/usr/share/massxpert/dictionaries/protein-3-to-1.dic is in massxpert-data 3.4.1-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 | # Converts from code on the left of '>' to code on the right.
# Number of letters allowed in each code is
# described with syntax 1>3 and that line should be the first
# non-comment line in this file.
3>1
Ala>A
Cys>C
Asp>D
Glu>E
Phe>F
Gly>G
His>H
Ile>I
Lys>K
Leu>L
Met>M
Asn>N
Pro>P
Gln>Q
Arg>R
Ser>S
Thr>T
Val>V
Trp>W
Tyr>Y
|