Appearance and Disappearance of Dendritic and Chiral Patterns in Domains of Langmuir Monolayers Observed with Brewster Angle Microscopy

Abstract

Investigations on the aggregation behavior and morphology of Langmuir films of enantiomeric (L) and racemic (DL) N-acyl amino acids on pure aqueous as well as metal cation containing subphases were carried out at the mesoscale level with the help of Brewster angle microscopy (BAM). In the case of N-hexadecanoyl alanine on a pure aqueous subphase at 298 K the L-enantiomer forms crystal platelets, while the irregular fractal-like shape of the domains of the racemic mixture can be explained by a diffusion limited aggregation (DLA) growth mechanism. At 303 K the L-enantiomer shows a dendritic growth pattern, which leads to explicitly chiral domain shapes that correspond with the chirality of the film-forming molecules and for which hydrogen bridges as directed attractive forces are assumed to be responsible. The compression of the L-enantiomer on a zinc ion containing subphase is accompanied by a remarkable metamorphosis of the condensed structure. Starting from torus-like domains they were at first converted into strongly wound S-shaped domains, finally turning into a seahorse-like appearance. The origin of these chiral shapes can be explained on the basis of an electrostatic growth model. The enantiomer of N-hexadecanoyl alanine methyl ester shows three different asymmetric dendritic growth patterns. The domains of the racemic mixture are dendritic too, but in contrast they are symmetric and have a notably low branching density. On a pure aqueous subphase the L-enantiomer of N-octadecanoyl valine exhibits dendritic growth as well, but the overall outer shape of the domains is not explicitly chiral.

Self-assembly, supramolecular organization, and phase transitions of a homologous series of N-acyl-l-alanines (n=8–20)

“…6A and B. All NAAs (n = [11][12][13][14][15][16][17][18][19][20] gave several sharp diffraction peaks in the 2Â range of 1-20 • . From the position of these diffraction peaks the d-spacings were calculated; in each case 4-6 peaks were used to estimate the d-spacing and the average values obtained together with standard deviations are given in Table S7.…”

“…Therefore, it is important to characterize them with respect to their structure and phase behavior and interaction with major membrane lipids such as phosphatidylcholines, phosphatidylethanolamines, and cholesterol. In previous work, it has been shown that N-palmitoyl alanine forms stable monolayers at the air-water interface where it exhibits chiral interactions that are temperature dependent and forms dendritic and chiral patterns as observed by Brewster angle microscopy [16,17]. In addition, some preliminary studies on the aggregation and calorimetric properties have been reported for a few NAAs [18,19].…”

“…However, there have been no detailed investigations on the phase transitions of NAAs containing different acyl chains in the dry and hydrated states and the supramolecular structures adopted by them. In the studies reported here, we synthesized NAAs with saturated acyl chains (n = [8][9][10][11][12][13][14][15][16][17][18][19][20] and investigated their aggregation behavior in water as well as in phosphate buffer. The phase transitions of NAAs were characterized in the dry state and upon complete hydration (for protonated acids as well as the carboxylates).…”

Systematic study of Langmuir films of different amino acid derivatives on several subphases

“…layers. L films of Ala [1][2][3], Asp [4], Ser [5][6], Val [3] etc. alkyl derivatives have been studied on different subphases.…”

Self-assembly properties of some chiral N-palmitoyl amino acid surfactants in aqueous solution

“…Specific interactions can take place between similar enantiomers or between enantiomers of opposite sign. As a result, the chirality of AAS influences the physical properties of Langmuir monolayers, the films being formed either on an aqueous medium or on various surfaces [7][8][9][10][11][12][13][14][15][16][17][18]. It also determines molecular packing in the solid state, influencing the melting point, Krafft temperature and enthalpy of solution [19][20][21].…”