Matrix-isolated monomeric tryptophan: electrostatic interactions as nontrivial factors stabilizing conformers.
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...O=C 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 Calpha-Cbeta-Cgamma=C and C1-Calpha-Cbeta-Cgamma angles (chi1 and chi2, respectively). In proteins, these two dihedral angles define the conformations of the amino acid residues. In monomeric Trp, chi1 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 chi2. 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 chi1 and chi2 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).
Kaczor
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Department of Chemistry, University of Coimbra, P-3004-535 Coimbra, Portugal. kaczor@chemia.uj.edu.pl
Reva
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Proniewicz
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Fausto
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D002627:Chemistry, Physical / Q000379:methods*; D019277:Entropy; D006860:Hydrogen Bonding; D008956:Models, Chemical; D008968:Molecular Conformation; D015394:Molecular Structure; D012984:Software; D013055:Spectrophotometry, Infrared; D017550:Spectroscopy, Fourier Transform Infrared; D055672:Static Electricity; D013816:Thermodynamics; D014364:Tryptophan / Q000737:chemistry*
D014364:Tryptophan