Molecular packing, hydrogen bonds

After direct high-resolution structural measurements on proteins became practicable, interest in crystalline amino acids decreased, but became renewed in attempts to simulate the distribution of electronic charge density in biopolymers using amino acids and simple peptides as model systems, to derive some transferable parameters and potentials [2-15]. Another research direction involved these systems and their packing patterns to mimic selected folds and interaction patterns of biopolymers. Typical molecular conformations and patterns of hydrogen bonds and packing in the structures of crystalline amino acids have been reviewed [16-19]. Similar analyses were performed for small peptides [18, 20-22]. 

Fig. 15 A portion of a layer containing the double-helix-like hydrogen-bonded molecular assembly in the crystal packing of 41-(CA)2...

It must be pointed out that deviations from such a simple relationship do occur. For example, since random copolymerisation tends to promote disorder, reduce molecular packing and also reduce the interchain forces of attraction, the Tg of copolymers is often lower than would be predicted by the linear relationship. Examples are also known where the Tg of the copolymer is higher than predicted. This could occur where hydrogen bonding or dipole attraction is possible between dissimilar comonomer residues in the chain but not between similar residues, i.e. special interchain forces exist with the copolymers. 

U shape. This has profound significance on molecular packing in membranes and on the positions occupied by fatty acids in more complex molecules such as phospholipids. Trans double bonds alter these spatial relationships. Trans fatty acids are present in certain foods, arising as a by-product of the samration of fatty acids during hydrogenation, or hardening, of natural oils in the manufacture of margarine. An additional small... 

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