There are several tools providing file conversion between Xplor-NIH and Cyana, but I found PdbStat is a pain-free one. Other scripts/tools, in my tests, remain issues sometimes.

PdbStat was developed by Guy Montelione’s group and was published in 2013 in J Biomol NMR. 2013 Aug; 56(4): 337–351 (doi: 10.1007/s10858-013-9753-7): PDBStat: A Universal Restraint Converter and Restraint Analysis Software Package for Protein NMR.

“Provides extensive tools for assessing NMR structures against data, including restraint violation analysis and RDC analysis, as well as tools for converting between coordinate and restraint formats, including NEF format, structural superimposition, and contact map generation.” quoted from PdbStat’s website.

PdbStat runs in Linux and Mac (OS arm64 or old x86_64). It can be downloaded at https://github.rpi.edu/RPIBioinformatics/PDBStat_public. The NESG Wiki for PdbStat is also very informative, useful, and a good reference. Highly recommended.

I use PdbStat to convert Cyana-generated NMR restraints (distance, coupling constants, sequence) to Xplor-NIH format for further calculation. Here are the steps to get the files converted smoothly.

1. Execute PdbStat in the X-terminal.

2. Load PDB coordinate or sequence file. This is required for restraint conversion as PdbStat needs to know the residue types and positions. I prefer structure PDB file.
In PdbStat, use “rea (or read)” to read file, use “save” to output file.

3. Load the Cyana NOE restraints (.upl), remember to choose “cyana” format which is not reminded in the listed options.

4. Save as Xplor-NIH NOE restraints (i save it as noe.tbl). PdbStat asked to estimate/add the range of upper/lower bound. Users can define the percentages. I often use 10-15%.

5. Load the Cyana TALOS-generated dihedral angle restraints (test.aco)

6. Save as Xplor-NIH dihedral angle restraints (I save it as dihedral.tbl).

7. Save the sequence in 3-letter code format for Xplor-NIH

Here are the text format of the files

Dihedral angles:
cyana:
   5 SER   PHI     -91.1   -54.5
   5 SER   PSI     126.6   159.3
   6 MET   PHI    -153.3   -96.8
   6 MET   PSI     108.9   157.9
   7 LYS   PHI    -129.7   -74.2
   7 LYS   PSI     118.2   158.6
   8 ILE   PHI    -161.3  -116.5
   8 ILE   PSI     139.5   173.1

XPLOR-NIH:
   assign (resid   4 and name C   )   (resid   5 and name N   )
          (resid   5 and name CA  )   (resid   5 and name C   )   1.0   -72.800   18.300   2 
   assign (resid   5 and name N   )   (resid   5 and name CA  )
          (resid   5 and name C   )   (resid   6 and name N   )   1.0   142.950   16.350   2 
   assign (resid   5 and name C   )   (resid   6 and name N   )
          (resid   6 and name CA  )   (resid   6 and name C   )   1.0  -125.050   28.250   2 
   assign (resid   6 and name N   )   (resid   6 and name CA  )
          (resid   6 and name C   )   (resid   7 and name N   )   1.0   133.400   24.500   2 
   assign (resid   6 and name C   )   (resid   7 and name N   )
          (resid   7 and name CA  )   (resid   7 and name C   )   1.0  -101.950   27.750   2 
   assign (resid   7 and name N   )   (resid   7 and name CA  )
          (resid   7 and name C   )   (resid   8 and name N   )   1.0   138.400   20.200   2 
   assign (resid   7 and name C   )   (resid   8 and name N   )
          (resid   8 and name CA  )   (resid   8 and name C   )   1.0  -138.900   22.400   2 
   assign (resid   8 and name N   )   (resid   8 and name CA  )
          (resid   8 and name C   )   (resid   9 and name N   )   1.0   156.300   16.800   2 
   assign (resid   8 and name C   )   (resid   9 and name N   )
          (resid   9 and name CA  )   (resid   9 and name C   )   1.0  -121.000   16.400   2 

NOE distances:
Cyana format:
 28 ILE  HB     57 ASP  HA      3.11  #peak 8 #plist 1 #SUP 0.87 #QF 0.87
 66 ILE  HB     66 ILE  QD1     3.23  #peak 12 #plist 1 #SUP 0.95 #QF 0.95
 23 GLU  HA     24 PRO  HD2     3.28  #peak 18 #plist 1 #SUP 0.98 #QF 0.98
 55 LEU  HA     55 LEU  QD2     2.90  #peak 20 #plist 1 #SUP 0.90 #QF 0.90
 53 LYS  QB     53 LYS  QG      2.77  #peak 22 #plist 1 #SUP 0.99 #QF 0.93
 31 VAL  HA     31 VAL  QG1     3.05  #peak 25 #plist 1 #SUP 0.99 #QF 0.29

Xplor-NIH format:
 assign (resid  28 and name HB  )  (resid  57 and name HA  )  2.701  0.901  0.720
   ! target: 2.70  lo: 1.80  up: 3.42 [1, idx:  401  895]
 assign (resid  66 and name HB  )  (resid  66 and name HD1*)  2.766  0.966  0.787
   ! target: 2.77  lo: 1.80  up: 3.55 [1, idx: 1032    0]
 assign (resid  23 and name HA  )  (resid  24 and name HD2 )  2.794  0.994  0.814
   ! target: 2.79  lo: 1.80  up: 3.61 [1, idx:  333    0]
 assign (resid  55 and name HA  )  (resid  55 and name HD2*)  2.585  0.785  0.605
   ! target: 2.58  lo: 1.80  up: 3.19 [1, idx:  861    0]
 assign (resid  53 and name HB* )  (resid  53 and name HG* )  2.513  0.713  0.534
   ! target: 2.51  lo: 1.80  up: 3.05 [1, idx:  825  828]
 assign (resid  31 and name HA  )  (resid  31 and name HG1*)  2.668  0.868  0.688
   ! target: 2.67  lo: 1.80  up: 3.36 [1, idx:  452    0]