A nanosecond fluorescence study of the simultaneous influx of Ca2+ and Cd2+ into liposomes

Nanosecond Dynamics of InfluenzaA/M2TM and an Amantadine Resistant Mutant Probed by Time-Dependent Red Shifts of a Native Tryptophan

Proteins involved in functions such as electron transfer or ion transport must be capable of stabilizing transient charged species on time scales ranging from picoseconds to microseconds. We study the influenza A M2 proton channel, containing a tryptophan residue that serves as an essential part of the proton conduction pathway. We induce a transition dipole in tryptophan by photoexcitation, and then probe the dielectric stabilization of its excited state. The magnitude of the stabilization over this time regime was larger than that generally found for tryptophan in membrane or protein environments. M2 achieves a water-like stabilization over a 25 nanosecond time scale, slower than that of bulk water, but sufficiently rapid to contribute to stabilization of charge as protons diffuse through the channel. These measurements should stimulate future MD studies to clarify the role of sidechain versus non-bulk water in defining the process of relaxation.

A proton-triggered ON-OFF-ON fluorescent chemosensor for Mg(II) via twisted intramolecular charge transfer

A novel fluorescent chemosensor has been synthesized and shows interesting fluorescent ON-OFF-ON processes through mediating its twisted intramolecular charge transfer (TICT) state. An exclusively fluorescent enhancement is observed for Mg (2+).

Nanosecond relaxation dynamics of protein GB1 identified by the time-dependent red shift in the fluorescence of tryptophan and 5-fluorotryptophan

The B1 domain of Streptococcal protein G (GB1) is a small, thermostable protein containing a single tryptophan residue. We recorded time-resolved fluorescence of the wild-type GB1 and its 5-fluorotryptophan (5FTrp) variant at more than 30 emission wavelengths between 300 and 470 nm. The time-resolved emission spectra reveal no signs of heterogeneity, but show a time-dependent red shift characteristic of microscopic dielectric relaxation. This is true for both 5FTrp and unmodified Trp in GB1. The time-dependent red shifts in the fluorescence of 5FTrp and unmodified Trp are essentially identical, confirming that the shift is caused by the relaxation of the protein matrix rather than by the fluorophore itself. The total amplitude (but not the rate) of the time-dependent red shift depends on the fluorophore, specifically, on the magnitude of the vector difference between its excited state and ground state electric dipole moments; for 5FTrp this is estimated to be about 88% of that for the unmodified Trp. The decay of the excited state fluorophore population is not monoexponential for either fluorophore; however, the deviation from the monoexponential decay law is larger in the case of unmodified Trp. The relaxation dynamics of GB1 was found to be considerably faster than that of other proteins studied previously, consistent with the small size, tightly packed core, and high thermodynamic stability of GB1.

Flipping the light switch 'on'--the design of sensor molecules that show cation-induced fluorescence enhancement with heavy and transition metal ions

Real-time and real-space analysis of heavy and transition metal ions employing fluorescent sensor molecules has received much attention over the past few years. Since many of these cations possess intrinsic properties that usually quench the fluorescence of organic dye molecules, a lot of research has lately been devoted to designing fluorescent probes that show complexation-induced fluorescence enhancement. Such an analytical reaction would be highly desirable in terms of increased sensitivity and selectivity. However, in this particular field of sensor research, the photophysical and photochemical mechanisms involved as well as the chemical constitutions of the sensor molecules employed are rather diverse and up to now, very few attempts have been made to establish some general concepts for rational probe design. By analyzing various systems published by other researchers as well as own work, this contribution aims at an elucidation of some of the underlying principles of heavy and transition metal ion-enhanced emission.

Hydrophobic clustering in acid-denatured IL-2 and fluorescence of a Trp NH-pi H-bond

The single tryptophan at position 121 of human interleukin-2 (IL-2) can form an NH-pi hydrogen bond with Phe 117 involving the indole nitrogen and the benzene aromatic ring. At pH 5.5, this type of aromatic interaction results in a fluorescence quantum yield three-fold lower than that of a fully solvent exposed tryptophan. At pH 2.1, IL-2 forms a compact denatured state with twice the emission intensity of the native protein. Global analysis of time-resolved fluorescence emission at multiple emission wavelengths shows that native and acid-denatured IL-2 can be described by four decay components. The fractional amplitudes of the shortest sub-nanosecond lifetimes are higher in the native state, suggesting rapid quenching due to the NH-pi hydrogen bond. In the denatured state, longer lifetimes have greater fractional amplitudes, indicating a smaller population of hydrogen-bonded species. Electrostatic-dipolar relaxation of the tryptophan microenvironment upon excitation is greater in the native-state of IL-2 than the acid-denatured state. This suggests that acid-denaturation sequesters Trp 121 from polar residues, while maintaining an interaction with Phe 117. This is consistent with the model of secondary structure preservation and hydrophobic clustering in molten-globule intermediates.