Dynamics of L-tryptophan in aqueous solution by simultaneous laser induced fluorescence (LIF) and photoacoustic spectroscopy (PAS)

Ultrafast Förster resonance energy transfer from tyrosine to tryptophan in monellin: potential intrinsic spectroscopic ruler

Ultrafast Förster Resonance Energy Transfer (FRET) between tyrosine (Tyr) and tryptophan (Trp) residues in the protein monellin has been investigated using picosecond and femtosecond time-resolved fluorescence spectroscopy. Decay associated spectra (DAS) and time-resolved emission spectra (TRES) taken with the different excitation wavelengths of 275, 290 and 295 nm were constructed via global analysis. At two of those three excitation loci (275 and 290 nm), earmarks of energy transfer from Tyr to Trp in monellin are seen, and particularly when the excitation is 275 nm, the energy transfer between Tyr and Trp clearly changes the signature emission DAS shape to that indicating excited state reaction (especially on the red side of fluorescence emission, near 380 nm). Those FRET signatures may overlap with the conventional signatory DAS in heterogeneous systems. When overlap and addition occur between FRET type DAS and "full positive" QSSQ (quasi-static self-quenching), mixed DAS shapes will emerge that still show "positive blue side and negative red sides", just with zero crossing shifted. In addition, excitation decay associated spectra (EDAS) taken with the different emission wavelengths of 330, 350 and 370 nm were constructed. In the study of protein dynamics, ultrafast FRET between Tyr and Trp could provide a basis for an intrinsic (non-perturbing) "spectroscopic ruler", potentially a powerful tool to detect even slight changes in protein structures.

Biomedical Application of Photoacoustics: A Plethora of Opportunities

The photoacoustic (PA) technique is a non-invasive, non-ionizing hybrid technique that exploits laser irradiation for sample excitation and acquires an ultrasound signal generated due to thermoelastic expansion of the sample. Being a hybrid technique, PA possesses the inherent advantages of conventional optical (high resolution) and ultrasonic (high depth of penetration in biological tissue) techniques and eliminates some of the major limitations of these conventional techniques. Hence, PA has been employed for different biomedical applications. In this review, we first discuss the basic physics of PA. Then, we discuss different aspects of PA techniques, which includes PA imaging and also PA frequency spectral analysis. The theory of PA signal generation, detection and analysis is also detailed in this work. Later, we also discuss the major biomedical application area of PA technique.

Prediction of absorption coefficients by pulsed laser induced photoacoustic measurements

In the current study, a pulsed laser induced photoacoustic spectroscopy setup was designed and developed, aiming its application in clinical diagnostics. The setup was optimized with carbon black samples in water and with various tryptophan concentrations at 281nm excitations. The sensitivity of the setup was estimated by determining minimum detectable concentration of tryptophan in water at the same excitation, and was found to be 0.035mM. The photoacoustic experiments were also performed with various tryptophan concentrations at 281nm excitation for predicting optical absorption coefficients in them and for comparing the outcomes with the spectrophotometrically-determined absorption coefficients for the same samples. Absorption coefficients for a few serum samples, obtained from some healthy female volunteers, were also determined through photoacoustic and spectrophotometric measurements at the same excitations, which showed good agreement between them, indicating its clinical implications.

Photoacoustic spectroscopy of ovarian normal, benign, and malignant tissues: a pilot study

Photoacoustic spectra of normal, benign, and malignant ovarian tissues are recorded using 325-nm pulsed laser excitation in vitro. A total of 102 (34 normal, 38 benign, and 30 malignant) spectra are obtained from 22 samples belonging to normal, benign, and malignant subjects. Applying multi-algorithm approach, comprised of methods such as, principal component analysis (PCA) based k-nearest neighbor (k-NN) analysis, artificial neural network (ANN) analysis, and support vector machine (SVM) analysis, classification of the data has been carried out. For PCA, first the calibration set is formed by pooling 45 spectra, 15 belonging to each of pathologically certified normal, benign, and malignant samples. PCA is then performed on the data matrix, comprised of the six spectral features extracted from each of 45 calibration samples, and three principal components (PCs) containing maximum diagnostic information are selected. The scores of the selected PCs are used to train the k-NN, ANN, and SVM classifiers. The ANN used is a classical multilayer feed forward network with back propagation algorithm for its training. For k-NN, the Euclidean distance based algorithm is used and for SVM, one-versus-rest multiclass kernel-radial basis function is used. The performance evaluation of the classification results are obtained by calculating statistical parameters like specificity and sensitivity. ANN and k-NN techniques showed identical performance with specificity and sensitivity values of 100 and 86.76%, whereas SVM had these values at 100 and 80.18%, respectively. In order to determine the relative diagnostic performance of the techniques, receiver operating characteristics analysis is also performed.

Femtosecond fluorescence spectra of tryptophan in human gamma-crystallin mutants: site-dependent ultrafast quenching

The eye lens Crystallin proteins are subject to UV irradiation throughout life, and the photochemistry of damage proceeds through the excited state; thus, their tryptophan (Trp) fluorescence lifetimes are physiologically important properties. The time-resolved fluorescence spectra of single Trps in human gammaD- and gammaS-Crystallins have been measured with both an upconversion spectrophotofluorometer on the 300 fs to 100 ps time scale, and a time correlated single photon counting apparatus on the 100 ps to 10 ns time scale, respectively. Three Trps in each wild type protein were replaced by phenylalanine, leading to single-Trp mutants: W68-only and W156-only of HgammaD- and W72-only and W162-only of HgammaS-Crystallin. These proteins exhibit similar ultrafast signatures: positive definite decay associated spectra (DAS) for 50-65 ps decay constants that indicate dominance of fast, heterogeneous quenching. The quenched population (judged by amplitude) of this DAS differs among mutants. Trps 68, 156 in human gammaD- and Trp72 in human gammaS-Crystallin are buried, but water can reach amide oxygen and ring HE1 atoms through narrow channels. QM-MM simulations of quenching by electron transfer predict heterogeneous decay times from 50-500 ps that agree with our experimental results. Further analysis of apparent radiative lifetimes allow us to deduce that substantial subpopulations of Trp are fully quenched in even faster (sub-300 fs) processes for several of the mutants. The quenching of Trp fluorescence of human gammaD- and gammaS-Crystallin may protect them from ambient light induced photo damage.

Quasi-static self-quenching of Trp-X and X-Trp dipeptides in water: ultrafast fluorescence decay

Time-resolved fluorescence decay profiles of N-acetyl-l-tryptophanamide (NATA) and tryptophan (Trp) dipeptides of the form Trp-X and X-Trp, where X is another aminoacyl residue, have been investigated using an ultraviolet upconversion spectrophoto fluorometer with time resolution better than 350 fs, together with a time-correlated single photon counting apparatus on the 100 ps to 20 ns time scale. We analyzed the set of fluorescence decay profiles at multiple wavelengths using the global analysis technique. Nanosecond fluorescence transients for Trp dipeptides all show multiexponential decay, while NATA exhibits a monoexponential decay near 3 ns independent of pH. In the first 100 ps, a time constant for the water "bulk relaxation" around Trp, NATA and Trp dipeptides are seen near 1-2 ps, with an associated preexponential amplitude that is positive or negative, depending on emission wavelength, as expected for a population conserving spectral shift. The initial brightness (sub-picosecond) we measure for all these dipeptides is less than that of NATA, implying even faster (<200 fs) intramolecular (quasi-) static quenching occurs within them. A new, third, ultrafast decay, bearing an exponential time constant of 20-30 ps with positive amplitude, has been found in many of these dipeptides. We believe it verifies our previous predictions of dipeptide QSSQ ("quasi-static self-quenching")-the loss of quantum yield to sub-100-ps decay process (Chen, R. F.; et al. Biochemistry 1991, 30, 5184). Most important, this term is found in proteins as well (Xu, J.; et al. J. Am. Chem. Soc. 2006, 128, 1214; Biophys. J. 2008, 94, 546; 2009, 96, 46a), suggesting an ultrafast quenching mechanism must be common to both.