An ab initio study of linear dihydrogen-bonded complexes containing LiH (acting as a proton acceptor) was undertaken. The complexes studied were LiH⋯HF, LiH⋯HCN, and LiH⋯HCCH. Equilibrium geometries and harmonic vibrational frequencies were computed at various levels of theory using a 6-31++G(d,p) basis set. It was found that the LiH⋯HF complex was not stable at all levels of theory. The relative stabilities of H-bonded and D-bonded isotopomers of LiH⋯HCN and LiH⋯HCCH were determined by differences in zero-point vibrational frequencies. For LiH⋯HCN isotopomers, it was found that the LiD⋯HCN was favored over LiH⋯DCN with a relative stability of 54 cm−1 calculated at the Quadratic Configuration Interaction—Singles and Doubles level. Similarly, LiD⋯HCCH is favored over LiH⋯DCCH, with a lower value for the relative stability. The relative stabilities of the H-bonded and D-bonded species LiH⋯HCCD and LiH⋯DCCH indicated that the D-bonded complex was energetically favored, in accordance with the Buckingham–Liu theory.

Complex systems of fluorophores undergoing energy transfer can exhibit a variety of anomalous lifetime behavior when probed with frequency domain methods. When presented in traditional apparent lifetime format the data from such systems can exhibit "nodal" behavior in which the computed lifetime approaches +/-infinity. The location of the nodes is system and frequency dependent. In addition, simpler systems, not undergoing energy transfer, show ill behavior in the region of zero lifetime (tau(m)) and long lifetime (tau(pi)) due to noise in typical measurements. Here, we systematically investigate systems of multiple fluorophores with and without energy transfer to provide insight into frequency domain investigations of complex systems of fluorophores. The results of simulations are compared to data collected from a multi-fluorophore system designed to exhibit fluorescence resonance energy transfer (FRET) using imaging spectroscopic fluorescence lifetime imaging microscopy (ISFLIM). The results are applicable to both cuvette and imaging arrangements.

Abstract:We report the acquisition and analysis of spectrally resolved photobleaching data from a model system designed to exhibit FRET. Spectrally resolved photobleaching can be used to determine the presence of FRET in these systems and to investigate multi-step mechanisms of energy transfer. The model system was a previously described set of fluorescent beads consisting of a system of 6 fluorophores. In standard photobleaching experiments to determine FRET, bleaching of an acceptor molecule resulting in recovery of donor intensity or changes in photobleaching kinetics are used as indicators of FRET. Here, we use the Bateman equations to model growth and decay in a photobleaching experiment. Linked donor-acceptor growth and decay is used as an indicator of FRET. The apparatus required is relatively simple when compared to lifetime imaging systems. Several data analysis strategies, rigorous model building, global fitting procedures, and error analysis are presented. Using these procedures a five-step sequential mechanism of energy transfer was selected for these beads.

No abstract

Background: Fluorescence imaging spectroscopy is a powerful but under-utilized tool. This article gives perspective on the use of imaging spectroscopy, and provides two examples of imaging spectroscopy done with a prism-based system. The intent is to give insight into the power of imaging spectroscopy when used in combination with other imaging techniques. In particular, studies of intact coral photobleaching and beads designed to show energy transfer are reported. In the bead study, spectroscopic lifetime imaging was performed at each photobleaching step. Results: Spectroscopic photobleaching of the hard coral, Montastrea annularis, revealed two spectral regions. A region in the red portion of the spectrum bleached rapidly while progressively increasing fluorescence was observed over a wide portion of the spectrum. This behavior is consistent with current theories for the role of fluorescent proteins in corals.