A pseudo-three-component synthesis of N-aroylmethylimidazoles 3 with three new C–N bonds formed regioselectively under microwave conditions was developed. Products were obtained by reacting two equivalents of aroylmethyl bromide (ArCOCH2Br, 1) with the appropriate amidine salt (RCN2H3.HX, 2) and with K2CO3 as a base in acetonitrile. The bicomponent reaction also occurred, giving the expected 4(5)-aryl-1H-imidazoles 4. Notably, the ratio of products 3 and 4 is governed by steric factors of the amidine 2 (i.e., R = H, CH3, Ph). Therefore, a computational study was carried out to understand the reaction course regarding product ratio (3/4), regioselectivity, and the steric effects of the amidine substituent group.
Lipid-membrane headgroups are invariably charged or zwitterionic. The headgroups can be weak acids (negatively charged when deprotonated), weak bases (positively charged when protonated), or zwitterionic (both types of groups present). Headgroups can also be salts, such as TMA þ Clin the lipid DOTAP þ. The screening of surface charge by dissociated counterions is usually negligible, as dissociated counterions are normally diluted to concentrations far below that of the ambient salt. However for a suspension of charged particles in a finite volume, the mean concentration of dissociated counterions depends on the suspension's surface charge/volume ratio, which for moderately concentrated suspensions of small particles can be quite high. Significant concentrations of dissociated counterions can screen and uncouple electrostatic interactions between like-charged particles. Such effects are readily observed in deionized suspensions where background salt is of order 10 À7 M. However in the presence of high salt, surface-dissociated counterions get masked by the salt counterions. Here we determine the maximum background salt concentrations at which dissociated-counterion screening can occur. For example, for 100 nm liposomes containing 20 mol% cationic or weak-acid lipids at 0.1 volume-fraction liposome concentration, dissociated counterions can reach mean concentrations of order 10 mM, or if the counterions are protons, mean pH$2. Although charge regulation can in some cases reduce the number of dissociated counterions, dissociated-counterion phenomena, i.e. screening-ion numbers proportional to total surface charge, can occur in the presence of surprisingly high values of ambient salt or acid.
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