The conformational landscape of isoleucine was investigated by a theoretical DFT and MP2 study. This investigation has revealed new important conformations in comparison to a previous study. Five conformations have been predicted with an abundance larger than 5% at both levels of theory. Among these, three different types of amino acid backbone were present, each characterized by a typical intramolecular H-bond. Due to the H-bonding, the frequencies of the involved groups are strongly shifted, and these modes were probably observed with FT-IR spectroscopy in an Ar matrix. The experimental abundances could be estimated and were in good accordance with the MP2 stabilities, whereas the accordance with the DFT abundances was poor. In contrast to this, the correspondence of the DFT and the experimental frequencies was excellent, as demonstrated by the mean frequency deviation of only 4.7 cm(-1).