The single residue vibrational spectra of tryptophan (Trp) and tyrosine (Tyr) residues in human adult hemoglobin (HbA), which play important roles in cooperative oxygen binding, were determined for the deoxy and CO-bound forms by applying UV resonance Raman spectroscopy to various variant Hbs. It was found that Trp beta 37, Tyr alpha 42, Tyr alpha 140, and Tyr beta 145 at the alpha(1)-beta(2) subunit interface underwent transitions between two contact states (named as T and R) upon ligand binding, while Trp alpha 14, Trp beta 15, and Tyr beta 35 displayed little changes. The corresponding spectral changes were identified only for the alpha(2)beta(2) tetramer, but not the isolated alpha and beta chains in the oligomeric forms, and therefore were exclusively attributed to a quaternary structure change. Ligand binding as well as allosteric effectors and pH altered only the number of the T-contacted Tyr and Trp residues without varying the two contact states themselves. A new method to semiquantitatively evaluate the amount of T-contacted Tyr and Trp residues in a given liganded form is here proposed, and with it a quaternary structure was determined for various symmetrically half-liganded forms obtained with ligand-hybrid, metal-hybrid, and valency-hybrid Hbs. It was found that ligand binding to the alpha or beta subunits yielded different subunit contacts and that the contact changes of the Tip and Tyr residues were not always concerted. The contact changes at the alpha(1)-beta(2) (alpha(2-)beta(1)) interface are correlated with the proximal strain exerted on the Fe-His(F8) bond, which is noted to be much larger in the alpha than beta subunits in the alpha(2)beta(2) tetramer.