Abstract
An integrated theoretical/experimental strategy has been applied to the study of environmental effects on the spectroscopic parameters of 4-(diphenylamino)phtalonitrile (DPAP), a fluorescent molecular rotor. The computational part starts from the development of an effective force field for the first excited electronic state of DPAP and proceeds through molecular dynamics simulations in solvents of different polarities toward the evaluation of Stokes shifts by quantum mechanics/molecular mechanics (QM/MM) approaches. The trends of the computed results closely parallel the available experimental results thus giving confidence to the interpretation of new experimental studies of the photophysics of DPAP in lipid bilayers. In this context, results show unambiguously that both flexible dihedral angles and global rotations are significantly retarded in a cholesterol/DPPC lipid matrix with respect to the DOPC matrix, thus confirming the sensitivity of DPAP to probe different environments and, therefore, its applicability as a probe for detecting different structures and levels of plasma membrane organization.
Highlights
Fluorescent molecular rotors (FMRs) are a class of chemical species capable of modulating their structural and optical properties in response to changes in the viscosity and polarity of the local environment, a feature that makes them suitable for sensing and imaging applications.[1−4] Such a peculiar capability mainly arises from the intrinsic structural flexibility of the FMRs: typically, this is ascribed to one or more unrestrained and environmentally sensitive dihedral angles, whose internal dynamics largely affects the FMR emission intensity and lifetime upon photoexcitation.[5]
Thanks to these remarkable properties, FMRs can act as viscosity sensors in different environments,[6] and they have been employed in recent years in order to report on local properties of various biophysical systems.[7−9] Among others, a very interesting application field concerns the investigation of lipid membrane structures
The optimized excited state structure of the DPAP molecular rotor adopts a propellerlike shape in order to minimize steric hindrance among the three phenyl rings, with the central moiety defined by the three ipso carbon atoms and the aminic nitrogen adopting a nearly planar conformation
Summary
Fluorescent molecular rotors (FMRs) are a class of chemical species capable of modulating their structural and optical properties in response to changes in the viscosity and polarity of the local environment, a feature that makes them suitable for sensing and imaging applications.[1−4] Such a peculiar capability mainly arises from the intrinsic structural flexibility of the FMRs: typically, this is ascribed to one or more unrestrained and environmentally sensitive dihedral angles, whose internal dynamics largely affects the FMR emission intensity and lifetime upon photoexcitation.[5]. The modern view identifies a spatially interlaced combination of liquid ordered (Lo) and liquid disordered (Ld) phases, enriched, respectively, in saturated and unsaturated lipids, together with different amounts of cholesterol.[10−12] This nanostructured dynamic assembly of Ld and Lo phases does not entail definite boundaries but is organized around the cytoskeletal network Such a dynamical membrane organization was proposed to be relevant for most membrane processes, such as formation of protein clusters, signal transduction, endocytosis, and cell polarization and motility.[11−15] In this context, it is not surprising that FMRs have been employed to detect the different phases of cell membranes[16−19] and to probe the transition from the gel-like to the liquid-crystal phase or, in general, to gain information on the microviscosity of the phospholipid bilayers.[20] In these studies, it was assumed that more viscous environments may slow down the FMR intramolecular motions,[21] leading to stronger intensities in the corresponding emission spectra and increased fluorescence lifetimes. An effective relation between solvent viscosity (η) and fluorescence quantum yield (φ) (or lifetime) is represented by the Forster−Hoffmann model[22] (i.e., log φ ∝ log η), which has been experimentally proved to hold over a wide range of viscosity and polarity scales.[23,24] It is worth
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