Resonance Raman spectra have been observed for NO adducts of wild-type (WT) sperm whale myoglobin (MbNO) and its H64G, H64L, L29W, V68W, and V68T mutants at neutral and acidic pH. Raman excitation in resonance with the Soret band enabled us to detect the Fe-NO stretching (nu(Fe-NO)), N-O stretching (nu(NO)), and Fe-N-O bending (delta(FeNO)) bands. The nu(Fe-NO), delta(FeNO), and nu(NO) bands of WT MbNO at neutral pH were observed at 560, 452, and 1613 cm(-1), respectively, and substitution of the distal His64 to Gly or Leu caused an upshift of nu(NO) to 1631-1635 cm(-1) but no change in nu(Fe-NO). This change in nu(NO) is considered to be due to the removal of hydrogen bonding between His64 and bound NO. Conversely, substitution of Leu29 with tryptophan (L29W) altered nu(Fe-NO) but caused no change in nu(NO) at neutral pH. This feature resembles that of MbO(2) but distinctly differs from that of MbCO, for which the Fe-CO and C-O stretching frequencies have an inverse linear correlation. The change in nu(Fe-NO) for L29W-MbNO is probably caused by tilting of the Fe-N bond from the heme normal on account of steric hindrance from the large indole ring but would not be due to changes in the Fe-N-O bond angle. When pH is lowered to 4, MbNO adopts the five-coordinate structure due to cleavage of the Fe-His bond. Accordingly, the heme maker bands such as nu(3) and nu(10), shifted from 1500 and 1636 cm(-1) at pH 7.4 to 1509 and 1646 cm(-1) at pH 4 which are in agreement with those of a five-coordinate Fe-protoporphyrin-NO complex in detergent micelles at neutral pH. The nu(Fe-NO) and nu(NO) bands of acidic MbNO were observed at 520 and 1668 cm(-1) and exhibited no shift when the distal His was replaced by Gly or Leu. The latter observation supports previous X-ray crystallographic, infrared, and resonance Raman spectroscopic measurements which show that the distal histidine becomes protonated at pH 4 and swings out into the solvent away from the bound ligand.