and FAD that are essential coenzymes in redox reactions in all living cells. In particular, NADH-alcohol dehydrogenase and NADPH-isocitrate dehydrogenase complexes could not be optically separated in our experimental conditions although fast protein liquid chromatography and mass spectrometry analysis undoubtedly indicated the presence of both enzymes in the molecular sample used. Also, the results of fluorescence, fast protein liquid chromatography, and mass spectrometry analysis revealed a significant contribution of the enzyme-bound coenzyme pyridoxal 5-phosphate to the fluorescence signal that could be separated from enzyme-bound NADH by using bandpass filters, but could effectively mask contribution from enzyme-bound FAD because the fluorescence spectra of the species practically overlapped. It was shown that enzyme-bound pyridoxal 5-phosphate fluorescence can be separated from enzyme-bound NAD(P)H and FAD through analysis of short fluorescence decay times of about tens of picoseconds. However, this analysis was found to be effective only at relatively high number of peak photon counts in recorded fluorescence signals. The results obtained in this study can be used for interpretation of fluorescence signals from a mixture of enzyme-bound fluorophores and should be taken into consideration when determining the intracellular NADH/FAD ratio using FLIM.

Two-Photon Excited Fluorescence Dynamics in Enzyme-Bound NADH: the Heterogeneity of Fluorescence Decay Times and Anisotropic Relaxation

The dynamics of polarized fluorescence in NADH in alcohol dehydrogenase (ADH) in buffer solution has been studied using the TCSPC spectroscopy. A global fit procedure was used for determination of the fluorescence parameters from experiment. The interpretation of the results obtained was supported by ab initio calculations of the NADH structure. A theoretical model was developed describing the polarized fluorescence decay in ADH-NADH complexes that considered several interaction scenarios. A comparative analysis of the polarization-insensitive fluorescence decay using multiexponential fitting models has been carried out. As shown, the origin of a significant enhancement of the decay time in the ADH-NADH complex can be attributed to the decrease of nonradiative relaxation rates in the nicotinamide ring in the conditions of the apolar binding site environment. The existence of a single decay time in the ADH-NADH complex in comparison with two decay times observed in free NADH was attributed to a single NADH unfolded conformation in the ADH binding site. Comparison of the experimental data with the theoretical model suggested the existence of an anisotropic relaxation time of about 1 ns that is related with the rotation of fluorescence transition dipole moment due to the rearrangement of the excited state NADH nuclear configuration.

Two-Photon Excited Fluorescence Dynamics in NADH in Water-Methanol Solutions: The Role of Conformation States

The dynamics of polarized fluorescence in reduced nicotinamide adenine dinucleotide (NADH) at 460 nm under two-photon excitation at 720 nm by femtosecond laser pulses in water-methanol solutions has been studied experimentally and theoretically as a function of methanol concentration. A number of fluorescence parameters have been determined from experiment by means of the global fit procedure and then compared with the results reported by other authors. A comprehensive analysis of experimental errors was made. Ab initio calculations of the structure of NADH in water and methanol and of β-nicotinamide mononucleotide (NMNH) in vacuum have been carried out for clarifying the role of decay time heterogeneity. The main results obtained are as follows. An explanation of the heterogeneity in the measured fluorescence decay times in NADH has been suggested based on the influence of the internal molecular electric field in the nicotinamide ring on nonradiative decay rates. We suggest that different charge distributions in the cis and trans configurations result in different internal electrostatic field distributions that lead to the decay time heterogeneity. A slight but noticeable rise of the fluorescence decay times τ₁ and τ₂ with methanol concentration was observed and treated as a minor effect of a nonradiative relaxation slowing due to the decrease in solution polarity. Relative concentrations of the folded and unfolded NADH conformations in solutions have been determined using a new method of analysis of the rotational diffusion time τ_r as a function of methanol concentration on the basis of the Stokes-Einstein-Debye equation. The analysis of the fluorescence anisotropy parameters obtained under linearly and circularly polarized excitation and the parameter Ω has been carried out and resulted in the determination of the two-photon excitation tensor components and suggested the existence of two excitation channels with comparable intensities. These were the longitudinal excitation channel dominated by the diagonal tensor component S_zz and the mixed excitation channel dominated by the off-diagonal tensor components |S_xz² + S_yz²|^1/2.

Characterization of endogenous fluorescence in nonsmall lung cancerous cells: A comparison with nonmalignant lung normal cells

Monitoring fluorescence properties of endogenous fluorophores such as nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD) in normal and cancerous cells provide substantial information noninvasively on biochemical and biophysical aspects of metabolic dysfunction of cancerous cells. Time-resolved spectral profiles and fluorescence lifetime images of NADH and FAD were obtained in human lung nonsmall carcinomas (H661 and A549) and normal lung cells (MRC-5). Both fluorophores show the fast and slowly decaying emission components upon pulsed excitation, and fluorescence spectra of NADH and FAD show blue- and red-shifts, respectively, during their decay. All identified lifetime components of NADH and FAD were found to be shorter in cancerous cells than in normal cells, no matter how they were measured under different extra-cellular conditions (cells suspended in cuvette and cells attached on glass substrate), indicating that the changes in metabolism likely altered the subcellular milieu and potentially also affected the interaction of NADH and FAD with enzymes to which these cofactors were bound. The intensity ratio of NADH and FAD of cancerous cells was also shown to be larger than that of normal cells.

Ultrafast Fluorescence Signals from β-Dihydronicotinamide Adenine Dinucleotide: Resonant Energy Transfer in the Folded and Unfolded Forms

β-Dihydronicotinamide adenine dinucleotide (NADH) plays a critical role in biological redox processes. Inside the cell, NADH can be in a folded or an 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 must compete with adenine's internal conversion channel, which is known to occur in the sub-picosecond time scale. 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 nucleotides. 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 by experiments in water and methanol solutions. Our femtosecond fluorescence results were complemented by the first nuclear magnetic resonance through space magnetization transfer measurements on NADH, which establish the solvent-dependent folded versus unfolded states. We determined the dynamics of the excited states by direct excitation of dihydronicotinamide at 380 nm and adenine at 266 nm. From this, we were able to measure for the folded state, a time constant of 90 fs for energy transfer. 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.

Polarized Two-Photon Absorption and Heterogeneous Fluorescence Dynamics in NAD(P)H

Two-photon absorption (2PA) finds widespread application in biological systems, which frequently exhibit heterogeneous fluorescence decay dynamics corresponding to multiple species or environments. By combining polarized 2PA with time-resolved fluorescence intensity and anisotropy decay measurements, we show how the two-photon transition tensors for the components of a heterogeneous population can be separately determined, allowing structural differences between the two fluorescent states of the redox cofactor NAD(P)H to be identified. The results support the view that the two states correspond to alternate configurations of the nicotinamide ring, rather than folded and extended conformations of the entire molecule.

Application of Fluorescence Lifetime Imaging (FLIM) to Measure Intracellular Environments in a Single Cell

Fluorescence lifetime imaging (FLIM) has now been used in many bioscience fields, which comes from the quantification of fluorescence lifetime. The procedure for obtaining lifetime images is very similar to that used in fluorescence microscopy. However, obtaining reliable lifetime images requires an understanding of the theory of fluorescence lifetime, principle of FLIM systems, and evaluation procedure of intracellular environments. In this chapter, the materials, methods, and notes on FLIM measurements have been described, in conjunction with a brief explanation of the background of FLIM.

Kinetics of Light-Induced Intramolecular Energy Transfer in Different Conformational States of NADH

When bound to a protein, the coenzyme NAD+/NADH typically exists in an extended conformation, while in aqueous solutions it can be characterized by an equilibrium of folded and unfolded structures. It was recognized long ago that in the folded conformation light absorption at the adenine ring initiates an effective energy transfer (ET) toward the nicotinamide group, but the mechanism of this process is still unexplored. Here we apply ultrafast transient absorption measurements on NADH combined with compartmental model analysis for following the kinetics of the ET. We find that the actual ET is extremely rapid (∼70 fs). The high rate can be well described by a Förster-type mechanism, promoted by both the special photophysical properties of adenine and the subnanometer inter-ring distance. The rapid ET creates a vibrationally hot excited state on nicotinamide, the vibrational and electronic relaxation of which is characterized by 1.7 and 650 ps, respectively.

Effects of Nanosecond Pulsed Electric Fields on the Intracellular Function of HeLa Cells As Revealed by NADH Autofluorescence Microscopy

The fluorescence lifetime of the endogenous fluorophore of reduced nicotinamide adenine dinucleotide (NADH) in HeLa cells is affected by the application of nanosecond pulsed electric fields (nsPEFs). In this study, we found that after nsPEF application, the fluorescence lifetime became longer and then decreased in a stepwise manner upon further application, irrespective of the pulse width in the range of 10-50 ns. This application time dependence of the NADH fluorescence lifetime is very similar to the time-lapse dependence of the NADH fluorescence lifetime following the addition of an apoptosis inducer, staurosporine. These results, as well as the membrane swelling and blebbing after the application of nsPEFs, indicate that apoptosis is also induced by the application of nsPEFs in HeLa cells. In contrast to the lifetime, the fluorescence intensity remarkably depended on the pulse width of the applied nsPEF. When the pulse width was as large as 50 ns, the intensity monotonically increased and was distributed over the entire cell as the application duration became longer. As the pulse width of the applied electric field became smaller, the magnitude of the field-induced increase in NADH fluorescence intensity decreased; the intensity was reduced by the electric field when the pulse width was as small as 10 ns. These results suggest that the mechanism of electric-field-induced apoptosis depends on the pulse width of the applied nsPEF.