Geometric bonding patterns

Abstract This tribute to the work by Carl Johan Ballhausen focuses on the emergence of quantitative means for the study of the electronic properties of complexes and molecules. Development, refinement, and application of the orbital picture elucidated electric and magnetic features of ranges of molecules when used for the interpretation of electronic transitions, electron spin resonance parameters, rotatory dispersion, nuclear quadrupole couplings as well as geometric bonding patterns. Ballhausen s profound impact on the field cannot be overestimated. 

Figure 9.4. The interplay between electronic structure (VB bonding pattern) and geometrical change on the potential energy surface.

In the following, the geometrical data obtained from a number of protein crystal structures determined at high resolution are analyzed in terms of hydrogen-bonding patterns, and then the data are compared metrically. This procedure and the data analysis are taken mainly from a review of Baker and Hubbard [596], who considered in detail the protein structures listed in Thble 19.1. 

The results obtained for the thymine 04-hydrogenated radical can be extended to 1-methylthymine and deoxythymidine since geometrical and electronic changes are expected to be small upon substitution at the N1 position. Comparison of calculated and experimental HFCCs indicates that the 04-hydrogen remains in the molecular plane and at an angle of approximately 60° out of the molecular plane in 1-methylthymine [25] and deoxythymidine [15,25] crystals, respectively. The differences in these systems relative to unsubstituted thymine arise due to the characteristic hydrogen bonding patterns in the crystals. 

The three-dimensional conformation of a protein is called its tertiary structure. An a-helix can be either twisted, folded, or folded and twisted into a definite geometric pattern. These structures are stabilized by dispersion forces, hydrogen bonding, and other intermo-lecular forces. 

The carbon chains of samrated fatty acids form a zigzag pattern when extended, as at low temperamres. At higher temperatures, some bonds rotate, causing chain shortening, which explains why biomembranes become thinner with increases in temperamre. A type of geometric isomerism occurs in unsaturated fatty acids, depending on the orientation of atoms or groups around the axes of double bonds, which do not allow rotation. If the acyl chains are on the same side of the bond, it is cis-, as in oleic acid if on opposite sides, it is tram-, as in elaidic acid, the tram isomer of oleic acid (Fig-... 

---