Fluorescence-based techniques are widely used in bioscience, offering a high sensitivity and versatility. In this work, fluorescence electronic energy migration/ transfer is applied to measure intramolecular distances in two types of systems and under various conditions. The main part of the thesis utilizes the process of donor-acceptor energy transfer to probe distances within the ribosomal protein S16. Proteins are essential to all organisms. Therefore, it is of great interest to study protein structure and function in order to understand and prevent protein malfunction. Moreover, it is also important to try to study the proteins in an environment which resembles its natural habitat. Here two protein homologs were investigated; S16Thermo and S16Meso, isolated from a hyperthemophilic bacterium and a mesophilic bacterium, respectively. It was concluded that the chemically induced unfolded state ensemble of S16Thermo is more compact than the corresponding ensemble of S16Meso. This unfolded state compaction may be one reason for the increased thermal stability of S16Thermo as compared to S16Meso. The unfolded state of S16 was also studied under highly crowded conditions, mimicking the environment found in cells. It appears that a high degree of crowding, induced by 200 mg/mL dextran 20, forces the unfolded state ensemble of S16Thermo to become even more compact. Further, intramolecular distances in the folded state of five S16 mutants were investigated upon increasing amounts of dextran 20. We found that the probed distances in S16Thermo are unaffected by increasing degree of crowding. However, S16Meso shows decreasing intramolecular distances for all three studied variants, up to 100 mg/mL dextran. At higher concentrations, the change in distance becomes anisotropic. This suggests that marginally stable proteins like s16Meso may respond to macromolecular crowding by fine-tuning its structure. More stable proteins like S16Thermo however, show no structural change upon increasing degree of crowding. We also investigated the possibility of local specific interactions between the protein and crowding agent, by means of fluorescence quenching experiments. Upon increasing amounts of a tyrosine labelled dextran, a diverse pattern of fluorescence quantum yield and lifetime suggests that specific, local protein-crowder interactions may occur. In a second studied system, electronic energy migration between two donor-groups, separated by a rigid steroid, was studied by two-photon excitation depolarization experiments. Data were analysed by using recent advances, based on the extended Förster theory, which yield a reasonable value of the distance between the two interacting donor-groups. To the best of our knowledge, this is the first quantitative analysis of energy migration data, obtained from two-photon excited fluorescence.

Somatostatin (SST) is a peptide hormone that was originally identified in the hypothalamus and subsequently found throughout the central nervous system and in various peripheral organs. Generally classified as an inhibitory factor, SST is secreted by endocrine, neuronal and immune cells and acts to regulate cell secretion, neurotransmission and cell proliferation. There are five receptor-subtypes known to engage SST, termed SSTR1-5, all belonging to the superfamily of G-protein coupled receptors (GPCRs). Within the past few years, there has been a prepondef8:llce of evidence to suggest the importance of GPCR dimerization in receptor-biogenesis, regulation and pharmacology. It has been previously reported in our laboratory, that human (h) SSTR5 homo- and heterodimerizes with hSSTR1 in an agonist-regulated manner. However, it was unclear as to the contribution of each subtype in the formation of the hSSTR1/hSSTR5 heterodimer, the possible molecular determinants involved and the effects of heterodimerization on the pharmacology of the receptors. Furthermore, the dimerization properties of other hSSTRs including their heterodimerization remain undetermined. Here, we demonstrate that agonist binding to hSSTR5 and not hSSTR1 modulates the formation of the heterodimer, with particular emphasis on its carboxyl-terminal tail in specifying the interaction. We also determined the mechanics of the hSSTR2 homodimer, unlike the previous hSSTRs investigated, forms constitutive dimers that dissociate into monomers following activation with agonist. This feature is important for receptor trafficking, as preventing their dissociation impairs agonist-mediated endocytosis. Lastly, we investigated the heterodimerization of hSSTR2 and hSSTR5, an interaction that, like the hSSTR1/hSSTR5 heterodimer, is subtype-specific, requiring selective-activation of hSSTR2 and not hSSTR5. The heterodimer exhibited enhanced signalling characteristics including, prolonged activation of MAP kinases and an increase in the induction of the cyclin-dependent kinase inhibitor p27Kip1. These enhanced properties of the heterodimer conferred an extended growth inhibitory response. Dimerization of GPCRs, with particular emphasis on heterodimers, generates novel receptors with unique properties distinct from those of the individual receptor monomers/homodimers. An understanding on the mechanisms involved in GPCR dimerization could provide a rationale in future drug design.

Imaging tools in cellular and molecular biology have long relied on organic fluorophores to observe microorganisms or various cell constituents. The advent of semiconductor nanoparticles known as quantum dots (QDs) has offered the possibility to use this new class of fluorescent probes with very advantageous optical properties in cell biology. The imaging of transmembrane potential and ionic currents is of significant importance for monitoring the activity of the cell. It remains possible with relatively complicated instruments and methods such as patch clamping. A complementary approach to view the dynamics of ion channels with modern and efficient fluorophores is therefore of great interest to the field of biology in general. / We developed a construct based on the FRET signal between QDs and organic fluorescent dyes to monitor the conformational changes of voltage gated sodium channels. The amino acid histidine was used as a "landing platform" for QDs and the bacterial sodium channel NaChBac was chosen for testing. This study focused on the preliminary steps of the project and aimed to characterize the electrophysiological behavior of the histidine-tagged channel. The whole-cell configuration of patch clamping was the tool we used to understand the differences between the wild-type and the histidine-tagged variants of the channels. We also explore the possibility to land QDs on the histidine tag.

Magic-sized nanocrystals (MSNs) are nanocrystals with a single size distribution. They have narrow spectral features that do not exhibit inhomogeneous line broadening. This enabled us to analyze homogeneous line broadening of CdSe and CdTe MSNs. In solution, we observed two aggregated configurations of CdSe and CdTe MSNs. Sub-peaks within MSN excitonic peaks were caused by these two aggregated configurations and surface states. A two-dimensional photoluminescence spectrum of a mixture of CdTe 427 nm and 500 nm MSNs gave direct evidence of Förster resonant energy transfer (RET) between the two sizes of MSNs. Normalized experimental overlap between donor emission and acceptor absorption spectra was on the order predicted by theory, confirming that there is sufficient overlap for RET to take place in this system. Additionally, within both aggregated configurations, the two sizes of MSNs were within sufficient distance from one another for RET to occur.

Magic-sized nanocrystals (MSNs) are nanocrystals with a single size distribution. They have narrow spectral features that do not exhibit inhomogeneous line broadening. This enabled us to analyze homogeneous line broadening of CdSe and CdTe MSNs. In solution, we observed two aggregated configurations of CdSe and CdTe MSNs. Sub-peaks within MSN excitonic peaks were caused by these two aggregated configurations and surface states. A two-dimensional photoluminescence spectrum of a mixture of CdTe 427 nm and 500 nm MSNs gave direct evidence of Förster resonant energy transfer (RET) between the two sizes of MSNs. Normalized experimental overlap between donor emission and acceptor absorption spectra was on the order predicted by theory, confirming that there is sufficient overlap for RET to take place in this system. Additionally, within both aggregated configurations, the two sizes of MSNs were within sufficient distance from one another for RET to occur.

Targeted Energy Transfer is a resonance phenomenon in coupled anharmonic oscillators. In this thesis we investigate if the concept of Targeted Energy Transfer is applicable to Bose-Einsteain condensates in optical lattices. The model used to describe Bose-Einstein condensates in optical lattices is based on the Gross-Pitaevskii equation. Targeted Energy Transfer in these systems would correspond to energy being transferred from one lattice site to another. We also try to expand the concept of Targeted Energy Transfer to a system consisting of three sites, where one of the sites are considered a perturbation to the system. We have concluded that it is possible to achieve Targeted Energy Transfer in a three-site system. The set-up of the system will in some of the cases studied lead to interesting properties, such as more energy being transferred to the acceptor site than what was initially localized on the donor site.

Understanding energy transfer at the molecular scale is both essential for the design of novel molecular level devices and vital for uncovering the fundamental properties of non-equilibrium open quantum systems. In this thesis, we first establish the connection between molecular scale devices -- molecular electronics and phononics -- and open quantum system models. We then develop theoretical tools to study various properties of these models. We extend the standard master equation method to calculate the steady state thermal current and conductance coefficients. We then study the scaling laws of the thermal current with molecular chain size and energy, and apply this tool to investigate the onset of nonlinear thermal current - temperature characteristics, thermal rectification and negative differential conductance. Our master equation technique is valid in the ``on-resonance" regime, referring to the situation in which bath modes in resonance with the subsystem modes are thermally populated. In the opposite ``off-resonance" limit, we develop the Energy Transfer Born-Oppenheimer method to obtain the thermal current scaling without the need to solve for the subsystem dynamics. Finally, we develop a mapping scheme that allows the dynamics of a class of open quantum systems containing coupled subsystems to be treated by considering the separate dynamics in different subsections of the Hilbert space. We combine this mapping scheme with path integral numerical simulations to explore the rich phenomenon of entanglement dynamics within a dissipative two-qubit model. The formalisms developed in this thesis could be applied for the study of energy transfer in different realizations, including molecular electronic junctions, donor-acceptor molecules, artificial solid state qubits and cold-atom lattices.

Lanthanide (III) complexes are used extensively in solar conversion devices, such as Luminescent Solar Concentrators (LSCs) and Luminescent Down-Shifting (LDS) for their peculiar characteristics of narrow band emission, avoidance of re-absorption losses due to large Stokes shift and possibility of high photoluminescence quantum yield (PLQY). The study has looked into the synthesis of Ln (III) complexes of the general formula: [Ln(hfac)3DPEPO], where DPEPO = bis(2-(diphenylphosphino)phenyl)ether oxide, and hfac = hexafluoroacetylacetonate. The work presented in this thesis focuses on the synthesis, and subsequent photophysical characterisation of these Ln(III) complexes, plus characterisation and spectroscopic study of [Tb(pobz)3(hacim)2], (where Hpobz = phenoxybenzoic acid, and Hacim = acetylacetone imine), yielding results that open new design of functional Ln(III) systems. Spectroscopic study of Chromium dioxalate and analogous compounds has revealed that with the appropriate design, Cr(III)Ln(III) energy transfer can be achieved, while study of polyaromatic hydrocarbons (PAH) such as coronene, enable to explore a ligand with better absorption in the whole UV region. These results open attractive perspectives for light-conversion systems, such as LSC devices.

Two chlorophylls of the FMO complex, the light-harvesting complex of the green sulfur bacteria, are modeled as two coupled qubits, each surrounded by one spin-bath simulating the environment. The dynamics of the system at a non-zero temperature provide exact analytical expressions for the transition probability and the coherence. It is shown that the decoherence-inducing interaction with the environment enhances the electronic energy transfer. Also the correlations in terms of entanglement and nonlocality are quantitatively studied, sensitively differing when introducing a decay term to resemble both chlorophylls being in their ground states. It is proved that nonlocality is a stronger form of correlation than entanglement.

The quaternary status of G protein-coupled receptors (GPCRs) is important, unknown and controversial. Estimates of size from numerous pharmacological, biochemical and biophysical studies range from monomers to octamers. Accounts of stability vary from constitutive oligomers to a spontaneous, ligand-regulated interconversion between monomers and dimers. In the present investigation, the oligomeric size of GPCRs in live Chinese hamster ovary (CHO) cells has been examined by two methods. Both are based on the efficiency of Förster resonance energy transfer (FRET) between fluorophore-tagged receptors, as determined from emission spectra via spectral deconvolution. In the first, the apparent FRET efficiency (Eapp) was measured for cells expressing eGFP- and eYFP-tagged M2 muscarinic receptors at different ratios of acceptor to donor. Eapp then was related to the pair-wise efficiency (Ep) according to a model that enumerates all pathways for the transfer of energy between single donors and acceptors within an oligomer of given size (n). Each value n returned a distinct and well-defined value of Ep. Fluorescence lifetime imaging provided an independent estimate of Ep that was in close agreement with the model-based value when n = 4, identifying the M2 receptor as a tetramer. In the second approach, the M1 and M2 muscarinic receptors and the β1 and β2 adrenergic receptors were tagged with GFP2 and eYFP, and the value of Eapp was estimated for each pixel in the image of a cell. The distributions of Eapp from 34–40 cells expressing each receptor were compared with those predicted for populations of dimers, trimers and tetramers, the latter configured as a square and a rhombus. In each case, the combined data were well described in terms of a rhombus. Distributions obtained for the M2 and β2 receptors were not affected by agonists or inverse agonists, nor was there evidence for appreciable numbers of dimers or larger oligomers. Taken together, the results suggest that GPCRs of Family 1 exist largely or wholly as constitutive tetramers.