Biomedical subjects

Jean Duhamel

Publications and source records attributed to Jean Duhamel.

6 recordsLinked to original sources

PubMed2006

Study of the semidilute solutions of poly(N,N-dimethylacrylamide) by fluorescence and its implications to the kinetics of coil-to-globule transitions.

The large scale motions of poly(N,N-dimethylacrylamide) chains randomly labeled with pyrene (Py-PDMA) were monitored by steady-state and time-resolved fluorescence in semidilute solutions of naked PDMA in acetone and DMF for polymer concentrations ranging from 0 to 550 g/L. Although increasing the polymer concentration of the solution led to a decrease of the mobility of the chromophore attached onto the PDMA backbone, this reduction was rather modest when compared to the large increase of the macroscopic viscosity. This result indicated that locally, the monomer constituting the chains experienced freedom of movement despite the high solution viscosity. The restricted mobility of the chromophore was characterized by the number of monomers occupying the volume probed by the excited chromophore during its lifetime, referred to as a fluorescence "blob". The number of monomers constituting a fluorescence blob, N(F)(-)(blob), and the volume of a fluorescence blob, V(F)(-)(blob), were found to decrease as the polymer concentration of the solution increased, reflecting the decreased mobility experienced by the chromophore. In DMF, the radius of an F-blob was found to scale as N(nu)(F)(-)blob, where nu equaled 0.66 +/- 0.03, very close to the expected value of the Flory exponent of 0.6 for a polymer in a good solvent. The combined knowledge of how N(F)(-)(blob) varies with the fluorescence lifetime of the chromophore and the coil density of the polymer was used to propose a new means of studying coil-to-globule transitions with potential implications for predicting the rate of protein folding.

Acetone↗

PubMed2006

Polymer chain dynamics in solution probed with a fluorescence blob model.

This Account describes a new fluorescence tool that characterizes the chain dynamics of polymers in solution. This new tool, coined the fluorescence blob model (FBM), is employed to analyze the fluorescence decays of polymers randomly labeled with pyrene. The FBM yields the number of monomers making up a blob (Nblob) where a blob is the volume probed by an excited pyrene. By establishing a relationship between Nblob and the lifetime of pyrene used as an internal clock, the FBM provides previously unavailable information about polymer chain dynamics and opens new venues of research, which are described in this Account.

Journal Article↗

PubMed2005

Correlations between the viscoelastic behavior of pyrene-labeled associative polymers and the associations of their fluorescent hydrophobes.

The viscoelastic behavior of a series of three pyrene-labeled hydrophobically modified alkali swellable emulsion copolymers (Py-HASEs) was investigated. All Py-HASEs thickened the aqueous solutions with viscosities orders of magnitude larger than that of a HASE control which displayed no pyrene hydrophobe. This fact demonstrated that the pyrene molecule is a good hydrophobe for associative thickeners such as HASEs. The Py-HASE solutions exhibited shear thinning, whose magnitude was found to increase with increasing pyrene content. A large shear-thinning effect indicates that a large fraction of the elastically active cross-links has been severed. Fluorescence measurements on the Py-HASEs confirmed that the smaller the pyrene content of the Py-HASE, the more intermolecular associations it formed, in agreement with the results obtained by rheology. Above the overlap concentration of the polymers, the zero-shear viscosity of the Py-HASE solutions increased steeply with increasing polymer concentration. The onset concentration where viscosity increases matches the onset concentration where intermolecular associations are being formed, as probed by fluorescence. Oscillatory rheological measurements were carried out to determine the terminal relaxation time, Td, and the storage modulus at the infinite time limit, G0, of the Py-HASE network. G0 was found to increase with decreasing pyrene contents, indicating that Py-HASEs with lower pyrene contents exhibited a higher density of elastically active chains. This result is in agreement with the trends obtained by the fluorescence and steady-state rheology measurements. A model is suggested that accounts for the fluorescence and rheology results.

Journal Article↗

PubMed2005

Comparison of the association level of a pyrene-labeled associative polymer obtained from an analysis based on two different models.

A hydrophobically modified alkali swellable emulsion copolymer (HASE) was labeled with pyrene and its fluorescence behavior was monitored by steady-state and time-resolved fluorescence as increasing amounts of the surfactant sodium dodecyl sulfate (SDS) were added to the solution. In aqueous solution, the pyrene pendants are aggregated. As SDS is added, the surfactant binds to the pyrene aggregates, which leads to their breakup at an onset SDS concentration of 1.25 x 10(-3) mol/L. The breakup of the pyrene aggregates is complete at 4.25 x 10(-3) mol/L, which is slightly larger than the critical micellar concentration of SDS in 0.01 M Na(2)CO(3) aqueous solution at pH 9 found to equal 3.5 x 10(-3) mol/L by surface tension measurements. The pyrene pendants were present as different species in solution, and the fractions representative of all emissive pyrene species were determined from the global analysis of the monomer and excimer fluorescence decays. Two analyses were applied to the decays. In the first analysis, the diffusional encounters between pyrene pendants were described by the blob model. In the second analysis, no assumptions were made on how the pyrene pendants encountered each other. Both analyses yielded identical results which demonstrate that the determination of the fractions of the different emissive pyrene species of a solution of a pyrene-labeled associative polymer does not depend on the model chosen to account for the diffusional encounters taking place between pyrene pendants.

Journal Article↗

PubMed2004

Self-assembling peptide as a potential carrier of hydrophobic compounds.

Microcrystals of a hydrophobic cargo were stabilized by EAK16 II, a self-assembling oligopeptide, and suspended in aqueous solution. Pyrene was used as a model hydrophobic compound. Egg phosphatidylcholine (EPC) vesicles were prepared to mimic a cell membrane. Pyrene was released from its EAK16 II coating into EPC vesicles. The excimer decay profiles were acquired. They showed that pyrene is present in the crystalline form when stabilized by EAK16 II, it is molecularly dispersed in EPC vesicles, and it is completely released from its EAK16 II coating into the membrane bilayers. The release of pyrene from the microcrystals coated with EAK16 II into the EPC membrane was followed by fluorescence as a function of time. The amount of pyrene released into the EPC vesicles at a given time was quantified using a calibration curve. The concentration of pyrene released was determined as a function of time, and the concentration-versus-time profile was fitted with one exponential. The rate of pyrene release was found to depend on the peptide-to-pyrene molecular ratio. Higher peptide-to-pyrene ratios lead to slower transfer of pyrene to the lipophilic environment. Scanning electron micrographs demonstrated that a thicker coating on the pyrene crystals results in a slower release. The data presented in this work demonstrate that the self-assembling EAK16 II can stabilize a hydrophobic cargo in aqueous solution and deliver it into a lipophilic environment, and that the rate of transfer can be adjusted by tuning the peptide-to-pyrene ratio.

Drug Carriers↗

PubMed2003

Side-chain dynamics of an alpha-helical polypeptide monitored by fluorescence.

The "blob" model, developed to analyze the fluorescence decays of polymers randomly labeled with pyrene, has been applied to a series of pyrene-labeled poly(glutamic acid)s (PyPGA) in DMF and carbonated buffer solutions at pH 9. Poly(glutamic acid) (PGA) exists in the ionized form in the buffer solutions as poly(sodium glutamate) (PGNa). PGA adopts an alpha-helical conformation in DMF, whereas in aqueous solution PGNa is a random coil. Fluorescence, UV-vis absorption, and circular dichroism measurements indicate that in our studies pyrene pendants attached themselves along PGA in a clustered manner. Simulations were carried out to establish that the geometry of the PGA alpha-helix induces the high level of pyrene clustering. Since the level of pyrene clustering decreased with lower pyrene content, information about naked PGA was retrieved by extrapolating the trends obtained by fluorescence to zero pyrene content. Analysis of the fluorescence decays demonstrated that during its lifetime an excited pyrene probes a 32 amino acid section of the PGA alpha-helix. This result was supported by molecular mechanics optimizations. This study establishes that the blob model, originally used to monitor the encounters between pyrenes attached randomly onto a polymer adopting a random coil conformation, can also be applied to study the dynamics of the side chains of structured proteins. Since the blob model helps in monitoring the encounters between amino acids in the initial state (i.e., random coil) and in the final state (i.e., structured protein) of the folding pathway of a protein, it could be applicable to the study of protein folding.

Circular Dichroism↗