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"CPK" stands for Corey-Pauling-Koltun. It commonly refers to spacefilling physical molecular models developed by Corey, Pauling and Koltun^[1][2] in the 1950's and 1960's. CPK also refers to the color scheme for the elements that they adopted.

CPK Color Scheme for the Elements

In this color scheme, carbon is gray or black, oxygen is red, nitrogen is blue, sulfur is yellow, and hydrogen is white. The command "color cpk" applies this color scheme in popular molecular visualization programs such as RasMol, Chime, and Jmol. The complete set of element colors used by Jmol is available at Jmol Colors, where the colors used by RasMol are compared with those used by Jmol. The CPK color scheme for the elements is also incorporated into a standard set of color schemes for macromolecules, DRuMS.

A color key such as the one below can be inserted into Proteopedia using existing templates (see Help:Color Keys):
C H O N S Se

References

1. ↑ Corey, RB and Pauling L (1953), Molecular models of amino acids, peptides and proteins. Rev. Sci. Instr. 24: 621-627.
2. ↑ Koltun WL. Precision space-filling atomic models. Biopolymers. 1965 Dec;3(6):665-79. PMID:4158989