Matrix-Isolated Monomeric Tryptophan: Electrostatic Interactions as Nontrivial Factors Stabilizing Conformers

Abstract

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···N intramolecular H-bond. Types II and III have the carboxylic group in the cis conformation and feature N−H···OC and N−H···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 Cα−Cβ−CγC and C1−Cα−Cβ−Cγ angles (χ1 and χ2, respectively). In proteins, these two dihedral angles define the conformations of the amino acid residues. In monomeric Trp, χ1 adopts the “+” (ca. +90°) and “−” (ca. −90°) orientations, while average values of −67.4, 170.5, and 67.6° (“a”, “b”, and “c”, respectively) were found for χ2. 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 χ1 and χ2 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).