Luminescence behaviour of 5-hydroxyindole in different environments

PRODAN Photophysics as a Tool to Determine the Bilayer Properties of Different Unilamellar Vesicles Composed of Phospholipids

Vesicles formed by phospholipids are promising candidates for drug delivery. It is known that the lipid composition affects properties such as the rigidity-fluidity of the membrane and that it influences the bilayer permeability, but sometimes sophisticated techniques are selected to monitor them. In this work, we study the bilayer of different unilamellar vesicles composed of different lipids (1,2-dioleoyl-sn-glycero-3-phosphocholine, DOPC, and lecithin) and diverse techniques such as extruder and electrospun templates and using 6-propionyl-2-(N,N-dimethyl) aminonaphthalene (PRODAN) and its photophysics. Moreover, we were able to monitor the influence of cholesterol on the bilayers. We demonstrate that the bilayer properties can be evaluated using the emission feature of the molecular probe PRODAN. This fluorescent probe gives relevant information on the polarity and fluidity of the microenvironment for unilamellar vesicles formed by two different methods. The PRODAN emission at 434 nm suggests that the bilayer properties significantly change if DOPC or lecithin is used in the vesicle preparation especially in their fluidity. Moreover, cholesterol induces alterations in the bilayer's structural and microenvironmental properties to a greater or lesser degree in both vesicles. Thus, we propose an easy and elegant way to evaluate physicochemical properties, which is fundamental for manufacturing vesicles as a drug delivery system, simply by monitoring the molecular probe emission band centered at 434 nm, which corresponds to the PRODAN species deep inside the bilayer.

Hydroxy-tryptophan containing derivatives of tritrpticin: modification of antimicrobial activity and membrane interactions

Tritrpticin is an antimicrobial peptide with a strong microbicidal activity against Gram-positive and Gram-negative bacteria as well as fungi. The 13-residue peptide is essentially symmetrical and possesses a unique cluster of three Trp residues near the center of its amino acid sequence. The mechanism of action of tritrpticin is believed to involve permeabilization of the cytoplasmic membrane of susceptible bacteria. However it has been suggested that intracellular targets may also play a role in its antimicrobial activity. In this work the mechanism of action of several tritrpticin derivatives was studied through substitution of the three Trp residues with 5-hydroxy-tryptophan (5OHW), a naturally occurring non-ribosomal amino acid. Although it is more polar, 5OHW preserves many of the biophysical and biochemical properties of Trp, allowing the use of fluorescence spectroscopy and NMR techniques to study the interaction of the modified peptides with membrane mimetics. Single or triple 5OHW substitution did not have a large effect on the MIC of the parent peptide against Escherichia coli and Bacillus subtilis. However, the mechanism of action was altered by simultaneously replacing all three Trp with 5OHW. Our results suggest that the inner membrane of Gram-negative bacteria did not constitute the main target of this particular tritrpticin derivative. Since the addition of a hydroxyl group to the indole motif of the Trp residue was able to modify the mechanism of action of the peptides, our data confirm the importance of the Trp cluster in tritrpticin. This work also shows that 5OHW constitutes a new probe to modulate the antimicrobial activity and mechanism of action of other Trp-rich antimicrobial peptides.

Full cLR-PCM calculations of the solvatochromic effects on emission energies

Are cLR-PCM excited-state geometries providing more accurate solvatochromic shifts?

A spectroscopic survey of substituted indoles reveals consequences of a stabilized 1Lb transition

Although tryptophan is a natural probe of protein structure, interpretation of its fluorescence emission spectrum is complicated by the presence of two electronic transitions, (1)L(a) and (1)L(b). Theoretical calculations show that a point charge adjacent to either ring of the indole can shift the emission maximum. This study explores the effect of pyrrole and benzyl ring substitutions on the transitions' energy via absorption and fluorescence spectroscopy, and anisotropy and lifetime measurements. The survey of indole derivatives shows that methyl substitutions on the pyrrole ring effect (1)L(a) and (1)L(b) energies in tandem, whereas benzyl ring substitutions with electrophilic groups lift the (1)L(a)/(1)L(b) degeneracy. For 5- and 6-hydroxyindole in cyclohexane, (1)L(a) and (1)L(b) transitions are resolved. This finding provides for (1)L(a) origin assignment in the absorption and excitation spectra for indole vapor. The 5- and 6-hydroxyindole excitation spectra show that despite a blue-shifted emission spectrum, both the (1)L(a) and (1)L(b) transitions contribute to emission. Fluorescence lifetimes of 1(0) ns for 5-hydroxyindole are consistent with a charge acceptor-induced increase in the nonradiative rate (1).

Binding of Human Serum Albumin to N-(p-Ethoxy-phenyl)-N'-(1-naphthyl)thiourea and Synchronous Fluorescence Determination of Human Serum Albumin

The binding of N-(p-ethoxy-phenyl)-N'-(1-naphthyl)thiourea (EPNT) to human serum albumin (HSA) was investigated under simulative physiological conditions by fluorescence spectra in combination with UV absorption spectroscopy and a molecular modeling method. A strong fluorescence quenching reaction of EPNT to HSA was observed, and the quenching mechanism was suggested to be static quenching according to the Stern-Volmer equation. The binding constants (K) at different temperatures as well as thermodynamic parameters, enthalpy change (DeltaH) and entropy change (DeltaS), were calculated according to relevant fluorescent data and the vant' Hoff equation. This indicated that a hydrophobic interaction was a predominant intermolecular force for stabilizing the complex, which is in agreement with the results of molecule modeling study. The effects of energy transfer and other ions on the binding constant were considered. In addition, synchronous fluorescence technology was successfully applied to the determination of HSA added into the EPNT solution.

Human serum albumin interaction with formononetin studied using fluorescence anisotropy, FT-IR spectroscopy, and molecular modeling methods

Interaction of formononetin with a model transport protein, human serum albumin (HSA), has been studied using fluorescence anisotropy, FT-IR spectroscopy, and molecular modeling methods. Upon binding with HSA, the fluorescence spectrum of formononetin exhibits appreciable hypsochromic shift along with an enhancement in the fluorescence intensity. Gradual addition of HSA led to a marked increase in fluorescence anisotropy (r). From the value of fluorescence anisotropy, it is argued that the drug is located in a restricted environment of protein. The binding constant (K approximately 1.6 x 10(5) M(-1)) and the standard free energy change (DeltaG(0) approximately -29.9 kJ/mol) of formononetin-HSA interaction have been calculated according to the relevant fluorescence data. Fourier transform infrared measurements have shown that the secondary structures of the protein have been changed by the interaction of formononetin with HSA. Computational mapping of the possible binding sites of formononetin revealed the molecule to be bound in the large hydrophobic cavity of subdomain IIA.

Identification and reactivity of the triplet excited state of 5-hydroxytryptophan

Both the neurotransmitter serotonin and the unnatural amino acid 5-hydroxytryptophan (5HT), contain the 5-hydroxyindole chromophore. The photochemistry of 5HT is being investigated in relation to the multiphoton excitation of this chromophore to produce a characteristic photoproduct with green fluorescence ('hyperluminescence'). Laser flash photolysis (308 nm) of 5HT in aqueous solution at neutral pH produces both the neutral 5-indoloxyl radical (lambda(max) 400-420 nm) and another transient absorption with lambda(max) 480 nm and lifetime of 2 micros in deaerated solutions. Based on quenching by oxygen and beta-carotene, the species at 480 nm is identified as the triplet excited state of 5HT. In acidic solution a new oxygen-insensitive intermediate with lambda(max) 460 is assigned to the radical cation of 5HT. Time-resolved measurements of luminescence at 1270 nm have shown that the triplet state of 5HT is able to react with oxygen to form singlet excited oxygen (1O2*) with a quantum yield of approximately 0.1. However, 5HT has also been found to be an effective quencher of singlet oxygen with a second order rate constant of 1.3 x 10(8) dm3 mol(-1) s(-1). The results are discussed in the light of recent observations on the multiphoton-excited photochemistry of serotonin.

Static and time-resolved fluorescence investigations of tryptophan analogues--a solvent study

The fluorescence properties of tryptophan, polytryptophan and seven of its analogues (7-azatryptophan, 5-hydroxytryptophan, 5-methoxytryptophan, 5-fluorotryptophan, 5-methyltryptophan, 5-bromotryptophan, and 6-fluorotryptophan) are studied using two novel fluorescence spectroscopic techniques for a wide range of solvent polarities. Two-dimensional mapping of all emission and all fluorescence spectra using excitation-emission spectroscopy (EES) has been used to determine quantum yields, positions of emission maxima, full widths at half maximum (FWHMs) as well as Stokes' shifts. Additionally, fluorescence lifetimes obtained from time-resolved experiments using a picosecond laser system are presented and compared with the data acquired from the static setup. This systematic study of the fluorescence characteristics is a prerequisite to assess the potential of these analogues to act as structure-conserving substitutes for tryptophan in protein fluorescence experiments. The potential of these analogues, to act as probes for the local environment, and allow estimation of the polarity in the vicinity of the fluorophore and its exposure to the solvent, is discussed.

Low temperature luminescence behaviours of 7-azatryptophan, 5-hydroxytryptophan and their chromophoric moieties

The non-natural amino acids 7-azatryptophan (7AT) and 5-hydroxytryptophan (5HT) have come into significant recent prominence as novel intrinsic luminescence probes for protein structure, function and dynamics. Here, we examine the low temperature luminescence behaviours of these molecules and their respective chromophoric moieties 7-azaindole (7AI) and 5-hydroxyindole (5HI) in representative solvent media. To ascertain, in particular, the potential usefulness of 7AT and 5HT as phosphorescence probes for exploring protein environments with different hydrogen bonding characteristics, a comparison is made of the phosphorescence properties of 7AI and 5HI chromophores in frozen solutions of ethanol and ethyl acetate at 77 K. These solvent media have been chosen as representative models for polar protic and aprotic environments in proteins, respectively. Our findings indicate that one or more of the phosphorescence emission parameters (phosphorescence emission maxima, relative yield and phosphorescence lifetime) of 7AI and 5HI chromophores can serve as sensitive and discriminating probes of hydrogen bonding and related aspects of their surrounding environments. Furthermore, in a model viscous environment (glycerol at low temperatures) significant temperature dependence and red edge excitation shift (REES) effects are observed for the fluorescence emission of 7AT and its chromophoric moiety 7AI. This is consistent with pronounced dipolar relaxation properties of these molecules, and suggests interesting possibilities for exploiting REES in exploring their environmental rigidity in motionally constrained situations.

Influence of reverse micellar environments on the fluorescence emission properties of tryptophan octyl ester

A number of recent studies have presented perspectives on the hydrophobic fluorescence probe tryptophan octyl ester (TOE). This molecule has attracted notable attention as a suitable model for the natural fluorophore tryptophan, in case of membrane proteins. We report here, for the first time, the fluorescence emission behaviour of TOE in reverse micelles of aerosol-OT (AOT) in n-heptane, containing different amounts of water. Relevant studies in representative homogeneous solvent media are also included for comparison. The fluorescence emission parameters (especially emission maximum, relative intensity, and anisotropy) of TOE are found to exhibit significant variation upon changes in the water/surfactant molar ratio (w(0)) of the reverse micelles. Fluorescence decay studies on TOE which we have also performed, indicate biexponential decay kinetics in reverse micelles as well as in homogeneous solvent media. The implications of these findings are examined in relation to the potentialities of TOE as a novel fluorescence probe for membrane proteins present in water restricted environments prevailing at the interfaces of biomembranes (for which reverse micelles serve as ideal model systems).