Hole-burning spectroscopy as a probe of nano-environments and processes in biomolecules: a review

From Melanocytes to Melanoma Cells: Characterization of the Malignant Transformation by Four Distinctly Different Melanin Fluorescence Spectra (Review)

The melanin fluorescence emitted by pigment cells of the human skin has been a central research topic for decades, because melanin, on the one hand, protects against (solar) radiation in the near-UV range, whereas on the other hand, melanocytes are the starting point for the malignant transformation into melanoma. Until recently, however, melanin fluorescence was not accessible in the context of conventional spectroscopy, because it is ultraweak and is overshadowed by the more intense so-called autofluorescence of endogenous fluorophores. The advent of a new method of laser spectroscopy has made this melanin fluorescence measurable in vivo. A stepwise two-photon absorption with 800 nm photons is used, which more selectively excites melanin (dermatofluoroscopy). Our review summarizes the experimental results on melanin fluorescence of the four types of cutaneous pigment cells from healthy and malignant tissues. Outstanding is the finding that different types of melanocytes (i.e., melanocytes of common nevi, versus dysplastic nevi or versus melanoma cells) show characteristically different fluorescence spectra. The possibilities of using this melanin fluorescence for melanoma diagnosis are shown. Moreover, the uniform fluorescence spectra emitted by different melanoma subtypes are essential. Conclusions are drawn about the molecular processes in the melanosomes that determine fluorescence. Finally, experimental suggestions for further investigations are given.

Effects of Chlorophyll Triplet States on the Kinetics of Spectral Hole Growth

Spectral hole burning has been employed for decades to study various amorphous solids and proteins. Triplet states and respective transient holes were incorporated into theoretical models and software simulating nonphotochemical spectral hole burning (NPHB) and including all relevant distributions, in particular the distribution of the angle between the electric field of light E and transient dipole moment of the chromophore μ. The presence of a chlorophyll a triplet state with a lifetime of several milliseconds explains the slowdown of NPHB (on the depth vs illumination dose scale) with the increase of the light intensity, as well as larger hole depths observed in weak probe beam experiments, compared to those deduced from the hole growth kinetics (HGK) measurements (signal collected at a fixed wavelength while a stronger burning beam is on) in cytochrome b₆f and chemically modified LH2. We also considered the solvent deuteration effects on triplet lifetime and concluded that both triplet states and local heating likely play a role in slowing down the HGK with increasing burn intensity.

Evidence of Simultaneous Spectral Hole Burning Involving Two Tiers of the Protein Energy Landscape in Cytochrome b6f

Cytochrome b₆f, with one chlorophyll molecule per protein monomer, is a simple model system whose studies can help achieve a better understanding of nonphotochemical spectral hole burning (NPHB) and single-complex spectroscopy results obtained in more complicated photosynthetic chlorophyll-protein complexes. We are reporting new data and proposing an alternative explanation for spectral dynamics that was recently observed in cytochrome b₆f using NPHB. The relevant distribution of the tunneling parameter λ is a superposition of two components that are nearly degenerate in terms of the resultant NPHB yield and represent two tiers of the energy landscape responsible for NPHB. These two components likely burn competitively; we present the first demonstration of modeling a competitive NPHB process. Similar values of the NPHB yield result from distinctly different combinations of barrier heights, shifts along the generalized coordinate d, and/or masses of the entities involved in conformational changes m, with md² parameter different by a factor of 2.7. Consequently, in cytochrome b₆f, the first (at least) 10 h of fixed-temperature recovery preferentially probe different components of the barrier- and λ-distributions encoded into the spectral holes than thermocycling experiments. Both components most likely represent dynamics of the protein and not of the surrounding buffer/glycerol glass.

Probing Energy Landscapes of Cytochrome b6f with Spectral Hole Burning: Effects of Deuterated Solvent and Detergent

In non-photochemical spectral hole burning (NPHB) and spectral hole recovery experiments, cytochrome b₆f protein exhibits behavior that is almost independent of the deuteration of the buffer/glycerol glassy matrix containing the protein, apart from some differences in heat dissipation. On the other hand, strong dependence of the hole burning properties on sample preparation procedures was observed and attributed to a large increase of the electron-phonon coupling and shortening of the excited-state lifetime occurring when n-dodecyl β-d-maltoside (DM) is used as a detergent instead of n-octyl β-d-glucopyranoside (OGP). The data was analyzed assuming that the tunneling parameter distribution or barrier distribution probed by NPHB and encoded into the spectral holes contains contributions from two nonidentical components with accidentally degenerate excited state λ-distributions. Both components likely reflect protein dynamics, although with some small probability one of them (with larger md²) may still represent the dynamics involving specifically the -OH groups of the water/glycerol solvent. Single proton tunneling in the water/glycerol solvent environment or in the protein can be safely excluded as the origin of observed NPHB and hole recovery dynamics. The intensity dependence of the hole growth kinetics in deuterated samples likely reflects differences in heat dissipation between protonated and deuterated samples. These differences are most probably due to the higher interface thermal resistivity between (still protonated) protein and deuterated water/glycerol outside environment.

Development of photoacoustic sensing platforms at the Army Research Laboratory

Traditionally, chemical sensing platforms have been hampered by the opposing concerns of increasing sensor capability while maintaining a minimal package size. Current sensors, although reasonably sized, are geared to more classical chemical threats, and the ability to expand their capabilities to a broader range of emerging threats is uncertain. Recently, photoacoustic spectroscopy, employed in a sensor format, has shown enormous potential to address these ever-changing threats. Photoacoustic spectroscopy is one of the more flexible infrared spectroscopy variants, and that flexibility allows for the construction of sensors that are designed for specific tasks. The Army Research Laboratory has, for the past 14 years, engaged in research into the development of photoacoustic sensing platforms with the goal of sensor miniaturization and the detection of a variety of chemical targets both proximally and at range. This paper reviews this work.

Indistinguishability and correlations of photons generated by quantum emitters undergoing spectral diffusion

Photon-based quantum information processing is based on manipulating multi photon interference. We focus on the Hong-Ou-Mandel (HOM) dip in the photon coincidence rate which provides a direct measure of interference of indistinguishable photons linked to their Bose statistics. The effect has been first observed with entangled photons generated by parametric down conversion and then extended to independent emitters. Fluctuations caused by coupling between emitters and a bath can erode the interference which causes the dip. Here we show how the magnitude and shape of the HOM dip is affected by spectral diffusion induced by coupling to a brownian oscillator bath. Conditions for maintaining and controlling the interference are specified.