The effect of ring currents on carbon chemical shifts in cytochromes.

Calculations suggest that some carbon chemical shifts in proteins should have large ring current shifts (> 1 ppm). We present 13C, 15N and 1H assignments for cytochrome C2 from Rhodospirillum rubrum, compare these with shifts for other cytochromes c, and show that the calculated ring current shifts are similar to experimentally observed shifts, but that there remain substantial conformation-dependent shifts of side-chain carbons. Ring current shifts as large as 6 ppm are observed. We show that the ring current effects do not seriously affect the Chemical Shift Index method for delineating secondary structure, but may have an impact on more precise methods for generating structural constraints.