Averaged length, net charge, and amino acid composition of 6616 peptides in the APD database:

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The average length of all 6616 is 28.02 residues.
The average net charge of all 6616 is 4.29

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I percent is 6.21%
V percent is 5.47%
L percent is 8.68%
F percent is 4.67%
C percent is 5.4%
M percent is 1.08%
A percent is 6.99%
W percent is 2.39%

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G percent is 10.3%
P percent is 4.65%

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T percent is 3.83%
S percent is 5.16%
Y percent is 2.32%
Q percent is 2.68%
N percent is 3.41%

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E percent is 2.36%
D percent is 2.25%
H percent is 2.06%
K percent is 11.81%
R percent is 8.12%

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If you want statistical information for a group of peptides, you can search the database first. Then select 'show statistic data' at the bottom of the 'query result' page.