Conformational studies of diosgenyl 2-amino-2-deoxy-b-D-glucopy- ranosides at the PM3 and DFT levels of theory Andrzej Nowacki ⇑ , Henryk Myszka, Beata Liberek Faculty of Chemistry, University of Gdan ´sk, Sobieskiego 18, PL-80-952 Gdan ´sk, Poland article info Article history: Received 13 April 2013 Received in revised form 17 May 2013 Accepted 18 May 2013 Available online 28 May 2013 Keywords: Diosgenyl glucosides 2-Amino-2-deoxy-b-D-glucopyranosides Conformation PM3 DFT Exo-anomeric effect abstract Geometry optimizations at the PM3 level were performed for diosgenyl 2-amino-2-deoxy-b-D-glucopy- ranoside and its N-protonated form. Next, B3LYP/6-311++G // level geometry optimizations were carried out, albeit on a simpler model. The relative Gibbs free energies and geometry parameters are presented for the optimized structures. Conformational analysis concerning the clockwise (cw) and counterclock- wise (ccw) arrangements of the OH groups as well as the three orientations of the NH 2 and CH 2 OH groups was performed. Furthermore, a full scan of all the possible locations of the diosgenyl moiety in relation to the sugar ring in the target compounds was done. The PM3 optimization results indicate that diosgenyl 2- amino-2-deoxy-b-D-glucopyranoside exists as a mixture of many rotamers. Of these, ccw conformers as well as the ac orientation of the aglycone (ca 100° for torsion angle /) are preferred. In the case of the N-protonated glucoside one rotamer is dominant in the mixture (population > 60%). Only the cw arrange- ment is stable in the protonated form, because of steric and electronic repulsion between the charged ammonium group and the hydrogen atom of the neighbouring 3-OH group. The gauche-trans (gt) orien- tation of the CH 2 OH group is favourable to both neutral and protonated forms of diosgenyl glucosides at the PM3 level. DFT studies also predict conformational mixtures for both neutral and protonated forms of simplified diosgenyl 2-amino-2-deoxy-b-D-glucopyranosides. These investigations showed unequivocally the preference for the ccw arrangement over cw, and that values of angle / concur with the exo-anomeric effect. The B3LYP functional predicts the greater stability of the gt and tg orientations for the neutral and protonated analogues respectively. According to both PM3 and B3LYP calculations, torsion angle w has only a minor influence on the conformational energy. The importance of the geometry parameters affect- ing the stability of these conformers appears to be as follows: angle /  NH 2 /ccw/cw > gg/gt/tg > angle w. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Saponins are steroid or triterpenoid glycosides widely distrib- uted in plants and in some marine organisms. 1 Diosgenyl glyco- sides are one of the most important groups of saponins. The structural diversity of naturally-occurring diosgenyl-type saponins is to be found mainly in the sugar moiety. The carbohydrate part of these compounds, usually a mono-, di-, tri- or tetrasaccharide, is covalently attached to the sapogenin backbone. Natural diosgenyl glycosides typically have b-D-glucopyranose attached directly to diosgenin. The b-D-glucopyranose residue, in turn, is very often substituted by a-L-rhamnopyranose at position 2 and another su- gar or sugar chain at position 3 or 4. 1 Diosgenyl glycosides are often found to be the major components in traditional oriental medicines used to treat malaria, helminth infections and snake bites. 2–4 They are also good antifungal and antibacterial agents. 5 Pharmacological studies have shown that diosgenin and its glycosides are cytotoxic towards hu- man cancer cells. 6,7 We have been synthesizing analogues of natural diosgenyl gly- cosides containing 2-amino-2-deoxy-D-glucopyranose attached di- rectly to the diosgenyl moiety in our laboratory for several years. 8,9 Biological tests have demonstrated that two such glycosides—dios- genyl 2-amino-2-deoxy-b-D-glucopyranoside hydrochloride and diosgenyl 2-deoxy-2-phenylureido-b-D-glucopyranoside—alone and in combination with 2-chlorodeoxyadenosine (2-CdA) induce apoptosis and necrosis in B cells isolated from the blood of patients with chronic lymphatic leukaemia. The results point to a support- ing role of synthetic diosgenin derivatives in the cytotoxic action of the 2-CdA towards cancer cells. 9 Since there is a direct relationship between the conformation of monosaccharides and their biological 10,11 and chemical 12–14 proper- ties, conformational studies of these carbohydrates have been car- ried out in considerable depth. To the best of our knowledge, glycosides of 2-amino-2-deoxy-b-D-glucopyranose, in contrast to other simple sugars, have never been subjected to detailed confor- mational studies. The present paper reports on the geometry 0008-6215/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carres.2013.05.013 ⇑ Corresponding author. Tel.: +48 58 5235461; fax: +48 58 5235472. E-mail address: anowacki@chem.univ.gda.pl (A. Nowacki). Carbohydrate Research 377 (2013) 4–13 Contents lists available at SciVerse ScienceDirect Carbohydrate Research journal homepage: www.elsevier.com/locate/carres