Hello,
I was reading about the coupling constants of alpha v.s. beta glucose and read that according to Karplus curve, for the Beta conformation where the Anomeric proton is at a 180 degree to the neighboring proton (axial-axial) the coupling is larger than that of the alpha-glucose when the anomeric proton is equitorial and the neighboring proton axial. The dihedral angle in this case would be 60. How is it so? Correct me if I am wrong but when two protons are further apart the coupling constant is larger reflecting their distance which makes sense in this case but how is it that two protons further apart give rise to a larger coupling constant, should it not be weaker? Maybe I can try to explain this and someone can confirm or correct me. If a proton is split by a neighbouring proton (so in this case the anomeric proton will be a doublet) the coupling (how far apart the two teeth of the doublet are) is purely an indication of how this other proton which is splitting it is?
Thanks in advance,
Nescafe.