Nanotechnology is a multidisciplinary field that covers a vast and diverse array of devices and machines derived from engineering, physics, materials science, chemistry and biology. These devices have found applications in biomedical sciences, such as targeted drug delivery, bio-imaging, sensing and diagnosis of pathologies at early stages. In these applications, nano-devices typically interface with the plasma membrane of cells. On the other hand, naturally occurring nanostructures in biology have been a source of inspiration for new nanotechnological designs and hybrid nanostructures made of biological and non-biological, organic and inorganic building blocks. Lipids, with their amphiphilicity, diversity of head and tail chemistry, and antifouling properties that block nonspecific binding to lipid-coated surfaces, provide a powerful toolbox for nanotechnology. This review discusses the progress in the emerging field of lipid nanotechnology.

We investigate the structure and electronic properties of phosphatidylcholine (PC) under different degrees of hydration at the single-molecule and monolayer type level by linear scaling ab initio calculations. Upon hydration, the phospholipid undergoes drastic long-range conformational rearrangements which lead to a sickle-like ground-state shape. The structural unit of the tilted gel-phase PC appears to be a water-bridged PC dimer. We find that hydration dramatically alters the surface potential, dipole and quadrupole moments of the lipids and consequently guides the interactions of the lipids with other molecules and the communication between cells.

A general computational method is introduced to estimate the Poisson’s ratio for membranes with small thickness. In this method, the Poisson’s ratio is calculated by utilizing a rescaling of inter-particle distances in one lateral direction under periodic boundary conditions. As an example for the coarse grained lipid model introduced by Lenz and Schmid, we calculate the Poisson’s ratio in the gel, fluid, and interdigitated phases. Having the Poisson’s ratio, enable us to obtain the Young’s modulus for the membranes in different phases. The approach may be applied to other membranes such as graphene and tethered membranes in order to predict the temperature dependence of its Poisson’s ratio and Young’s modulus.

ABSTRACT:We report the complete assignment of the vibrational spectrum of dipalmitoylphosphatidylcholine (DPPC), which belongs to the most ubiquitous membrane phospholipid family, phosphatidylcholine. We find that water hydrating the lipid headgroups enables efficient energy transfer across membrane leaflets on sub-picosecond time scales. The emergence of spatially extended vibrational modes upon hydration, underlies this phenomenon. Our findings illustrate the importance of collective molecular behavior of biomembranes and reveal that hydrated lipid membranes can act as efficient media for the transfer of vibrational energy. ■ INTRODUCTIONCell membranes are subject to energy input from various sources such as the sun, exothermic chemical reactions, and mechanical perturbation by the cytoskeleton and the extracellular matrix. To understand the stability and functioning of membranes, it is thus important to acquire a better understanding of how the membrane deals with the influx of energy. An important role is played by the water molecules hydrating the membrane phospholipid molecules.1,2 Hence to gain a deeper understanding of energy-transfer phenomena, we have to know the molecular mechanisms of energy storage and transfer of the combined phosphilipid−water system. 3 This is also highly relevant in nanotechnology where membranes are often the soft material of choice for nanoscale devices. 4 To obtain molecular insights into energy transfer in combined lipid−water systems is challenging, as lipid headgroups are complex structures, involving hydrophobic patches with an otherwise highly hydrophilic area. Interfacial water on the hydrophilic parts has characteristic structures and dynamics, distinct from those of water layers on hydrophobic surfaces. Experimentally, progress in the study of assembled interfacial water structures has been made using ultrafast electron crystallography, and it was found that the energy transfer in these structures happens on ultrashort time scales. 5 The energytransfer dynamics of a lipid monolayer interfacing aqueous environment has also been studied with time-resolved surface sum-frequency generation. 6 The results of this study indicated the presence of two distinct categories of water molecules, showing fast and slow energy-transfer dynamics, and fast energy transfer across a monolayer of lipids. 7 However, a molecularscale assignment of the mechanism behind the energy transfer and the different types of water has remained challenging. 8Classical (molecular dynamics and Monte Carlo) approaches have been extensively used to probe energy transfer in a wide range of materials with significant success. 4 Density functional theory (DFT) treatments are in principle more accurate 9,10 but have not been tractable for large molecular systems because of the enormous computational cost as a result of the cubic scaling of the cost with the number of atoms. Here, we circumvent those limitations by adopting an approach developed recently in the realm of condensed matter physics. 11,12 In this a...

ABSTRACT:The structure and autoionization of water at the water−phospholipid interface are investigated by ab initio molecular dynamics and ab initio Monte Carlo simulations using local density approximation (LDA) and generalized gradient approximation (GGA) for the exchange−correlation energy functional. Depending on the lipid headgroup, strongly enhanced ionization is observed, leading to the dissociation of several water molecules into H + and OH − per lipid. The results can shed light on the phenomena of the high proton conductivity along membranes that has been reported experimentally. ■ INTRODUCTIONPhospholipids are amphiphilic molecules that, in an aqueous environment, self-assemble into bilayers and form the major structural constituents of biomembranes. Although the affinity of the lipid's headgroup to interfacial water has been widely addressed in the literature, 1,2 the exact chemical nature of this coupling is not fully understood and its strength remains to be quantified. In particular, it is not clear how this coupling affects the structure and dynamics of the interfacial water layer and the proton transport. Enhanced proton conduction along phospholipid−water interfaces was first observed in the mid 1980s.3,4 Recent studies by means of scanning tunneling microscopy (STM) confirm a significant lateral conductivity. This conductivity is believed to be of functional importance because lateral proton diffusion along membrane surfaces represents the most efficient pathway for H + transport between protein pumps. 6 The molecular mechanism underlying the high lateral proton conductivity has not yet been resolved. 7−11In this paper we report ab initio molecular dynamics (AIMD) and ab initio Monte Carlo (AIMC) simulations of interfacial water covering the headgroup of zwitterionic dipalmitoyl−phosphatidylcholine (DPPC) molecule. We perform calculations within the local density approximation and generalized gradient approximation for the exchange−correlation energy functional, with Ceperley−Alder 12 and Perdew− Burke−Ernzerhof 13 parametrizations, respectively. We show that the interfacial water exhibits a strongly enhanced autoionization that is caused by the presence of strong local electric field as well as strong hydrogen bonding. ■ COMPUTATIONAL DETAILSGround State Calculations. To calculate the ground state structure of the DPPC molecule, we first build the molecule by putting atoms together with coordinations according to the bonding rules from chemistry. To relax this structure, density functional theory (DFT) is applied as implemented in the SIESTA code.14 To this end, the DPPC molecule is placed in a unit cell with dimensions much larger than the size of the molecule. The spatial extension of the pure DPPC structure is ≈4 × 29 × 8 Å. We choose a box of 20 × 40 × 20 Å dimensions. As a consequence, because we are dealing with a molecule (a cluster), no periodic boundary conditions are adopted and only the Γ point in the reciprocal space is needed for the energy integration. The mesh cutoff is set to...