Chain-Length Dependence of Insertion, Desorption, and Translocation of a Homologous Series of 7-Nitrobenz-2-oxa-1,3-diazol-4-yl-Labeled Aliphatic Amines in Membranes

We present a complete characterization of the kinetics of interaction between the homologous series of fluorescent fatty amines with the fluorescent moiety 7-nitrobenz-2-oxa-1,3-diazol-4-yl covalently bound to the amine group, NBD-C-n (n = 8-16), and a lipid bilayer in the liquid disordered phase. The insertion into and the desorption from the lipid bilayer, as well as the rate of translocation across the two bilayer leaflets, has been measured at different temperatures, allowing an estimation of the thermodynamic parameters in the formation of the transition state. This is the first report on the complete characterization of the kinetics of the interaction of a large series of structurally homologous amphiphiles. In a recent paper from this research group, the equilibrium interaction of NBD-C-n (n = 4-10) with POPC bilayers and serum albumin was reported. This information allows the calculation of the equilibrium distribution of the amphiphiles among the aqueous phase, serum proteins, and biomembranes. The data presented in this manuscript complement its characterization with information on the kinetics of the interactions, making possible the quantitative evaluation of their pharmacokinetics. The rate of translocation is shown to decrease with increasing alkyl chain length up to n = 12, becoming relatively insensitive to further increases in n. The Gibbs free energy variation associated with the rate of desorption from the lipid bilayer increased linearly with n, with Delta Delta G double dagger(0) = 3.4 +/- 0.5 kJ mol(-1) per methylene group. It was also found that the process of insertion in the lipid bilayer is not diffusion-limited, although it is close to this limit for the smaller amphiphiles in the homologous series at high temperatures.