Raman spectroscopy study of the interaction of 3,5,3′‐triiodo‐L‐thyronine with phosphatidylglycerol lipid bilayers

Abstract

A Raman spectroscopic study of anionic model membranes and their structural alterations exerted by a relatively small biomolecule, such as the hormone 3,5,3′-triiodo-L-thyronine (T3), is presented. Spectral differences between pure dipalmitoylphosphatidylglycerol (DPPG) multilamellar vesicles and DPPG-T3 mixture and between pure dilauroylphosphatidylglycerol (DLPG) and DLPG-T3 mixture were evaluated in order to determine the response of lipid membranes in gel and liquid-crystalline phases to the hormone incorporation. Density functional theory (DFT) calculations support the band analysis of the complex 1150-1050 cm À1 Raman region. Geometry optimizations and vibrational behaviors of a model charged molecule that mimics the phosphatidylglycerol lipid moiety in solvated state were taken into account for the spectral interpretation of this specific region. The anionic nature of the lipid polar head plays an important role in the interaction with the hormone, as is evidenced by the C¼O and PO 2 À stretching bands. In addition, the differential penetration of T3 into the hydrophobic region of the membranes shows to be dependent on the lipid phase. The spectral data were compared with those previously obtained for zwitterionic membranes.

Structural characterization of phosphatidylglycerol model membranes containing the antibiotic target lipid II molecule: a Raman microspectroscopy study

“…Our computational results show that the υ s PO 2 − tends to increase the wavenumbers when is involved in intermolecular interactions while downshifts would be expected upon intramolecular associations; a compromise between both trends is observed in mixed H‐bond interactions. Although the experimental evidence does not support a rigorous differentiation between intra‐ and intermolecular associations, we propose a detailed assignment of the bands belonging to the polar headgroups based on the computational predictions and previous studies of lipid membranes upon interaction with biomolecules . Thus, the bands at 1100 cm −1 (DLPG spectrum) and at 1098 cm −1 (DPPG spectrum) are tentatively assigned to υ s PO 2 − of intermolecular H‐bonded groups, while the bands at 1092 and 1093 cm −1 (in DLPG and DPPG spectra, respectively) are assigned to the stretching of phosphate groups in intramolecular associations with the glycerol OH groups.…”

“…The relevance of these bands is based on the fact that they directly reflect intramolecular trans– gauche conformational changes within the hydrocarbon chains region of the lipid matrix . In addition, the symmetric stretching of the PO 2 − groups (υ s PO 2 − ) and the stretching of the glycerol C―O bonds (υC―O) of the lipid polar heads are also expected around 1000–1100 cm −1 in the membrane Raman spectra . Band decomposition by applying a curve fitting procedure was required to perform a detailed analysis of this highly complex spectral region.…”

“…The spectral profile delimited by these two strong bands characterizes the lipid disorder because it contains the bands corresponding to C―C stretching modes of gauche defects in the acyl chains. Because these defects can be localized at different positions along the acyl chain, they originate different features whose intensities are strongly dependent on the lipid phase: the band at ~1084 cm −1 , associated to gauche conformers at the chain ends (end‐ gauche , υ(C―C) e‐G ), is observable even in the ordered phase; the band at ~1077 cm −1 , corresponding to gauche –trans– gauche (υ(C―C) G–T–G ) conformations in the body of the chain is typical for the L α phase; a third type of kinked‐chain structure, the double‐ gauche (υ(C―C) G‐G ) conformation, can be also observed at ~1070 cm −1 , although this conformation is considerably less abundant and further studies should be perform to reach a more confident characterization. It is worth mentioning that the spectral decomposition and subsequent interpretation show consistency with previous results we obtained for DLPC and DPPC lipid membranes in pure state and in interaction with several thyroid hormones .…”

“…The peak fitting procedure applied to the spectral range comprised between 1000 and 1150 cm −1 reveals also the presence of several overlapped bands attributable to the υ s PO 2 − and υC―O modes of the lipids polar headgroups (see Figs and ). The assignment of these features is based on reported experimental values for PG and PC model membranes in different phase states; quantum chemical calculations for the charged molecule [CH 3 ―O―P(O) 2 ―O―CH 2 ―CHOH―CH 2 OH] − that mimics the PG moiety; and MD simulations of pure PG membranes in liquid crystalline and gel states . Predicted wavenumbers for the hydroxyl C―O bonds and the υ s PO 2 − mode are listed in Table .…”

“…Interactions at the interfacial level are evaluated in based on the CO band that appeared around 1740 cm −1 . This vibration appears in the Raman spectra of lipid membranes as a weak band with complex contour because of the different conformations and hydration degree . MD simulations of DOPG and DPPG bilayers in the liquid crystalline and gel phases, respectively, described different environments surrounding the ester CO groups .…”