Studies on O-H-O Hydrogen Bonds and conformation of cyclic hexa depsipeptides

Abstract

The working criteria which have been developed for both O-H...O and N-H...O types of hydrogen bond can be utilised. FIG. 3.5. FLOW CHART OF THE PROGRAM MODULE Read input data: (a) coordinates of the hydrogen bond forming atoms; (b) type of donor group; (c) type of acceptor group. Read ranges for the criteria of various parameters. Does the acceptor group belong to carbonyl type?

Yes: Calculate the direction cosines l, m, n for the best plane passing through the group by least squares method and fix orbitals. No: Calculate the direction cosines l, m, n of acceptor plane after fixing the orbitals by usual tetrahedral fixing method.

Calculate the donor and acceptor end parameters. Choose proper range of criteria depending upon the classification of the donor and acceptor group. Apply the criteria for N-H or O-H hydrogen bond. In any conformational calculation involving hydrogen bond search, a computer programme written in the Fortran language for the IBM 360/44 computer available at the Institute can be used in conjunction with any conformational programme and the qualitative strength of the existing hydrogen bonds can be known. The flow chart of the programme is shown in Fig. 3.5. This will give a more complete test for the hydrogen bond since both the donor and acceptor ends are involved in the test. However, such a test will only give the qualitative strength of the hydrogen bond. To get the quantitative strength of the hydrogen bonds, new potential functions are to be proposed utilising the data available here and the constants can be obtained which will take into account the geometries of both the donor and acceptor groups. The materials presented in this chapter have been collected from various reported literature citations and are intended to serve as a background to the study described in the next two chapters. The crux of the study has been a systematic theoretical conformational study on cyclic hexadepsipeptide and in this, a beginning has been made by studying the conformations which have a three-fold symmetry in the ring. Secondly, the material presented in this chapter brings out the biological importance of the family of cyclic hexadepsipeptides though, due to the limitation of time, a direct correlation of the conformations that are obtained in the present study with their biological functional properties has not been attempted.