Fluorescence methods have found a wide range of important applications in science and technology. Compared with absorption, fluorescence gives a many-fold higher sensitivity and specificity. In addition, fluorescence possesses several important parameters such as intensity, excitation and emission spectra, polarisation, lifetime, and quantum yield. These parameters are functions of solvent nature, temperature, polarity and viscosity, and can be used to research the structure and function of various systems and for analytical applications. For example, fluorescence polarisation and non-radiative resonance energy transfer are used for detection of biomarkers in in vitro medical diagnostics. However, the use of fluorescence in in vivo diagnostics is hindered by the limited penetration of the visible light into living tissue or requires the use of fibre-optic probes. At the same time, proteins, being a major component of living organisms, should have diagnostic and prognostic potential. The intrinsic fluorescence of proteins is caused by three amino acid residues with aromatic side chains: phenylalanine, tyrosine and tryptophan. Out of these three, the latter plays the most important role due to its excitation and emission spectra having the longest wavelength (near the UV range) and longest lifetime. These features simplify measurements of its fluorescence and allow for its selective detection. However, contrary to its universal nature, tryptophan fluorescence of proteins has been predominantly used only in research to study protein conformational changes or protein–protein interactions. A question arises whether protein fluorescence can be used in medical diagnostics? Success in understanding the role of protein conformation in homeostasis of living organisms2 could provide solid support to this type of diagnostics. It has become apparent that protein post-translational modifications (PTM), causing protein mis-folding and aggregation, are responsible for a broad range of diseases, such as Parkinson’s and Alzheimer’s produced by degeneration in the brain tissue, cardiomyopathy caused by degeneration of the heart muscles or cataract caused by colouration and aggregation of the eye lens proteins.

Student ratings of instruction are an important means of assessment within universities and have been the focus of much study over the last 50 years. Until very recently it has been difficult to perform meaningful analysis of student narrative comments given that most universities collected them as hand-written notes. This work uses statistical and text mining techniques to analyze a data set consisting of over 1 million student comments that were collected using an online process. The methodology makes use of positive and negative “category vectors” representing instructor characteristics and a domain-specific lexicon. Sentiment analysis is used to detect and gauge attitudes embedded in comments about each category. The methodology is validated using three approaches, two quantitative and one qualitative. While useful to individual instructors and administrators, it is only through data mining that student perceptions of teaching can be analyzed en masse to inform and influence the educational process.

The equilibrium unfolding transitions of Cro repressor variants, dimeric variant Cro F58W and monomer Cro K56[DGEVK]F58W, have been studied by urea and guanidine hydrochloride to probe the folding mechanism. The unfolding transitions of a dimeric variant are well described by a two state process involving native dimer and unfolded monomer with a free energy of unfolding, ΔG0,un0, of ∼10–11kcal/mol. The midpoint of transition curves is dependent on total protein concentration and ΔG0,un0 is independent of protein concentration, as expected for this model. Unfolding of Cro monomer is well described by the standard two state model. The stability of both forms of protein increases in the presence of salt but decreases with the decrease in pH. Because of the suggested importance of a N2⇔2F dimerization process in DNA binding, we have also studied the effect of sodium perchlorate, containing the chaotropic perchlorate anion, on the conformational transition of Cro dimer by CD, fluorescence and NMR (in addition to urea and guanidine hydrochloride) in an attempt both to characterize the thermodynamics of the process and to identify conditions that lead to an increase in the population of the folded monomers. Data suggest that sodium perchlorate stabilizes the protein at low concentration (<1.5M) and destabilizes the protein at higher perchlorate concentration with the formation of a “significantly folded” monomer. The tryptophan residue in the “significantly folded” monomer induced by perchlorate is more exposed to the solvent than in native dimer.

These are some thoughts to introduce this volume on protein fluorescence. The following articles will describe several specific protein systems and fluorescence techniques. There will be examples that focus on understanding the fluorescence properties of a protein, articles that exploit fluorescence to gain information about protein dynamics, and articles that apply the fluorescence of tryptophan or other fluorophores to gain kinetic or thermodynamic information. The applications of fluorescence are vast.

The sodium perchlorate-induced conformational transition of Staphylococcal nuclease has been monitored by both circular dichroism (CD) and fluorescence spectroscopy. The perchlorate-induced transition is cooperative as observed by both spectroscopic signals. However, the protein loses only about one-third of its native far-UV CD signal at high perchlorate concentrations, indicating that a significant amount of secondary structure remains in the post-transition state. The remaining CD signal can be further diminished in a cooperative manner by the addition of the strong denaturant, urea. Near-UV CD spectra clearly show that the protein loses its tertiary structure in the perchlorate-induced denatured state. The perchlorate-induced transition curves were fit to the standard two-state model and the standard free energy change and m value of the transition are 2.3kcal/mol and 1.8kcal/(molM), respectively. By comparison, the urea-induced unfolding of Staphylococcal nuclease (in the absence of perchlorate) yields an unfolding free energy change, DeltaG(0,un), of 5.6kcal/mol and an m value of 2.3kcal/(molM). Thus, the thermodynamic state obtained in the post-transition region of perchlorate-induced conformation transition has a significantly lower free energy change, a high content of secondary structure, and diminished tertiary structure. These results suggest that the perchlorate-induced denatured state is a partially folded equilibrium state. Whether this intermediate is relevant to the folding/unfolding path under standard conditions is unknown at this time.

This chapter discusses the various ways that fluorescence techniques can be used to obtain thermodynamics information about biological macro-molecules. It discusses that one might think of fluorescence as a spectroscopic method, which of course it is, some of the most valuable applications of fluorescence are to determine thermodynamic and kinetic information about proteins, nucleic acids, and membrane systems. Some of the advantages of fluorescence, over other experimental methods such as scanning or titration microcalorimeter or plasmon resonance, are that (1) fluorescence permits study with an extremely wide range of macromolecule concentrations, (2) fluorescence measurements are amenable to a wide range of solution conditions, (3) it is a rapid monitoring method, which makes it possible to have a rapid throughput in data collection, and (4) fluorescence can provide selectivity by focusing on a relatively few fluorescing centers within a macromolecule. This latter feature can be important, because it helps in understanding of what molecular events are happening and it permits some thermodynamics studies to be carried out even when the samples are not 100% pure. It also reviews that many biological macromolecules contain intrinsic fluorescing groups, or these groups can be attached as extrinsic probes. Also, in cases involving a small molecule as a ligand, the ligand may be the fluorescing species.

The biophysical characterization of globular proteins will almost always include some type of study of the unfolding of protein to obtain thermodynamic parameters. The basic idea is that a transition between a native and unfolded state, induced by temperature, pH, or denaturant concentration, can serve as a standard reaction for obtaining a thermodynamic measure of the stability of the native state. For example, the free energy change for the unfolding reaction can be used to compare the stability of a set of mutant forms of a protein (1-4). This type of analysis is based both on assumptions of the thermodynamic model for the unfolding process and on assumptions in the way the data are analysed; some of these assumptions and their limitations will be discussed below. There are a variety of methods that can be used to monitor an unfolding process. A common method is differential scanning calorimetry, DSC, which measures the variation in the specific heat of a protein-containing solution as a protein is thermally unfolded (5-7). DSC is a popular method for this purpose, but optical methods can also provide suitable information for tracking the unfolding of a protein The spectroscopic signals for the native and unfolded states of a protein can give some insight regarding the structure of the states, and often can provide advantages of economy, ease of measurement and amenability to a wide range of sample concentration. The optical spectroscopic methods that have been used most often for this purpose are absorption spectroscopy, circular dichroism and fluorescence, which will be discussed in this chapter. A key to each of these methods and their use in protein unfolding studies is that the signal is a mole fraction weighted average of the signals of each thermodynamic state. That is, the observed signal, S, can be expressed as . . . S = ∑XiSi . . . . . . 1 . . . where Xi is the mole fraction of species i and si is the intrinsic signal of species i. In order for a particular spectroscopic signal to be useful for tracking a N ↔ U transition of a protein, the signal must be sufficiently different for the N and U states.

We have biosynthetically incorporated several tryptophan analogues into three forms of Staphylococcal nuclease to investigate the spectroscopic characteristics of these "intrinsic" probes and their effect on the structure of the proteins. The set of tryptophan analogues includes 5-hydroxytryptophan, 7-azatryptophan, 4-fluorotryptophan, 5-fluorotryptophan, and 6-fluorotryptophan. 5-Hydroxytryptophan and 7-azatryptophan have red-shifted absorbance spectra, and the latter has a red-shifted fluorescence, which is very sensitive to its environment (being heavily quenched in water). The fluorotryptophans can serve as 19F NMR probes, and 4-fluorotryptophan has a very low fluorescence quantum yield, thus making it a "knock-out" fluorescence analogue. The set of proteins studied includes wild-type nuclease, which has a single tryptophan site at position 140; its V66W mutant, which has a second tryptophan at position 66; and the Delta 137-149 fragment, V66W', which only has a tryptophan at position 66. The environments of positions 66 and 140 are significantly different; position 140 is near the end of the long C-terminal alpha-helix and is moderately solvent-exposed, whereas position 66 is in the beta-barrel core region of the protein and is surrounded by apolar side chains. Absorbance and 19F NMR spectra are used to estimate the extent of analogue incorporation for each protein. Steady-state and time-resolved fluorescence data are reported to characterize the emission of the analogues in these positions in the three proteins and to develop the use of the analogues as probes of protein structure and dynamics. Circular dichroism spectra are reported to show that, in all but a couple of cases, the secondary structure of the proteins containing the analogues is not significantly perturbed by the probes. Additionally, fluorescence anisotropy decay data show the variants of wild-type nuclease to have a rotational correlation time similar to that of tryptophan-containing nuclease.

We report a general route for the synthesis of E and Z isomers of indol-3-yl cyclopropyl amines, carboylic acids, and esters. These cyclopropane containing molecules are of interest as conformationally constrained analogues of tryptamine and indole propionic acid, biologically active indoles. The route involves reaction of vinyl indole with ethyl diazoacetate, chromatographic separation of the E and Z stereoisomers of the resulting cyclopropane esters, hydrolysis to form the E and Z cyclopropane acids, and formation of amines by the Curtius reaction.

Phosphorescence and optically detected magnetic resonance (ODMR) measurements are reported on the triplet states of the tryptophan analogues, 7-azatryptophan (7AW), 5-hydroxytryptophan (5HW), and 4-, 5-, and 6-fluorotryptophan (4FW, 5FW, 6FW), when incorporated at position 140 of wild-type Staphylococcal nuclease (7AW-nuclease, etc. ), positions 66 and 140 of its V66W mutant (7AW-V66W, etc.), and the deletion fragment of the latter, Delta 137-149 (7AW-V66W', etc.). These measurements point to the retention of protein structure at position 140 in each of the wild-type nuclease analogues. Substitution of the analogue at both tryptophan sites of V66W leads to structured sites with differentiated triplet-state properties for all analogues except 7AW-V66W, whose structure is destabilized. 5HW-V66W' is the only fragment that apparently lacks structure at position 66. All other V66W' analogues exhibit a structured environment at position 66 (4FW-V66W' was not studied), but in each case this site can be differentiated readily from the corresponding site in intact V66W. 7AW-V66W' is resolved by ODMR into two discrete structures with slightly differing zero field splittings (ZFS). Interaction of the protein with 5HW at position 66 of 5HW-V66W induces a 2-fold increase in the ZFS E parameter, which is reduced to its normal value upon formation of the fragment, 5HW-V66W'. Analogous effects occur for 5FW, but on a smaller scale.

Horse liver alcohol dehydrogenase contains two tryptophan residues per subunit, Trp-15 on the surface of the catalytic domain and Trp-314 buried in the interface between the subunits of the dimer. We studied the contributions of the tryptophans to fluorescence and catalytic dynamics by substituting Trp-314 with a leucine residue and making two compensatory mutations that were required to obtain a stable protein, leading to the triple mutant M303F-L308I-W314L enzyme. The substitutions increased by two- to sixfold the turnover numbers for ethanol oxidation, acetaldehyde reduction, and the dissociation constants of the coenzymes. The rate of the exponential burst phase for the transient oxidation of ethanol increased slightly, but the rate of dissociation of the enzyme-NADH complex still limited turnover of ethanol, as for wild-type enzyme. The three substitutions at the dimer interface apparently activate the enzyme by allowing more rapid conformational changes that accompany coenzyme binding, probably due to movement of the loop containing residues 293 to 298. The emission spectrum of M303F-L308I-W314L enzyme, which contains Trp-15, was redshifted compared to wild-type enzyme. Time-resolved fluorescence measurements with the triple mutant show that the decay of Trp-15 is dominated by a approximately 7-ns component. In the mutant enzyme with Trp-15 substituted with phenylalanine, the decay of Trp-314 is dominated by a approximately 4-ns component. Solute quenching data for wild-type enzyme and the mutants show that only Trp-15 is exposed to iodide and acrylamide, whereas Trp-314 is inaccessible. The luminescence properties of the tryptophan residues in the mutated enzymes are consistent with conclusions from studies of the wild-type enzyme [M. R. Eftink, 1992, Adv. Biophys. Chem. 2, 81-114].

The tryptophan analogues, 5-hydroxytryptophan, 7-azatryptophan, 4-fluorotryptophan, 5-fluorotryptophan, and 6-fluorotryptophan, have been biosynthetically incorporated into Staphylococcal nuclease, its V66W mutant, and the Delta 137-149 fragment of the latter mutant. The guanidine-HCl induced unfolding and thermal unfolding of these proteins were studied to characterize the effect of incorporation of these tryptophan analogues on the thermodynamic stability of the proteins. The three proteins have tryptophan residues at positions 140 (in wild type) and 66 (in the Delta 137-149 fragment of V66W) and at both positions (in V66W). The unfolding data show that 5-hydroxytryptophan does not perturb the stability of wild-type nuclease, but it destabilizes the fragment and causes the V66W mutant to unfold in a more cooperative manner. 7-Azatryptophan is found to destabilize all three proteins. 4-Fluorotryptophan is slightly stabilizing of the three proteins, but the other two fluorotryptophans do not alter the stability of the proteins.