Fluorescence of nitrobenzoxadiazole (NBD) labeled lipids in model membranes is connected not to lipid mobility, but to probe location

0453494 - ÚFCH JH 2017 RIV GB eng J - Journal Article
Amaro, Mariana - Filipe, H. A. - Ramalho, J. P. P. - Hof, Martin - Loura, L. M. S.
Fluorescence of nitrobenzoxadiazole (NBD) labeled lipids in model membranes is connected not to lipid mobility, but to probe location.
Physical Chemistry Chemical Physics. Roč. 18, č. 10 (2016), s. 7042-7054. ISSN 1463-9076. E-ISSN 1463-9084
R&D Projects: GA ČR GBP208/12/G016
Institutional support: RVO:61388955
Keywords : fluorescence * nitrobenzoxadiazole
Subject RIV: CF - Physical ; Theoretical Chemistry
Impact factor: 4.123, year: 2016

Nitrobenzoxadiazole (NBD)-labeled lipids are popular fluorescent membrane probes. However, understanding of important aspects of NBD photophysics remains incomplete, including the observed shift in the emission spectrum of NBD-lipids to longer wavelengths following excitation at the red-edge of the absorption spectrum (red-edge excitation shift or REES). REES of NBD-lipids has been previously interpreted as reflecting restricted mobility of solvent surrounding the fluorophore in membrane environments. However, this requires a large change in dipole moment (Δμ) of NBD upon excitation. Previous calculations of Δμ of NBD in the literature have been carried out using outdated semiempirical methods, leading to conflicting values. Using up-to-date density functional theory methods, we recalculated Δμ confirming a rather small value (~2 D). Fluorescence measurements confirmed a REES of ~16 nm for 1,2-dioleoyl-sn-glycero-3-phospho-L-serine-N-(NBD) (NBD-PS) in dioleoylphosphatidylcholine vesicles. However, the observed shift is independent on both temperature and presence of cholesterol, and is therefore insensitive to membrane mobility and hydration. Moreover, red-edge excitation leads to an increased contribution of the shorter lifetime decay component, while time-resolved emission spectra of NBD-PS showed an atypical blue-shift following excitation. This excludes solvent relaxation restrictions as cause for the measured NBD REES and TRES pointing instead to heterogeneous probe transverse location as the origin of these effects. The latter hypothesis was confirmed by molecular dynamics simulations, from which calculated NBD hydration/location heterogeneity correlated with measured fluorescence lifetimes/REES. Altogether, our combination of theory and experiment-based techniques has led to a considerably improved understanding of NBD photophysics and a reinterpretation of its REES in particular.
Permanent Link: http://hdl.handle.net/11104/0254355