An extensive analysis of the conformational space of tryptophan (Trp) was performed at the B3LYP/6-311++G(d,p) level and verified by comparison with the infrared spectra of the compound isolated in low-temperature argon and xenon matrixes. Different types of conformers have been unequivocally identified in the matrixes. Type I exhibits the trans arrangement of the carboxylic group and is stabilized by an O-H center dot center dot center dot N intramolecular H-bond. Types II and III have the carboxylic group in the cis conformation and feature N-H center dot center dot center dot OC and N-H center dot center dot center dot O-C hydrogen bonds, respectively. Three individual conformers of type I were identified in the matrixes. Other conformational degrees of freedom are related with the C-alpha-C-beta-C-gamma=C and C-1-C-alpha-C-beta-C-gamma angles (chi(1) and chi(2), respectively). In proteins, these two dihedral angles define the conformations of the amino acid residues. In monomeric Trp, chi(1) adopts the "+" (ca. +90 degrees) and "-" (ca. -90 degrees) orientations, while average values of -67.4, 170.5, and 67.6 degrees ("a", "b", and "c", respectively) were found for chi(2). Theoretical analysis revealed two important factors in stabilizing the structures of the Trp conformers: the H-bond type and electrostatic interactions. Classified by the H-bond type, the most stable are forms I, followed by II and III. Out of possible combinations of the chi(1) and chi(2) dihedral angles, "a+", "b+", and "c-" were theoretically found more stable than their "a-", "b-", and "c+" counterparts. Thus, the stabilizing effect of interactions involving the pyrrole ring (which are possible in Ia+, Ib+, and Ic- conformers) is considerably higher compared to those in which the phenyl ring is engaged (existing in the Ia-, Ib-, and Ic+ forms).