Conformational Study of Histidine- and Tryptophan-Containing Peptides in Solution Based upon the Combination of Nuclear Magnetic Resonance, Circular Dichroism, and Fluorescence Measurements. 13

Article abstract of DOI:10.1021/j150653a021

Oligopeptides containing both histidine and tryptophan of the series H-Gly-His-(Gly)n-Trp-Gly-OH (n = 0, 1, 2), together with the dipeptide H-His-Trp-OH and the tripeptide H-His-Trp-Gly-OH, have been prepared and characterized, and their conformational behaviors in dilute solution have been investigated.In particular, our study was aimed at clarifying whether and under which conditions intramolecular interactions between the two aromatic rings are present, and how these affect the behavior of the whole molecule.Our strategy consisted of the simultaneous study by several spectroscopic techniques (UV absorption, fluorescence, circular dichroism (CD), 1H NMR analysis) of the compounds in their different ionization states (pH 2-11) and over a wide temperature range (-70 to +50 deg C).This approach permits reasonable guesses about some of the stablest conformers in solution, in particular, for ⁺H2-His⁺-Trp-O^- and H-Gly-His-Trp-Gly-O^-.In the first case the two aromatic rings are relatively close to each other and form an angle of ca. 45 deg with respect to one another.In the latter case they are almost perpendicular to each other, the H_5' proton of His being the closest to the indole ring; the main chain rather closely follows a β-turn.In no case did we find evidence of parallel stacking between the two rings.We also considered in detail the question of charge-transfer (CT) complexes between histidine and indole, and on the basis of the pH dependence of both UV-absorption and NMR data we could rule out the existence of CT complexes in the ground state.The marked quenching of the indole fluorescence upon protonation of the histidine ring for both H-Gly-His-Trp-Gly-OH and H-Gly-His-Gly-Gly-Trp-Gly-OH is ascribed to a dynamic process.More generally, the data and the strategy presented in this paper are used to critically outline the potential and limits of the various spectroscopic techniques to the problem of conformation of flexible molecules in solution.