Heteronuclear directed site‐selective NOESY spectroscopy by polarization transfer or multiple quantum filtering

Two methods are examined which allow a NOESY experiment to be performed involving only a specific site of the molecule labelled by a heteronucleus. They involve the merging of a classical phase‐sensitive NOESY experiment with either an INEPT sequence, which transfers 1 H magnetization to a coupled heteronucles, or a multiple quantum filtering sequence which allows the selection of only the 1 H resonances which are coupled to the heteronucles. The two methods are first compared on two small molecules, non‐labelled alanine and [U‐ 13 C]alanine, particularly with respect to the signal‐to‐noise ratio. An example of the application of these methods on a 25000 molecular weight protein system (elastase‐peptide complex) is also given. The limitations of the methods, in particular in terms of T 2 , are discussed.