Amino acid Amphiphilic molecule

Proteins are biopolymers of some 22 different amino acids. Because of the variation in physical-chemical properties, mainly polarity and electrical charge, between the constituent amino acids, protein molecules are am-pholytic (i.e., containing positively and negatively charged groups) and more or less amphiphilic (i.e. comprising polar and apolar domains). These properties, in turn, lead to the formation of complex three-dimensional (3D) structures. 

In this chapter, we have surveyed a wide range of chiral molecules that self-assemble into helical structures. The molecules include aldonamides, cere-brosides, amino acid amphiphiles, peptides, phospholipids, gemini surfactants, and biological and synthetic biles. In all of these systems, researchers observe helical ribbons and tubules, often with helical markings. In certain cases, researchers also observe twisted ribbons, which are variations on helical ribbons with Gaussian rather than cylindrical curvature. These structures have a large-scale helicity which manifests the chirality of the constituent molecules. 

Hydroxy stearic acid and polybenzylglutamate were the classical cases of relatively simple molecules that produced twisted fibers, till the burst of reports on other examples, viz. amino acid amphiphiles, diacetylenic phospholipids, and gluconamides started in the mid-1980s. The handedness of chiral assemblies can be determined from electron micrographs, provided that care is taken to manipulate grids, specimens, films, and image scanners in a consistent way. It was not till the... 

The structure of the major trimeric LHCII complex has been recently obtained at 2.72 A (Figure 7.3) (Liu et al., 2004). It was revealed that each 25kDa protein monomer contains three transmembrane and three amphiphilic a-helixes. In addition, each monomer binds 14 chlorophyll (8 Chi a and 6 Chi b) and 4 xanthophyll molecules 1 neoxanthin, 2 luteins, and 1 violaxanthin. The first three xanthophylls are situated close to the integral helixes and are tightly bound to some amino acids by hydrogen bonds to hydroxyl oxygen atoms and van der Waals interactions to chlorophylls, and hydrophobic amino acids such as tryptophan and phenylalanine. 

Fig. 4. Alignment of amino acid residues surrounding the binding sites of iGluRl-7. Function lock, interdomain hydrogen bond Pkt, amphiphilic or hydrophobic residues bordering the pocket -NH3+, COO, and distal anion bind the respective charged moieties of glutamate Wat, binds to a binding-site water molecule in iGluR2.

These possibilities rectify the proposed subsequent appearance and amplification of chiral autocatalytic molecules and hypercydes. [190] Any autocatalytic systems would propagate [191] throughout an extensive adjoining hydrated porous network already rich in layered amphiphiles, lipids, polymeric materials, amino acids, thiols, and so forth. In addition, amphiphiles are known to be organized into lipid membranes by interaction with the inner surfaces of porous minerals. [136] It is a small organizational jump from these membranes to frilly formed lipid vesides. 

The effect of chain length on surface tension arises from the fact that, as the hydrophobicity increases with each -CH2- group, the amphiphile molecule adsorbs more at the surface. This will thus be a general trend in more complicated molecules also, such as proteins and other polymers. In proteins, the amphiphilic property arises from the different kinds of amino acids (25 different amino acids). Some amino acids have lipophilic groups (such as phenylalanine, valine, leucine, etc.), while others have hydrophilic groups (such as glycine, aspartic acid, etc.) (Figure 3.4). 

Laboratory experiments have shown that radiation processing of simulated presolar ices leads to more complex molecular species [25-27]. Hundreds of new compounds are synthesized, although the starting ices contain only a few simple common interstellar molecules. Many of the compounds formed in these experiments are also present in meteorites and cometary and asteroidal dust (interplanetary dust particles - IDPs), and some are presumably relevant to the origin of life, including amino acids [28,29], quinines [30], and amphiphilic material [31]. 

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