Peptides (version 1.2.1)

pI: Compute the isoelectic point (pI) of a protein sequence

Description

The isoelectric point (pI), is the pH at which a particular molecule or surface carries no net electrical charge.

Usage

pI(seq, pKscale = "EMBOSS")

Arguments

seq

An amino-acids sequence

pKscale

A character string specifying the pK scale to be used; must be one of "Bjellqvist", "EMBOSS", "Murray", "Sillero", "Solomon", "Stryer", "Lehninger", "Dawson" or "Rodwell"

Details

The isoelectric point (pI) is the pH at which the net charge of the protein is equal to 0. It is a variable that affects the solubility of the peptides under certain conditions of pH. When the pH of the solvent is equal to the pI of the protein, it tends to precipitate and loose its biological function.

Examples

# COMPARED TO ExPASy ProtParam
# http://web.expasy.org/cgi-bin/protparam/protparam
# SEQUENCE: QWGRRCCGWGPGRRYCVRWC
# Theoretical pI: 9.88

pI("QWGRRCCGWGPGRRYCVRWC","Bjellqvist")
# [1] 9.881

# COMPARED TO EMBOSS PEPSTATS
# http://emboss.bioinformatics.nl/cgi-bin/emboss/pepstats
# SEQUENCE: QWGRRCCGWGPGRRYCVRWC
# Isoelectric Point = 9.7158

pI("QWGRRCCGWGPGRRYCVRWC","EMBOSS")
# [1] 9.716

# OTHER SCALES

pI("QWGRRCCGWGPGRRYCVRWC","Murray")
# [1] 9.818
pI("QWGRRCCGWGPGRRYCVRWC","Sillero")
# [1] 9.891
pI("QWGRRCCGWGPGRRYCVRWC","Solomon")
# [1] 9.582
pI("QWGRRCCGWGPGRRYCVRWC","Stryer")
# [1] 9.623
pI("QWGRRCCGWGPGRRYCVRWC","Lehninger")
# [1] 9.931
pI("QWGRRCCGWGPGRRYCVRWC","Dawson")
# [1] 9.568
pI("QWGRRCCGWGPGRRYCVRWC","Rodwell")
# [1] 9.718