Effects of fluorescent probe NBD-PC on the structure, dynamics and phase transition of DPPC. A molecular dynamics and differential scanning calorimetry study.

We present a combined theoretical (molecular dynamics, MD) and experimental (differential scanning calorimetry, DSC) study of the effect of 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) acyl chain-labeled fluorescent phospholipid analogs (C6-NBD-PC and C12-NBD-PC) on 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers. DSC measurements reveal that <1 mol% of NBD-PC causes elimination of the pre-transition and a large loss of cooperativity of the main transition of DPPC. Labeling with C6-NBD-PC or C12-NBD-PC shifts the main transition temperature to lower or higher values, respectively. Following our recent report on the location and dynamics of these probes (BBA 1768 (2007) 467-478) in fluid phase DPPC, we present a detailed analysis of 100-ns MD simulations of systems containing either C6-NBD-PC or C12-NBD-PC, focused on their influence on several properties of the host bilayer. Whereas most monitored parameters are not severely affected for 1.6 mol% of probe, for the higher concentration studied (6.2 mol%) important differences are evident. In agreement with published reports, we observed that the average area per phospholipid molecule increases, whereas DPPC acyl chain order parameters decrease. Moreover, we predict that incorporation of NBD-PC should increase the electrostatic potential across the bilayer and, especially for C12-NBD-PC, slow lateral diffusion of DPPC molecules and rotational mobility of DPPC acyl chains.