Spatial dependency of the PPG morphology at right carotid common artery.

PhotoPlethysmoGraphy (PPG) is ubiquitously employed in wearable devices for health monitoring. Photodiode signal inversion is observed in rare occasions, most of the time when the sensor is pressed against the skin. We report in this article such observations made at the right common carotid artery site. Indeed we have systematically observed a photodiode signal inversion when the PPG sensor is placed where the pulse is the best felt at the carotid. In addition to be inverted, the pulse is steeper during the systolic phase. Such inversion has implications in terms of pulse arrival time (PAT) measurements In our experiments, this causes a difference of 20 ms in the carotid PAT when measured at the absolute maximum slope. The mechanical and optical properties of tissues must be better accounted to explain the PPG signal morphology. Clinical Relevance- Understanding the role of mechanical tissue properties seems relevant in order to obtain more reproducible results in PPG signal analysis.

Quantitative phase imaging of adherent mammalian cells: a comparative study.

The Quantitative phase imaging methods have several advantages when it comes to monitoring cultures of adherent mammalian cells. Because of low photo-toxicity and no need for staining, we can follow cells in a minimally invasive way over a long period of time. The ability to measure the optical path difference in a quantitative manner allows the measurement of the cell dry mass, an important metric for studying the growth kinetics of mammalian cells. Here we present and compare cell measurements obtained with three different techniques: digital holographic microscopy, lens-free microscopy and quadriwave lateral sheering interferometry. We report a linear relationship between optical volume density values measured with these different techniques and estimate the precisions of this measurement for the different individual instruments.

Multispectral total-variation reconstruction applied to lens-free microscopy.

Lens-free microscopy multispectral acquisitions are processed with an inverse problem approach: a multispectral total variation criterion is defined and minimized with the conjugate gradients method. Reconstruction results show that the method is efficient to recover the phase image of densely packed cells.

Visible supercontinuum generation controlled by intermodal four-wave mixing in microstructured fiber.

We present an experimental and numerical study of supercontinuum generation extended in the visible part of the spectrum by using a selective optical coupling of the pump wave in the largely anomalous dispersion regime. The broadband frequency generation is induced by an initial four-wave mixing process that converts the pump wave at 1064 nm into 831 nm anti-Stokes and 1478 nm Stokes wavelengths. Phase matching is ensured on such a large frequency shift thanks to a microstructured multimodal fiber with a specific design. Continuum generation is therefore enhanced around the two generated sidebands.

Diode-pumped passively mode-locked Nd:YVO4 laser at 914 nm.

We demonstrate, for the first time, to our knowledge, a diode-pumped passively mode-locked Nd:YVO4 laser, operating on the 4F(3/2)-4I(9/2) transition of the neodymium ion at 914 nm. We obtained 8.8 ps pulses at approximately 914 nm at a repetition rate of 94 MHz, and an averaged output power of 87 mW by using a semiconductor saturable absorber mirror.

Steady state and time-resolved fluorescence study of residual structures in an unfolded form of yeast phosphoglycerate kinase.

A previous study performed using steady state fluorescence has revealed the existence of residual structures surrounding the two tryptophan residues in an unfolded form of yeast phosphoglycerate kinase [Garcia, P., et al. (1995) Biochemistry 34, 397-404]. In this paper, we present a more detailed characterization of these residual structures, through the study of two single tryptophan-containing mutants of yPGK, W333F and W308Y. Denaturation experiments have first been performed at low temperatures to assess the nature of the interactions stabilizing these residual structures. On the other hand, the compactness and dynamics of the protein matrix were probed using tryptophan fluorescence quenching by acrylamide at various denaturant concentrations. Taking into consideration the changes in sample viscosity induced by addition of guanidinium chloride made feasible the use of this technique during the denaturation process. These different approaches have shown that the residual structures around tryptophan 308 are mainly stabilized by hydrophobic interactions and are more compact and less fluctuant than the ones surrounding tryptophan 333. Native and denatured yPGK have also been studied by time-resolved fluorescence spectroscopy. In the native protein, tryptophan buried in the core, W333, is mainly associated with a lifetime close to 0.1 ns, whereas tryptophan that is partially accessible to the solvent, W308, has a lifetime close to 0. 5 ns. The time-resolved tryptophan fluorescence emission of wild-type yPGK can be accounted for quantitatively by the summed emissions of its two single tryptophan mutants. The significance of minor long lifetime components is discussed for the two tryptophan residues. This new assignment of fluorescent decay times has allowed for the detection of a folding intermediate in which the environment of tryptophan 333 is modified for denaturant concentrations lower than those for tryptophan 308.

Aggregation ofNaja nigricollis cardiotoxin: Characterization and quantitative estimate by time-resolved polarized fluorescence.

After purification to homogeneity by Bio-Rex 70 ion exchange chromatography, micromolar solutions ofNaja nigricollis cardiotoxin were found to contain significant amounts of aggregates, as detected by time-resolved polarized fluorescence of its single tryptophan residue. The level of cardiotoxin aggregation depends strongly and reversibly on the protein concentration and pH. However, supplementary reverse-phase HPLC completely suppresses this aggregation, resulting in all cases in fluorescence anisotropy decays characteristic of the pure cardiotoxin monomer. The self-association properties of cardiotoxin, in the presence of a possible cofactor eliminated by the HPLC step, may be functionally relevant, and would deserve further investigation. The physical heterogeneity of the cardiotoxin samples required an appropriate model for the analysis of fluorescence depolarization, which was iteratively improved by comparison with experimental results. In this way, an approximate molar fraction of 10-15% aggregated cardiotoxin at a 90ΜM total protein concentration, pH 7, was determined. The fluorescence of the partly aggregated samples is significantly perturbed as compared to the HPLC-treated monomer, indicating that the cardiotoxin aggregate must have an increased average fluorescence lifetime and a strongly decreased initial anisotropy. The decrease in initial anisotropy suggests either an increased mobility of the tryptophan residue upon aggregation or fast energy transfers between residues of different cardiotoxin molecules brought within a short distance in the aggregate. This study illustrates the high sensitivity of the time-resolved fluorescence technique, through both total fluorescence and anisotropy parameters, to low levels of physical or chemical heterogeneity in a protein sample.

Dynamics of the active loop of snake toxins as probed by time-resolved polarized tryptophan fluorescence.

The local environment and dynamics of the single tryptophan residue in the respective active loops of cardiotoxin and alpha-neurotoxin from Naja nigricollis and of erabutoxin b from Laticauda semifasciata have been studied by steady-state and time-resolved polarized fluorescence and analyzed with distributions of decay times. Trp11 in loop I of cardiotoxin exhibits a very broad and complex distribution of fluorescence lifetimes at 20 degrees C. Despite its relatively external location in the toxin, the residue appears to be partly shielded from water and shows restricted but significant conformational fluctuations on the picosecond and nanosecond time scales. The thermal stability of cardiotoxin allowed a study of its static and dynamic fluorescence properties over a large range of temperatures. Interconversions in the intermediate nanosecond range lead to a thorough reorganization of the cardiotoxin fluorescence lifetime distribution with temperature. On the contrary, the fluorescence kinetics of Trp29 in loop II of the two neurotoxins is dominated by about 80% of a major decay time, which suggests that a nearly unique local conformation of the toxin is maintained over all time scales above the sub-nanosecond range. The fluorescence anisotropy decays show that the residue also has extremely limited rotational freedom down to the picosecond time scale. These findings are in good agreement with structural and dynamic information previously reported on the different toxins from NMR and X-ray crystallographic studies. The different dynamic properties around the tryptophan residue of the cardiotoxin and neurotoxin active loops can be analyzed within the frame of their different respective mechanisms of toxicity.