Water and Membrane Dynamics in Suspensions of Lipid Vesicles Functionalized with Poly(ethylene glycol)s

The present work was aimed at studying the molecular dynamics at different levels of model membranes having a simulated glycoclix, with focus on the molecular crowding conditions at the lipid-water interfacial region. Thus, binary mixtures of dipalmitoylphosphatidylcholine (dpPC) and a poly(ethylene glycol) (PEG(n)) derivative of dipalmitoylphosphatidylethanolamine (PE) (where n = 350, 1000, and 5000, respectively, refer to PEG molecular masses) were submitted to H-1 spin-lattice relaxation time (T-1) and P-31 NMR spectra analysis. H-1 NMR relaxation times revealed two contributing components in each proton system (PEG, phospholipids, and water), for all the mixtures studied, exhibiting values of T-1 with very different orders of magnitude. This allowed identifying confined and bulk water populations as well as PEG moieties becoming more disordered and independent from the phospholipid moiety as n increased. P-31 spectra showed a typical broad bilayer signal for n = 350 and 1000, and an isotropic signal characteristic of micelles for n = 5000. Surface pressure (pi)-molecular area isotherms and compressional modulus measurements provided further structural information. Moreover, epifluorescence microscopy data from monolayers at pi similar to 30 mN/m, the expected equilibrium pi in lipid bilayers, allowed us to postulate that both H-1 populations resolved through NMR in phospholipids and lipopolymers corresponded to different phase domains.