Racemic mixtures, monolayers

The implications for films cast from mixtures of enantiomers is that diagrams similar to those obtained for phase changes (i.e., melting point, etc.) versus composition for the bulk surfactant may be obtained if a film property is plotted as a function of composition. In the case of enantiomeric mixtures, these monolayer properties should be symmetric about the racemic mixture, and may help to determine whether the associations in the racemic film are homochiral, heterochiral, or ideal. Monolayers cast from non-enantiomeric chiral surfactant mixtures normally will not exhibit this feature. In addition, a systematic study of binary films cast from a mixture of chiral and achiral surfactants may help to determine the limits for chiral discrimination in monolayers doped with an achiral diluent. However, to our knowledge, there has never been any other systematic investigation of the thermodynamic, rheological and mixing properties of chiral monolayers than those reported below from this laboratory. 

The Yl/A isotherms of the racemic and enantiomeric forms of DPPC are identical within experimental error under every condition of temperature, humidity, and rate of compression that we have tested. For example, the temperature dependence of the compression/expansion curves for DPPC monolayers spread on pure water are identical for both the racemic mixture and the d- and L-isomers (Fig. 13). Furthermore, the equilibrium spreading pressures of this surfactant are independent of stereochemistry in the same broad temperature range, indicating that both enantiomeric and racemic films of DPPC are at the same energetic state when in equilibrium with their bulk crystals. 

Taken together, the equilibrium spreading pressures of films spread from the bulk surfactant, the dynamic properties of the films spread from solution, the shape of the Ylj A isotherms, the monolayer stability limits, and the dependence of all these properties on temperature indicate that the primary mechanism for enantiomeric discrimination in monolayers of SSME is the onset of a highly condensed phase during compression of the films. This condensed phase transition occurs at lower surface pressures for the R( —)- or S( + )-films than for their racemic mixture. 

Conversely, the racemic film system appears to be solubilized by the achiral fatty acid component. At compositions of 10-33% palmitic acid, the ESP of the racemic system varies linearly with film composition, indicating that the monolayer in equilibrium with the racemic crystal is a homogeneous mixture of racemic SSME and palmitic acid. At compositions of less than 33% palmitic acid, the ESP is constant, indicating that three phases consisting of palmitic acid monolayer domains, racemic SSME monolayer domains, and racemic SSME crystals exist in equilibrium at the surface. 

No discrimination in the pressure/area characteristics was seen for diastereomeric monolayer films spread from all possible mixtures of pure racemates (R- and S-) and their racemic mixtures (R-, S-) of stearoylalanine, stearoylserine, stearoyltyrosine, and stearoyltryptophan methyl esters. The one exception was heterochiral and homochiral mixtures of stearoylalanine methyl esters and stearoylserine methyl esters at 35°C. The force/area... 

The experimentally estimated k2 value (= 0s tr[Os(II)]// ru) (36X107 s-1) for the A- Ru(II)/A-Os(II) pair is much larger than that (8.5 X 107 s-1) for the A- Ru(II)/A-Os(II) pair. This result is consistent with the chirality effect on the surface pressure versus molecular area (tt-A) isotherms, in which the A-ruthen-ium(II) complex forms a more compact monolayer than the racemic mixture of the ruthenium(II) complex in other words, the racemic mixture induces some steric repulsion between A- and A-ruthenium(II) complexes. 

To test the basic idea, some experiments are possible. When the side chains are long and flexible, there will be little difference between the energy of interaction between one molecule and another either of the same type or of the opposite enantiomorphic form, irrespective of the backbone directions. For example the properties of poly(c-benzyloxy-carbonyl-L-lysine) can be compared with a 1 1 mixture with the D-lysine enantiomorph. When side chains are short and inflexible, as in poly(L-alanine), significant differences should exist between its monolayer properties and that of a 1 1 racemic mixture, in this case of poly(L-alanine) and poly(D-alanine). 

Monolayers of Racemic Mixtures Poly (alanine), Poly (y-benzyl Glutamate), Poly ( -benzyl Aspartate), Poly (Benzyloxy carbonyl Lysine). Experimental Results. The pressure-area and surface potential results for the two enantiomorphic forms of a given polymer were virtually identical except for poly (benzyl aspartate) where the plateau of poly( -benzyl-D-aspartate) was about 2 dynes/cm higher than that of the L enantiomorph (Figure 6). This may result from the incomplete ben-zylation of the o-aspartate. 

Electron diffraction patterns have been obtained from collapsed monolayers of poly (alanine) and poly (y-benzyl glutamate). The enantiomorphic forms of the other two polymers give patterns with very poor crystallinity, and their racemic mixtures have not therefore yet been investigated. The principal features of the diffraction pattern of poly-... 

No differences can be observed between the n (A) isotherms for pure enantiomers of a particular phospholipid and racemic mixtures. Apparently, energetic differences are too small to give significantly different isotherms. However, the morphological properties of the monolayers show that chirality can affect both the shape and inner structure of the LC phase domains ). 

One of the earliest studies on the use of BAM examined monolayers of the racemic mixture and of the pure l- and D-enantiomers of N-dodecylgluconamide. This system presented jt-A isotherms at 10 °C that were more condensed for the racemic mixture than for the pure enantiomers, which were equivalent however, at 25 °C, the opposite was true, although the isotherms crossed at a certain point. Significant surface pressure relaxation at fixed area was observed, which differed in extent between the enantiomers and the racemic mixture and was much greater at 25 °C than at 10 °C, as expected on the basis of the short chain length. BAM observation during compression of either of the pure enantiomers at 25 °C showed extensive growth of two-dimensional dendritic domains. Differences could not be found between the dendritic forms of the two enantiomers. The monolayers of the racemic... 

In a following study, monolayers of the enantiomers of A-dodecytmannonamide and of the racemic mixture were examined and very different results were obtained than those found for the enantiomers of N-dodecylgluconamide. In this case, the surface pressure isotherms clearly show homochiral discrimination. BAM observation of the monolayers of the pure enantiomers showed formation of feather-like dendrites with curved main growth axes and with curved side branches. The side arms were observed to curve exclusively counterclockwise for the L-enantiomer and clockwise for the o-enantiomer. 

Figure 10 Images of domains of monolayers of the o-enantiomer, L-enantiomo-, and racemic mixture of A-stearoylserine methyl ester clearing showing chiral discrimination effects. The domains of the enantiomers display unique curvature, and the domain of the racemic mixture shows featnres with both senses of curvature and hence evidence for chiral segregation. Reproduced from Ref. 62. American Chemical Society, 2003, and the figure caption reads as follows Chiral discrimination of the condensed-phase domains of N-stearoyl serine methyl ester monolayers spread on pH 3 water, (a) D-enantiomer (b) L-enantiomer (c) and (d) 1 1 dl racemate. Image size 80 x 80 pm.

The tt-A isotherms for these monolayers indicated homochiral discrimination. Similar curved spiral domains were seen in monolayers of A-pahnitoylaspartic acid. Very striking, curved, condensed-phase domains were observed by BAM in monolayers of V-a-palmitoylthreonine, curving with opposite sense for the two pure enantiomers and showing twin-like structures with one arm curving in each sense for the racemic mixture paper. ... 

In the case of monolayers of the compound A-tetradecyl-y, 5-dihydroxy-pentanoic acid amide (TDHPAA), chiral discrimination effects in the domain structures were observed by BAM. The small, spear-like crystals showed a longer and shorter branch at one end and were seen to be mirror images of each other when the two enantiomers were compared. The racemic mixture formed a symmetric, almost straight and narrow crystallite for its condensed phase. 

The influence of the head group size on the chain packing is illustrated in a GIXD study of DPPC and DPPE in contact with the -alkane dodeceuie. At surface pressures higher than ca. 8 and 5 mN/m DPPC and DPPE, respectively, formed non-tilted phases. For the DPPE monolayer, but not for the DPPC monolayer, the alkane can be squeezed out at elevated surface pressure [141]. In monolayers of DPPC, an oblique structure is observed for both the racemic mixture and the enantiomer [139]. Chirality of phospholipids has a strong influence on the functional properties of these molecules in a biological membrane. 

The transformation of racemic chemistry to chiral biology remain an unsolved mystery of nature, but the spontaneous segregation of a racemic mixture into enantiomers in 2D and 3D might have played an important role [33]. To address this problem the molecular structures (at the air-water interface) of racemic and enantiomerically resolved monolayers have been explored, e.g., a-amino acids with a long alkyl chain residue. A brief overview of pure derivatives of amino acids, peptides and proteins characterised at the air-water interface is presented. 

The Z)-enantiomer of N-docosyl-leucine 2D-crystallises in an oblique unit cell, as expected. However, GIXD revealed a different oblique unit cell for the racemic mixture suggesting miscibility of the two enantiomers [152]. For myristoyl alanine monolayers, GIXD data indicated that a racemic mixture separated into 2D-crystalline islands of opposite chirality [153]. 

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