The 3D HNCA experiment is specifically designed to correlate ¹⁵N and NH chemical shifts with the intra- and interresidue ¹³CA carbon shifts by means of the ¹J(NH) and ^1,2J(N,CA) coupling constants. Interresidue correlations can be assigned from a 3D HN(CO)CA experiment.

Implementation on AVANCE spectrometers equipped with a third channel. Improved versions using pulsed field gradients (PFGs) are also available and, therefore, in such cases gradient technology is required.

The experiment is applied on ¹⁵N,¹³C-labeled proteins. Because the amide (NH) protons are involved, the HNCA experiment must be recorded in H₂O.

The 3D HNCA pulse sequence 92JB195 and 94JACS6464 , 93ANG1489 , 94JMRB203-103 , and 94JMRA129-109 ) is closely analog to the 3D HNCO experiment and consisted of the following out-and-back steps:

1. Initial transfer from ¹HN to ¹⁵N via ¹J(NH) using an INEPT pulse sequence.
2. Fixed evolution delay to achieve antiphase ¹⁵N magnetization with respect to ¹³CA via ¹J(N,CA) and refocusing of ¹J(NH).
3. ¹³CA chemical shift evolution during the variable evolution t₁ period in an HSQC-type way followed by ¹⁵N chemical shift evolution during a constant-time evolution t₂ period with evolution of ¹J(NH) and refocusing of ¹J(N,CA).
4. Magnetization is finally transferred back to the NH protons by applying a retro-INEPT scheme and proton acquisition is recorded under ¹⁵N decoupling.

Several improved versions have been proposed incorporating the following modifications:

• The original sequence used a different pathway in which ¹⁵N chemical shift evolution takes place before the ¹³CA chemical shift evolution delay ( 90JMR496-89 ).
• Constant-time period (CT) in the F₁(¹⁵N) dimension, HSQC-like transfer in F₂(¹³CA) dimension and optional ¹H decoupling instead of 180º ¹H pulses was first described in 92JMR432-96 .
• A refocused and optimized version ( 92JB195 ).
• Incorporation of PFGs ( 92JB395 ).
• Improved sensitivity incorporating the PEP methodology in phase-cycled ( 92JMR431-100 ) and gradient-enhanced versions ( 94JACS6464 ) as described for the HNCO experiment ( 93ANG1489 , 94JMRB203-103 , and 94JMRA129-109 ).
• Selective CB-CO decoupling
• Improved sensitivity by ²H decoupling during the Constant-Time CA evolution period in  ¹⁵N,¹³C,²H-labeled proteins ( 94JACS6464 and 94JACS11655 ). The use of selective ¹³CB decoupling during this period has also been proposed ( 96JMRB91-113 ).
• Improved sensitivity and resolution for large biomolecules using the TROSY approach ( 98PNAS13585 , 99JB85 , 99JB181-15 , 01JB127-20 , 01JB177-20 , 02JMR123-155 ). A version has been proposed to improve the sensitivity when conformational exchange-induced (CSX) is present ( 01JB161-21 )
• Improved sensitivity by simultaneous acquisition of two coherence pathways in the so-called HNCA+ experiment ( 00JMR223-143 ).
• Sequential HNCA experiment to obtain only interresidue correlations ( 01JMR100-150 ) as an alternative when the HN(CO)CA experiment fails. Otherwhise, intraresidue HNCA connectivities could be identified from 3D intra-HNCA
• For small and medium-size proteins: 2D H[NCA] ( 94JB325 ).
• The same connectivities can be traced out from 3D (HA)CANH and 3D(HA)CA(CO)NH experiments.