Pseudo-Contact Shift PCS

Coupling constants are routinely used to determine the side-chain conformation of amino acids in peptides and proteins. Whereas proteins nowadays are almost exclusively studied as C- and N-labeled isotopomers, peptides usually have these isotopes in natural abundance, i.e. the magnetically active heteronuclei are highly diluted. Most amino acids contain a methylene group at the ji-position for which the X angle is determined by the conformation of the Ca—Cp bond. Two vicinal Jhh coupling constants can be measured Ha to and H to Usually 

Recently it has been found that couplings between and C across H-bonds, e.g. in systems containing N—H..-0= C units (in proteins), can be directly detected and provide evidence for H-bonds in proteins and nucleic acids [89]. Although this technology is now standard for larger biomolecules it is rarely used for smaller molecules [90]. 

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