Nanosecond time-resolved circular polarization of fluorescence: study of NADH bound to horse liver alcohol dehydrogenase.

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Circularly polarized luminescence (CPL) spectroscopy provides information on the excited-state chirality of a lumiphore analogous but complementary to information regarding the ground-state chirality derived from circular dichroism. The sensitivity of CPL spectra to molecular conformation makes this technique uniquely suited for the study of biomolecular structure, as extensively demonstrated in earlier studies. Unfortunately, the CPL spectra of many biomolecules often contain significantly overlapping contributions from emitting species either because multiple lumiphores are present (e.g., tryptophan residues in a protein) or because multiple conformations of the biomolecule simultaneously exist, each with a unique CPL spectrum. Increased resolution between individual contributions to the CPL may be achieved by time-resolving this signal, thus taking advantage of the fact that, as a rule, each of the emitting species also has a characteristic decay time associated with its electronically excited state. In addition, the time resolution provides information regarding dynamics associated with the different chiral states of the system. The present study describes an instrument for the determination of time-resolved CPL (TR-CPL) with subnanosecond resolution and its application to several chiral systems. The technique was first demonstrated on a model system with a strong time-dependent CPL signal. Subsequently, the circularly polarized component in the fluorescence of reduced nicotinamide adenine dinucleotide (NADH) bound to liver alcohol dehydrogenase was time-resolved. The CPL of NADH in the binary enzyme-coenzyme complex is time-dependent, reflecting structural differences around the reduced nicotinamide possibly due to a dynamic restructuring. In contrast, the CPL of the coenzyme in the ternary complex formed with enzyme and the substrate analog isobutyramide is essentially time-independent, likely reflecting a more rigid binding domain. Since the linear polarization of the fluorescence of the two complexes did not show any local flexibility of the NADH chromophore, the excited-state conformational rearrangement of the binary complex indicates a subtle change in its interactions with group(s) in direct contact with it.

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