Analogy parameters joining gaff2 terms for modified residues
MASS

BOND
C -n2   490.0   1.335   ! C-N
c1-N    490.0   1.335   ! C-N

ANGLE
CX-C -n2   70.0       116.60    ! CX-C-N
C -n2-c3   50.0       121.90    ! C-N-CT
C -n2-hn   50.0       120.00    ! C-N-H
c1-N -H    50.0       120.00    ! C-N-H
o -c1-N    80.0       122.90    ! N-C-O 
O -C -n2   80.0       122.90    ! N-C-O
c3-c1-N    70.0       116.60    ! CT-C-N
c1-N -CX   50.0       121.90    ! C-N-CX

DIHE
CX-C -n2-c3     4   10.00   180.0    2.   ! X-C-N-X
CX-C -n2-hn     4   10.00   180.0    2.   ! X-C-N-X
O -C -n2-c3     4   10.00   180.0    2.   ! X-C-N-X
O -C -n2-hn     1    2.50   180.0   -2.   ! H-N-C-O term 2
O -C -n2-hn     1    2.00     0.0    1.   ! H-N-C-O term 1
c3-c1-N -H      4   10.00   180.0    2.   ! X-C-N-X
c3-c1-N -CX     4   10.00   180.0    2.   ! X-C-N-X
o -c1-N -H      1    2.50   180.0   -2.   ! H-N-C-O term 2
o -c1-N -H      1    2.00     0.0    1.   ! H-N-C-O term 1
o -c1-N -CX     4   10.00   180.0    2.   ! X-C-N-X
CX-C -n2-c3     4   10.00   180.0    2.   ! X-C-N-X
CX-C -n2-hn     4   10.00   180.0    2.   ! X-C-N-X

IMPROPER

NONBON