The effect of the orientation of CO bound to the heme of two c-type cytochromes on the CO-stretch frequency nu(CO) has been investigated using molecular mechanics and finite difference Poisson-Boltzmann calculations. Our approach treats the charge distribution of the protein as an external electric field capable of inducing Stark frequency shifts, Results show that modifying the Fe-C-O bending angle (beta) does not change the CO stretch frequency within a range of 100-175 degrees, equivalent to a bending motion away from the propionic acid chains, The calculated Stark shifts range from 5.4 to 8.6 cm(-1) and are in good agreement with the experimentally observed shift (6 cm(-1)). However, motion of the CO toward the propionics exerts significant influence on the calculated shifts (-4.5 to -1.8 cm(-1)), which are then in total disagreement with experiment, not only in magnitude but also in predicting the wrong direction for the shift. The Stark shifts calculated for the tilt angle (tau) show that it has no significant effect on nu(CO) within a 15 degrees distortion range. With respect to the proximal histidine displacement angle (omega), only a complete (and unlikely) 90 degrees rotation about the Fe-N bond was effective in significantly affecting the Stark shift, and we accordingly rule out a major contribution to the CO distortion from the coordination geometry of the proximal histidine. Overall, the calculations show that the CO ligand could thus enjoy a significant amount of flexibility in the heme pocket-as required to approach and leave the heme group.