A photodepletion spectrum of gaseous protonated tyrosine molecules was obtained at 150 K by a UV laser spectroscopic technique in conjunction with mass spectrometry and interpreted by theoretical methods. The spectrum exhibits three distinct bands separated each other by about 800 cm(-1). Stable conformers of the molecular ion were determined by quantum mechanical density functional theory, and their electronic transition energies were obtained by a semiempirical quantum chemistry calculation. The whole pattern of the spectrum was reasonably reproduced by a combination of theoretical methods, the second-order cumulant expansion, a semiempirical quantum chemistry method, molecular dynamics simulation, and a semiclassical time-correlation function approach. The three spectral bands turned out to arise from the vibronic transition of two vibrational modes constituted by the "benzene breathing" mode and a torsional mode of the amino acid backbone. It is suggested that the major factor in spectral broadening is not conformational disorder or lifetime broadening but thermal fluctuation of the stable conformers. The good agreement between experimental and theoretical spectra exemplifies the validity of the theoretical methods applied for the present molecular system.