ABSTRACT
Thin film bulk acoustic wave resonator (FBAR) devices supporting simultaneously multiple resonance modes have been designed for gravimetric sensing. The mechanism for dual-mode generation within a single device has been discussed, and theoretical calculations based on finite element analysis allowed the fabrication of FBARs whose resonance modes have opposite reactions to temperature changes; one of the modes exhibiting a positive frequency shift for a rise of temperature whilst the other mode exhibits a negative shift. Both modes exhibit negative frequency shift for a mass load and hence by monitoring simultaneously both modes it is possible to distinguish whether a change in the resonance frequency is due to a mass load or temperature variation (or a combination of both), avoiding false positive/negative responses in gravimetric sensing without the need of additional reference devices or complex electronics.
Subject(s)
Acoustics/instrumentation , Biosensing Techniques/instrumentation , Proteins/chemistry , Adsorption , Animals , Equipment Design , Gravitation , Humans , Sound , Temperature , Transducers , X-Ray Diffraction , Zinc Oxide/chemistryABSTRACT
A thermodynamic approach is proposed to quantitatively analyze the binding isotherms of peptides to model membranes as a function of one adjustable parameter, the actual peptide charge in solution z(p)+. The main features of this approach are a theoretical expression for the partition coefficient calculated from the molar free energies of the peptide in the aqueous and lipid phases, an equation proposed by S. Stankowski [(1991) Biophysical Journal, Vol. 60, p. 341] to evaluate the activity coefficient of the peptide in the lipid phase, and the Debye-Hückel equation that quantifies the activity coefficient of the peptide in the aqueous phase. To assess the validity of this approach we have studied, by means of steady-state fluorescence spectroscopy, the interaction of basic amphipathic peptides such as melittin and its dansylcadaverine analogue (DNC-melittin), as well as a new fluorescent analogue of substance P, SP (DNC-SP) with neutral phospholipid membranes. A consistent quantitative analysis of each binding curve was achieved. The z(p)+ values obtained were always found to be lower than the physical charge of the peptide. These z(p)+ values can be rationalized by considering that the peptide charged groups are strongly associated with counterions in buffer solution at a given ionic strength. The partition coefficients theoretically derived using the z(p)+ values were in agreement with those deduced from the Gouy-Chapman formalism. Ultimately, from the z(p)+ values the molar free energies for the free and lipid-bound states of the peptides have been calculated.
Subject(s)
Dansyl Compounds , Lipid Bilayers/chemistry , Melitten/analogs & derivatives , Melitten/chemistry , Protein Conformation , Substance P/analogs & derivatives , Substance P/chemistry , Amino Acid Sequence , Electrochemistry , Molecular Sequence Data , Spectrometry, Fluorescence , Thermodynamics , TransglutaminasesABSTRACT
The interaction of monocationic quinine with zwitterionic dimyristoyl phosphatidylcholine (DMPC) and mixed negatively-charged dimyristoylphosphatidyl glycerol (DMPG)/DMPC small unilamellar vesicles in the liquid-crystalline phase was investigated by steady-state fluorescence spectroscopy at pH 7 and 37 degrees C. The maximum fluorescence emission peak at 383 nm, upon excitation at 335 nm, shifts to lower wavelength and decreases its intensity as the ratio between the total lipid and quinine concentrations increases. This indicates that in the membrane-bound state quinine is in an environment of low polarity, more deeply buried when anionic DMPG is present in the vesicle. For monoprotonated quinine/DMPC system the corresponding association isotherms show that the extension of binding is slightly enhanced as the ionic strength decreases, whereas for mixed DMPG/DMPC vesicles at low ionic strength, the association of the drug is favoured as the percentage of anionic DMPG increases. The binding curves have been quantitatively analyzed by the binding and the partition models including in this latter an activity coefficient, gamma, to account for non ideal quinine interactions. It is demonstrated for both neutral and anionic membranes that the activity coefficient approaches the unity and that the deviation from ideality is mainly due to electrostatic forces.
