Real-time light-driven dynamics of the fluorescence emission in single green fluorescent protein molecules

Real-time single-molecule fluorescence detection using confocal and near-field scanning optical microscopy has been applied to elucidate the nature of the "on-off" blinking observed in the Ser-65 --> Thr (S65T) mutant of the green fluorescent protein (GFP). Fluorescence time traces as a function of the excitation intensity, with a time resolution of 100 micros and observation times up to 65 s, reveal the existence of a nonemissive state responsible for the long dark intervals in the GFP. We find that excitation intensity has a dramatic effect on the blinking. Whereas the time during which the fluorescence is on becomes shorter as the intensity is increased, the off-times are independent of excitation intensity. Statistical analysis of the on- and off-times renders a characteristic off-time of 1.6 +/- 0.2 s and allows us to calculate a transition yield of approximately 0.5 x 10(-5) from the emissive to the nonemissive state. The saturation excitation intensity at which on- and off-times are equal is approximately 1.5 kW/cm(2). On the basis of the single-molecule data we calculate an absorption cross section of 6.5 x 10(-17) cm(2) for the S65T mutant. These results have important implications for the use of the GFP to follow dynamic processes in time at the single-molecular level.

Visualising individual green fluorescent proteins with a near field optical microscope

The use of the green fluorescence protein (GFP) as an individual marker for applications in molecular biology requires detailed understanding of its photophysical and photodynamical properties. We investigated individual S65T mutants of GFP both on a glass surface and embedded in a water-pore gel. An aperture-type near field scanning optical microscope (NSOM) with two polarisation detection channels was applied to afford high spatial (approximately 70 nm) and temporal (0.5 ms) resolution. Shear-force and near field fluorescence imaging were performed simultaneously, allowing direct correlation between topographic and optical features. Polarisation data showed that the emission dipole moment of the proteins is fixed in space within both the barrel structure of the protein and the gel matrix used for spatial confinement of the proteins. The photophysical behaviour of the S65T-GFP mutants was monitored in time, with 500-micros real-time resolution and continuous imaging for periods of more than 2 h. Our results show the reversible on-off behaviour on a time scale that spans from 10(-4) to 10(3) s. Even a process generally identified as "bleaching" turns out to be reversible if a sufficient long observation time is allowed. As such, the photodynamics of individual GFPs appear to be much more complex than the properties deduced from ensemble-averaged measurements.

Resonance Raman microspectroscopy of myeloperoxidase and cytochrome b558 in human neutrophilic granulocytes

With (resonance) Raman microscospectroscopy, it is possible to investigate the chemical constitution of a very small volume (0.5 fl) in a living cell. We have measured resonance Raman spectra in the cytoplasm of living normal, myeloperoxidase (MPO)-deficient, and cytochrome b558-deficient neutrophils and in isolated specific and azurophilic granule fractions, using an excitation wavelength of 413.1 nm. Similar experiments were performed after reduction of the redox centers by the addition of sodium dithionite. The specific and azurophilic granules in both redox states appeared to have clearly distinguishable Raman spectra when exciting at a wavelength of 413.1 nm. The azurophilic granules and the cytochrome b558-deficient neutrophils showed Raman spectra similar to that of the isolated MPO. The spectra of the specific granules and the MPO-deficient neutrophils corresponded very well to published cytochrome b558 spectra. The resonance Raman spectrum of the cytoplasmic region of normal neutrophilic granulocytes could be fitted with a combination of the spectra of the specific and azurophilic granules, which shows that the Raman signal of neutrophilic granulocytes mainly originates from MPO and cytochrome b558, at an excitation wavelength of 413.1 nm.

Evidence for Hoogsteen GC base pairs in the proton-induced transition from right-handed to left-handed poly(dG-dC).poly(dG-dC)

The structure of double-helical poly(dG-dC).poly(dG-dC) is investigated at various pH values with Raman spectroscopy, absorption spectroscopy, and circular dichroism. A comparison is made between the B-form with Watson-Crick base pairing at 1 mM [Na+] and pH 7.2, the Z-form with Watson-Crick base pairing at 4 M [Na+] and pH 7.2, and a different structure at 1 mM [Na+] and pH 4.5 as well as at 150 mM [Na+] and pH 3.1. The CD spectrum of poly(dG-dC). poly(dG-dC) under the latter conditions does not show a negative band at 290 nm. The structure is a double-helical structure different from the B-form and the Z-form according to circular dichroism, Raman, and absorption spectroscopic studies. The Raman spectra evidence that the structure contains Hoogsteen base pairing. This can be accommodated in the double helix when the cytosine group is protonated and the sugar-guanine conformer has adopted a C2'-endo/syn conformation. It is shown that this antiparallel-stranded Hoogsteen base paired structure can be maintained under varying conditions, balancing the decrease in pH with an increased salt concentration. It is further concluded that the proton-induced transition from a Watson-Crick to a Hoogsteen base pair is aided by a decrease of [Na+] at pH 4.5 and occurs prior to a conversion from a right-handed helix to a left-handed helix.

Raman microspectroscopic approach to the study of human granulocytes

A sensitive confocal Raman microspectrometer was employed to record spectra of nuclei and cytoplasmic regions of single living human granulocytes. Conditions were used that ensured cell viability and reproducibility of the spectra. Identical spectra were obtained from the nuclei of neutrophilic, eosinophilic, and basophilic granulocytes, which yield information about DNA and protein secondary structure and DNA-protein ratio. The cytoplasmic Raman spectra of the three cell types are very different. This was found to be mainly due to the abundant presence of peroxidases in the cytoplasmic granules of neutrophilic granulocytes (myeloperoxidase) and eosinophilic granulocytes (eosinophil peroxidase). Strong signal contributions of the active site heme group(s) of these enzymes were found. This paper illustrates the potentials and limitations for Raman spectroscopic analysis of cellular constituents and processes.

Laser irradiation and Raman spectroscopy of single living cells and chromosomes: sample degradation occurs with 514.5 nm but not with 660 nm laser light

In Raman spectroscopic measurements of single cells (human lymphocytes) and chromosomes, using a newly developed confocal Raman microspectrometer and a laser excitation wavelength of 514.5 nm, degradation of the biological objects was observed. In the experiments high power microscope objectives were used, focusing the laser beam into a spot approximately 0.5 micron in diameter. At the position of the laser focus a paling of the samples became visible even when the laser power on the sample was reduced to less than 1 mW. This was accompanied by a gradual decrease in the intensity of the Raman signal. With 5 mW of laser power the events became noticeable after a period of time in the order of minutes. It is shown that a number of potential mechanisms, such as excessive sample heating due to absorption of laser light, multiple photon absorption, and substrate heating are unlikely to play a role. In experiments with DNA solutions and histone protein solutions no evidence of photo damage was found using laser powers up to 25 mW. No degradation of cells and chromosomes occurs when laser light of 660 nm is used. The most plausible explanation therefore seems to be that the sample degradation is the result of photochemical reactions initiated by laser excitation at 514.5 nm of as yet unidentified sensitizer molecules or complexes present in chromosomes and cells but not in purified DNA and histone protein samples.