Electrostatic interactions in phospholipid membranes revealed by coherent 2D IR spectroscopy

Abstract

The inter-and intramolecular interactions of the carbonyl moieties at the polar interface of a phospholipid membrane are probed by using nonlinear femtosecond infrared spectroscopy. Twodimensional IR correlation spectra separate homogeneous and inhomogeneous broadenings and show a distinct cross-peak pattern controlled by electrostatic interactions. The inter-and intramolecular electrostatic interactions determine the inhomogeneous character of the optical response. Using molecular dynamics simulation and the nonlinear exciton equations approach, we extract from the spectra short-range structural correlations between carbonyls at the interface.carbonyl ͉ interface ͉ intermolecular ͉ nonlinear exciton equations ͉ molecular dynamics A s a constituent organelle in a cell, the membrane sets the information and energy gradients necessary for life. Carbonyl, phosphate, and choline are the common structural moieties in the polar surface of cellular membranes (1), mediating molecular recognition and signal transduction (1-4). Unfortunately, our knowledge on their arrangement and dynamics is rather limited due to experimental difficulties. In a lipid bilayer, lateral irregularity smears the diffraction pattern in neutron scattering measurements, and NMR resonances are broad because of the restricted motions that result in incomplete motional narrowing. IR spectroscopy, on the other hand, is known to be helpful in application to such systems. Since 1972, the IR resonance of carbonyl moieties in phospholipid membranes has attracted considerable attention (5-9). The absorption band shows a clear inhomogeneous character and can be described as a superposition of several substates (5-7). The carbonyl stretching line-shapes in membranes could yield direct information about molecular architecture and fluctuations in the membrane interface (10, 11), provided that the origin of the spectral inhomogeneity of the carbonyl IR response in phospholipid membranes is understood. The inhomogeneity was attributed to differences in the local environment of the sn-1 and sn-2 carbonyl moieties stemming from the packing arrangements (6, 8), the local chain conformations (8,9,(12)(13)(14), the relative positions of the two CAO groups with respect to the interface (9, 15), and the degree of hydration (16,17). In an elegant work, Blume et al. (16) ruled out all of the scenarios involving local structural differences except hydrogen bonding. In a recent study, we eliminated the variance in hydration as a possible source of inhomogeneity (18).Here we employ 2D IR spectroscopy (19,20) to explore whether and to what extent the inhomogeneities of the carbonyl absorption can be attributed to electric field fluctuations. 2D IR techniques are femtosecond optical analogues of 2D NMR that generate 2D correlation plots that can separate homogeneous and inhomogeneous broadenings along the diagonal and antidiagonal axis and provide a rich cross-peak pattern (21). Electrostatic interactions strongly contribute to the optical response of molecular crystals (2...

Distinct Water Species Confined at the Interface of a Phospholipid Membrane

“…As a consequence, the spectral inhomogeneity decays with a comparatively slow rate, dictated by the time scale of the phospholipid rotational dynamics and lateral mobility. This long time scale inhomogeneity is similar to that observed for the carbonyl stretching modes reported elsewhere [21]. The complexity of water's optical response at the interface of dehydrated membranes [15] reflects the presence of several aqueous molecular forms.…”

Residue-Specific Vibrational Echoes Yield 3D Structures of a Transmembrane Helix Dimer

“…Mackenzie et al detected inter-helical contacts between nearby amides in the NMR structure of the TM domain of the homodimeric protein, glycophorin (13). This finding suggested that it might be possible to solve structures of membrane proteins by 2D-IR if two methodological and theoretical challenges could be met.…”

“…Based on knowledge of the typical sequences of interacting regions of TM dimers (13) and mutagenesis studies of the αIIb homodimer (8) it was anticipated this region would encompass some amides on different helices that would have measurable interactions. We then applied 2D IR spectroscopy to micelle embedded samples of these peptides in solution.…”

Kohärente mehrdimensionale Schwingungsspektroskopie von Biomolekülen: Konzepte, Simulationen und Herausforderungen

“…Ein 2DIR-Spektrum der C = O-Streckschwingung dieser Doppelschicht ist in Abbildung 22 gezeigt. [107] Die Entartung der sn-1-und sn-2-Carbonylgruppen wurde durch 13 C-Markierung der sn-2-Kette aufgehoben, sodass zwei breite Banden bei 1697 und 1740 cm À1 beobachtet wurden.…”

“…[106] Righini und Mitarbeiter untersuchten die lokale Struktur von Lipiden in Dimyristoylphosphatidylcholin(DMPC)-Membranen durch Isotopenmarkierung der Carbonylgruppen der Membran. [107,108] Durch Signalformanalyse der 2D-Spektren der Amid-I-Banden (Streckschwingung der Rückgrat-Carbonylgruppen) von elf entlang eines Transmembranpeptidbündels ausgerichteten Aminosäureresten konnten Zanni et al die homogenen und inhomogenen Signalverbreiterungen bestimmen, die Auskunft über die Strukturverteilungen und die Pikosekundendynamik von Peptiden und deren Umgebung geben. [93] Auch 2DIR-Studien fehlgefalteter Peptidaggregate (Amyloidfibrillen) wurden beschrieben.…”