The effect of ring currents on carbon chemical shifts in cytochromes

Calculations suggest that some carbon chemical shifts in proteinsshould have large ring current shifts (>1 ppm). We present¹³C, ¹⁵N and ¹H assignments forcytochrome c₂ from Rhodospirillum rubrum, compare these withshifts for other cytochromes c, and show that the calculated ring currentshifts are similar to experimentally observed shifts, but that there remainsubstantial conformation-dependent shifts of side-chain carbons. Ringcurrent shifts as large as 6 ppm are observed. We show that the ring currenteffects do not seriously affect the Chemical Shift Index method fordelineating secondary structure, but may have an impact on more precisemethods for generating structural constraints.