The biological activity of porphyrins and related tetrapyrroles in proteins may be modulated by nonplanar conformational distortions; consequently, two aspects of nonplanarity have been investigated in the highly nonplanar octaalkyltetraphenylporphyrins (OATPPs). In the first part, the effect of the central metal ion (M = Ni(II), Co(II), Cu(II), Zn(II), Co(III), Fe(III)) on the conformation of the OATPP macrocycle has been determined. Crystallographic studies reveal that the sterically encumbered, nonplanar porphyrin 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin (OETPP) remains sufficiently flexible to show a small decrease in nonplanarity for large metal ions. This decrease in nonplanarity for the OETPP metal complexes is predicted by using a molecular mechanics force field derived from structural and vibrational data for planar metalloporphyrins. A detailed analysis of the crystal structures of the Co(II) and Cu(II) complexes of OETPP reveals that the metal-dependent changes in bond lengths and bond angles are qualitatively similar to the changes observed for the OEP complexes. As the metal size increases, both OEPs and OETPPs exhibit expansion of the meso bridges (increases in the Cα-Cm bond length and the Cα-Cm-Cα bond angle) and a movement of the coordinating nitrogen atoms away from the metal atom (increases in the M-N bond length and the Cα-N-Cα bond angle and a decrease in the N-Cα bond length). Furthermore, the frequencies of several structure-sensitive Raman lines correlate with structural parameters obtained from these crystallographic studies. In the second part, a combination of molecular mechanics and INDO/CI molecular orbital calculations successfully predicts the optical spectra of a series of highly substituted OATPPs with increasing nonplanar distortion. The success of these calculations indicates the importance of including both the macrocycle conformation and the peripheral substituents in the INDO calculations.