title: Unblocked statistical-coil tetrapeptides in aqueous solution: quantum-chemical computation of the carbon-13 NMR chemical shifts.
abstract: We recently reported a theoretical characterization of representative ensembles of statistical-coil conformations for tetrapeptides with unblocked termini in aqueous solution, at pH 7. The results showed good agreement between the computed Boltzmann-averaged and experimentally-determined values for both the vicinal coupling constants (3)J(NHalpha) and the alpha-proton chemical shifts. Here, we carry out a cluster analysis of the ensembles of conformations generated in that study, and use them to compute the Boltzmann-averaged values of the quantum-chemical (13)C chemical shifts for different amino acids in the unblocked tetrapeptides GGXA (where X stands for Phe, Arg, His, Glu, Ile, Lys, Gln, Tyr, Leu, Thr, Ala, Gly and Val). The values of the (13)C chemical shifts in these thirteen amino acids (for which experimental data are available) were computed by using Density Functional Theory with a 6-311+G(2d,p) basis set. Good agreement is found in terms of both the correlation coefficient (R) and standard deviations of the difference between the computed Bolztmann-averaged and the NMR-determined values for the (13)C chemical shifts. These results suggest that it may be possible to build a reliable theoretically-derived database of chemical shifts for statistical-coil residues. The results of the current study contribute to our understanding of the relations between chemical shifts, dihedral angles and vicinal coupling constants, (3)J(NHalpha). In addition, they can shed light as to how the statistical-coil conformation is related to the conformational preference of more structured states, such as the alpha-helical conformation.
authors.affiliation: Universidad Nacional de San Luis, Facultad de Ciencias Físico Matemáticas y Naturales, Instituto de Matemática Aplicada San Luis, CONICET, Ejército de Los Andes 950, 5700 San Luis, Argentina.
authors.forename: Jorge A
authors.initials: JA
authors.lastname: Vila
authors.forename: Héctor A
authors.initials: HA
authors.lastname: Baldoni
authors.forename: Daniel R
authors.initials: DR
authors.lastname: Ripoll
authors.forename: Harold A
authors.initials: HA
authors.lastname: Scheraga
mesh_terms: D000595:Amino Acid Sequence; D002247:Carbon Isotopes; D016000:Cluster Analysis; D008962:Models, Theoretical; D019906:Nuclear Magnetic Resonance, Biomolecular / Q000379:methods*; D009842:Oligopeptides / Q000737:chemistry*; D017433:Protein Structure, Secondary; D011789:Quantum Theory; D012996:Solutions
chemical_list: D002247:Carbon Isotopes; D009842:Oligopeptides; D012996:Solutions