Amino Acid Crystal Structures Research Articles

Study of hydrogen bonds in amino acids and peptides.

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.

The Molecular Structure of Simple Substances Related to Proteins

Publisher Summary This chapter deals with structure of molecules related to protein. Biological reactions are closely related to the molecular configurations of proteins. The structural studies of proteins may be divided into two categories: (1) investigations made directly on proteins, and (2) determinations of molecular and crystal structures of amino acids, amides, and other simple molecular and crystal structures of amino acids, amides, and other simple substances closely related to proteins. The experimental methods are the most important ones for the determination of the fine structure of protein molecules. The reasonable models of polypeptides cannot differ much from one another in the atomic distances and bond angles. The essential point in their difference lies in the manner of the chain-folding due to the internal rotation about single bonds contained in the polypeptide chain. Evidence based on spectroscopic, electric, and thermal measurements indicates that rotation is not free and that there is more than one potential in one complete rotation about a single bond axis. The excess of energy arising from the internal rotation potential must be compensated by the secondary forces which act upon the part of the polypeptide chain from within and without hydrogen bonds. The experimental methods for the determination of stable positions of internal rotation consist of the measurements of the Raman effect, infrared absorption, dielectric constants, electron diffraction, entropies, and specific heats. N -Acetylglycine and β -glycylglycine contains a peptide group.