A Karplus Equation for the Conformational Analysis of Organic Molecular Crystals

In the liquid state, vicinal J couplings (3J) have been extensively used for conformational analysis of organic compounds, peptides and proteins through the use of empirical Karplus equations. Contrastingly, there are essentially no examples of the use of 3J for quantitative structural investigation of solids. With the support of DFT calculations, we show that 13C-13C 3J couplings measured on a series of 13C-enriched solids can be related to the dihedral angle of the corresponding molecular fragment by a simple Karplus relationship. We show that such function could be successfully used to deduce dihedral angles on a solid compound with known structure.

A Karplus Equation for the Conformational Analysis of Organic Molecular Crystals

In the liquid state, vicinal J couplings (3J) have been extensively used for conformational analysis of organic compounds, peptides and proteins through the use of empirical Karplus equations. Contrastingly, there are essentially no examples of the use of 3J for quantitative structural investigation of solids. With the support of DFT calculations, we show that 13C-13C 3J couplings measured on a series of 13C-enriched solids can be related to the dihedral angle of the corresponding molecular fragment by a simple Karplus relationship. We show that such function could be successfully used to deduce dihedral angles on a solid compound with known structure.

Conformation of 5-Amino-5-deoxypentonolactams

Four configuration isomers of 5-amino-5-deoxy-D-pentonolactam 1a-4a and their tri-O-acetyl derivatives 1b-4b were studied using NMR and CD spectroscopy. For all compounds chemical shifts of the 1H and 13C nuclei as well as of vicinal coupling constants were obtained. Comparison of the observed 3J(H,H) with those calculated for various conformations by a modified Karplus relationship led to the assignment of predominant conformation 3H4(D) or 4H3(D) to the lactams 1a-4a and 1b-4b in solution. The most important factor for determining the conformation seems to be the pseudoequatorial position of the substituent on the carbon next to the carbonyl group. The results of the CD spectra of the lactams 1a-4a in water, interpreted according to the currently used rules, agreed with the NMR results.