Interactions of Tyrosine in Leu-Enkephalin at a Membrane−Water Interface: An Ultrafast Two-Dimensional Infrared Study Combined with Density Functional Calculations and Molecular Dynamics Simulations

Abstract

The interactions of neuropeptides and membranes play an important role in peptide hormone function. Our current understanding of peptide-membrane interactions remains limited due to the paucity of experimental techniques capable of probing such interactions. In this work, we study the nature of opioid peptide-membrane interactions using ultrafast two-dimensional infrared (2D IR) spectroscopy. The high temporal resolution of 2D IR is particularly suited for studying highly flexible opioid peptides. We investigate the location of the tyrosine (Tyr) side chain of leucine-enkephalin (Lenk) in lipid bilayer membranes by measuring spectral diffusion of the phenolic ring vibrational mode in three different systems: Lenk in lipid bilayer membranes (bicelles), Lenk in deuterated water, and p-cresol in deuterated water. Frequency-frequency correlation functions obtained from waiting-time-dependent 2D IR spectra reveal an ultrafast decaying component with an approximately 1 ps time constant that is common for all three systems. On the basis of density functional theory calculations and molecular dynamics simulations, this spectral diffusion component is attributed to hydrogen-bond dynamics of the phenolic hydroxyl group interacting with bulk water. Unlike p-cresol in water, both Lenk systems exhibit static spectral inhomogeneity, which can be attributed to conformational distributions of Lenk that do not interconvert within 4 ps. Our results suggest that the Tyr side chain of Lenk in bicelles is located at the water-abundant region at the membrane-water interface and not embedded into the hydrophobic core.

The Magic of Bicelles Lights Up Membrane Protein Structure

“…It was concluded that the tyrosine side chain is not embedded in the hydrophobic core of the lipid bilayer. 180 The partitioning of another neuropeptide, 11-residue substance P, into isotropic bicelles was studied by PFG methods. 181 A conformational change of substance P was observed when using bicelles that contain ganglioside GM1.…”

Two-dimensional infrared study of 3-azidopyridine as a potential spectroscopic reporter of protonation state

“…For example, the amide I mode of proteins and peptides has been used extensively to probe protein and peptide structure and dynamics using 2D IR spectroscopy. [1][2][3][4][5][6][7][8][9][10][11][12][13] Similarly, the O-H͑D͒ stretching vibration of water, alcohols, and acids has been used to probe hydrogen-bond dynamics. [14][15][16][17][18][19][20][21][22][23] In other cases, external probes can be introduced to the system.…”

Anharmonic overtone and combination states of glycine and two model peptides examined by vibrational self-consistent field theory

“…8 Examples of 2D IR studies of biomolecules have been documented in recent literature. [9][10][11][12][13][14][15][16][17][18][19][20] In 2D IR, the anharmonic interaction between two vibrators can be directly shown as off-diagonal anharmonicity, which is an important parameter that may serve as a probe of molecular structure. [21][22][23] Multi-quantum vibrational transitions are very common due to the anharmonic nature of molecular vibrations.…”