Hydrogen bonding in globular proteins

Of late, the most bountiful natural biopolymer chitin and chitosan have become cynosure of all party because of an unusual combination of biological activities plus mechanical and physical properties. However applications of chitin are limited due to its inherent insoluble and intractable nature. Chitosan, alkaline hydrolytic derivative of chitin has better solubility profile, less crystallinity and is amenable to chemical modifications due to presence of functional groups as hydroxyl, acetamido, and amine. The chemical modification of chitosan is of interest because the modification would not change the fundamental skeleton of chitosan, would keep the original physicochemical and biochemical properties and finally would bring new or improved properties. In view of rapidly growing interest in chitosan its chemical aspects and chemical modification studies is reviewed. The several chemical modifications such as oligomerization, alkylation, acylation, quternization, hydroxyalkylation, carboxyalkylation, thiolation, sulfation, phosphorylation, enzymatic modifications and graft copolymerization along with many assorted modifications have been carried out. The chemical modification affords a wide range of derivatives with modified properties for specific end use applications in diversified areas mainly of pharmaceutical, biomedical and biotechnological fields. Assorted modifications including chitosan hybrids with sugars, cyclodextrin, dendrimers, and crown ethers have also emerged as interesting multifunctional macromolecules. The versatility in possible modifications and applications of chitosan derivatives presents a great challenge to scientific community and to industry. The successful acceptance of this challenge will change the role of chitosan from being a molecule in waiting to a lead player.

Chitosan-modifications and applications: Opportunities galore

Of the truly abundant polysaccharides in Nature, only chitin has yet to find utilization in large quantity. Chitin is the second most abundant natural biopolymer derived from exoskeletons of crustaceans and also from cell walls of fungi and insects. Chitin is a linear beta 1,4-linked polymer of N-acetyl-D-glucosamine (GlcNAc), whereas chitosan, a copolymer of GlcNAc (approximately 20%) and glucosamine (GlcN, 80%) residues, is a product derived from de-N-acetylation of chitin in the presence of hot alkali. Chitosan is, in fact, a collective name representing a family of de-N-acetylated chitins deacetylated to different degrees. Both chitin/chitosan and their modified derivatives find extensive applications in medicine, agriculture, food, and non-food industries as well. They have emerged as a new class of physiological materials of highly sophisticated functions. Their application versatility is a great challenge to the scientific community and to industry. All these are the result of their versatile biological activity, excellent biocompatibility, and complete biodegradability in combination with low toxicity. Commercial availability of high-purity forms of chitin/chitosan and the continuous appearance of new types of chitin/chitosan derivatives with more and more useful and specific properties have led to an unlimited R&D efforts on this most versatile amino polysaccharide, chitin to find new applications, which are necessary to realize its full potential. Incidentally, this too has become an environmental priority. No doubt, chitin is surely an undisputed biomolecule of great potential.

Chitin--the undisputed biomolecule of great potential

The structure of alpha-chitin.

The three dimensional structure of human carbonic anhydrase C has been determined at 2.0 A resolution. The active site has been identified by the binding of inhibitors and the location of the zinc ion.

Crystal structure of human carbonic anhydrase C.

An analysis of the various parameters associated with N-H… O type of hydrogen bonds has been made using data from reported crystal structures of amino acids and simple peptides. The different parameters at the donor and the acceptor ends have been suitably defined and evaluated. In some cases the analysis is done depending upon the chargedness or other characteristics of the donor and acceptor groups. Histograms giving the distribution of these parameters have been drawn and possible conclusions arrived at:



1. The distribution shows a maximum between 2.8 A and 2.9 A for the charged donor group and 2.9 A and 3.0 A for the uncharged donor group and is probably not dependent upon the charge on the acceptor group.



2. The angle between the directions CO and O. N tends to lie between two cones about C O with semi-vertical angles 40 and 70°. The orientation of the directions O. N and O. H with respect to the lone pair orbital directions on the acceptor oxygen atoms are analysed in detail using spherical polar coordinates. The analysis indicates that the group NH has a very strong tendency to point towards the acceptor oxygen atom. A general feature has been found in that the direction N-H tends to be closer to an orbital if the oxygen is an acceptor of two hydrogen bonds, while the direction tends to lie in between the orbitals when the acceptor oxygen is the receipient of only one hydrogen bond. The possible explanation of this on the basis of lone pair interaction is briefly discussed.

Nageshwar Prasad

Papers

Study of hydrogen bonds in amino acids and peptides.

Rigid-body refinement and conformation of α-chitin