Effect of Conditioning Solvent on the Orientation of Bonded Hydrocarbon Moieties in Totally Aqueous Mobile Phases

Hybrid-Lipid Bilayers Induce n-Alkyl-Chain Order in Reversed-Phase Chromatographic Surfaces, Impacting their Shape Selectivity for Aromatic Hydrocarbon Partitioning

“…The Raman spectra of C 18 particles and four hybrid supported phospholipid bilayers prepared by deposition of DOPC, POPC, DMPC, and DPPC on the interior surfaces of porous C 18 -modified chromatographic particles are plotted in Figure ; see Figure S-2 for the lipid structures. These spectra allow the ordering of the n -alkyl chains of the C 18 ligands and lipid-modified surfaces to be determined from the intensity ratio of the asymmetric C–C trans stretching frequency (1061 cm –1 ) to the symmetric C–C stretching of the gauche conformer (1086 cm –1 ). ,− As shown in the Figure , the low density n -alkyl chains (3.4 μmol/m 2 ) of the unmodified C 18 particles under aqueous (5% methanol) conditions are dominated by gauche conformers (trans-to-gauche ratio of ≤1), consistent with disordered chains. , In contrast, C 18 particles modified with phospholipids having saturated acyl chains (DPPC and DMPC) resulted in C–C stretching modes that are dominated by trans-conformers, indicative of extended, ordered, and interdigitated hydrocarbon chains (trans-to-gauche ratios >5) indicating even greater chain order than corresponding lipid vesicle bilayer membranes, which are not interdigitated . Double-bonds in the phospholipid acyl chains (POPC and DOPC) lead to less ordered chains in the hybrid bilayers with increased gauche conformers (Figure ), which is consistent with these structures at or above their melting phase transitions at room temperature, typical of unsaturated phospholipid bilayers. , With variation in the phospholipid acyl chain length and saturation, a systematic variation in alkyl-chain order as reported by the ratio of trans-to-gauche C–C stretching intensities is achieved, from the highly ordered DPPC hybrid bilayer to the DOPC hybrid bilayer, whose relative gauche population is equivalent to the unmodified C 18 stationary phase (Figure ).…”

“…27,54−58 As shown in the Figure 1, the low density n-alkyl chains (3.4 μmol/m 2 ) of the unmodified C 18 particles under aqueous (5% methanol) conditions are dominated by gauche conformers (trans-to-gauche ratio of ≤1), consistent with disordered chains. 59,60 In contrast, C 18 particles modified with phospholipids having saturated acyl chains (DPPC and DMPC) resulted in C−C stretching modes that are dominated by trans-conformers, indicative of extended, ordered, and interdigitated hydrocarbon chains (trans-to-gauche ratios >5) indicating even greater chain order than corresponding lipid vesicle bilayer membranes, which are not interdigitated. 39 Double-bonds in the phospholipid acyl chains (POPC and DOPC) lead to less ordered chains in the hybrid bilayers with increased gauche conformers (Figure 1), which is consistent with these structures at or above their melting phase transitions at room temperature, typical of unsaturated phospholipid bilayers.…”

NMR Wool Tube: a novel method for NMR solution analysis of derivatized glass surfaces

“…7a) and their respective idealized drawings (Fig. 7b) follows the "bristle brush", "haystack", "ink bottle", and other models presented by Halasz and Sebestian [21], Hemetsberger et al [22], Lochmuller and Wilder [27] and others [23][24][25]. These experiments reiterate the solution NMR experiments done on alkyl-chain derivatized HPLC silica particles packed within an NMR tube [9,12,[15][16][17][18]26,28,29,31,32], albeit, with the NMR Wool Tube experiments, more solvent mixtures were examined.…”

A molecular dynamics study of a reversed-phase liquid chromatography model

“…Chain structure An important question related to the retention mechanism in RPLC is the effect of different mobile phases on the ordering of the stationary phase. The expectation has been that disordered or folded states predominate with poor chain solvents such as water, while solvation of the chains with an organic component favors chain extension and ordering [26,57,58]. Three quantities which give a measure of the chain structure from the simulations are the distribution of the angle x, defined as cosx = ri8 -z. where rig = rgri and z is a unit normal to the wall, the distribution of the end-end distance of the alkyl chains, and the center of mass of the hydrocarbon layer, r^c / fzc…”