The 2D Heteronuclear Cross-Polarization (HCP) experiment is an heteronuclear correlation experiment based on the polarization transfer by means of the so-called heteronuclear cross-polarization (HCP) procedure (see HCP building block).

Implementation on any AVANCE spectrometer is feasible. Accurate power level and pulse width optimizations in proton and X-channels are required.

The principles of HCP in liquid state was proposed many years ago ( 78JACS5227 ). The basic pulse sequence of the 2D HCP experiment is very simple:
 
• A 90º ¹H pulse creates transverse magnetization which evolves during the variable evolution period. A 180º ¹³C pulse is applied at the middle od this period to refocus heteronuclear coupling evolution.
• Heteronuclear polarization transfer is achieved by applying simultaneous pulse trains in both ¹H and ¹³C channels in order to match the Hartmann-Hahn condition for both nuclei.
• Finally, carbon resonances are detected under broadband proton decoupling.

Good heteronuclear polarization transfer can be achieved using isotropic mixing sequences as MLEV, WALTZ-16 ( 89CPL432 and 90JMR533-89 ) and DIPSI, and the efficiency of these schemes have been analyzed in detail ( 91MP219 , 91JMR413-91 and 00JMR369-142 ). New broadband heteronuclear Hartmann-Hahn sequences have been designed ( 94JMRA115-111 and 97JMR110-126 ).

Long-range connectivites can also be traced out using a longer mixing time ( 89CPL432 , 91MRC83 , 92MRC662 , 92JMR139-100 , and 92JACS10651 ).

Analogs proton-detected experiments have also been reported ( 92JMR176-98 and 93JB577 ).

Presently, the HCP procedure is largely applied in multidimensional NMR experiments of labeled biomolecules instead of the conventional INEPT-like transfers:

• A suite of novel 2D correlation HCp experiments have been proposed to achive spin-state selective excitation for IS and I2S spin systems ( 04JB37-29 ). Thus, HCP-E.COSY, HCP-TROSY and HCP-S3 spectra can be obtained as with the usual HSQC analogues.

The 2D HCP experiment can be recorded in routine/automation modes. The most important parameters to optimize are the power level and pulse width in both proton and X-channels in order to match the Hartmann-Hahn condition. In addition, mixing time must be optimized to 1/J(CH).

The 2D HCP spectrum is an heteronuclear correlation map showing direct or long-range heteronuclear connectivities depending of the mixing time optimization. However, undesired cross-peaks can be observed due to homonuclear TOCSY transfer when long mixing times are used.

Also see 1D HCP experiment

Related experiments:

2D X-detected experiments