Due to the importance of the structure of amino acids for the folding and functionality of proteins, the conformational behavior of lysine has been investigated. Experimental matrix-isolation FT-IR spectra have been recorded. These spectra were interpreted using an extended theoretical DFT and MP2 study. Theoretically, 28 (DFT) and 18 (MP2) conformations were found with Delta E < 10 kJ mol(-1). Incorporation of the entropy term changed the relative order of the stability because of the large unfavorable effect of this term for the conformations with one or two intramolecular H-bonds. As a matter of fact, the predicted abundances are strongly temperature dependent. The abundant conformations of lysine at sublimation temperature can be characterized by the type of amino acid backbone and the eventual additional H-bond in four groups. These groups are predicted to be detectable in the matrix, as their abundances are all larger than 5%. The theoretical spectral data of the most abundant conformation of a particular group are used to represent the group. In the matrix-isolation FT-IR spectrum all the important, H-bonded involved modes (nu(OH), nu(NH2), nu(C=O), gamma(OH), delta(OH) and gamma(NH2)) of the four conformational groups were observed. A linear correlation between the stretching frequency shift nu(XH) and the elongation of the XH distance Delta r(XH) in different conformations of lysine and other amino acids has been observed. The experimental frequencies are in good relationship with the theoretically obtained data, which is proven by a mean frequency deviation for the most abundant conformation is 12.6 cm(-1).