RESUMEN
Asparagine (Asn) is a powerful turn-inducer residue, with a large propensity to occupy the second position in the central region of ß-turns of proteins. The present work aims at investigating the role of a local anchoring between the Asn side chain and the main chain in this remarkable property. For this purpose, the H-bonding patterns of an asparagine residue in an isolated protein chain fragment forming a γ- or a ß-turn have been determined using IR/UV double resonance gas phase spectroscopy on laser-desorbed, jet-cooled short models in conjunction with relevant quantum chemistry calculations. These gas phase data provide evidence for an original double anchoring linking the Asn primary amide side chain (SC), which adopts a gauche+ rotameric form, to its main chain (MC) local environment. From both IR spectroscopic evidence (H-bond induced red shifts) and quantum chemistry, Asn SC is found to behave as a stronger H-bond acceptor than donor, resulting in stronger MCâSC H-bonds than SCâMC ones. These gas phase structural data, relevant to a hydrophobic environment, have been used as a reference to assess the anchoring taking place in high resolution crystallized proteins of the Protein Data Bank. This approach reveals that, when the SC adopts a gauche+ orientation, the stronger MCâSC bonds are preserved in many cases whereas the SCâMC bonds are always disrupted, in qualitative agreement with the gas phase ranking of these interactions. Most interestingly, when Asn occupies the second position of central part of a ß-turn (i.e., the very turn-inducer position), the MCâSC H-bonds are also disrupted and replaced by a water-mediated SC to MC anchoring. Owing to the specific features of the hydrated Asn side chain, we propose that it could be a turn precursor structure, able to facilitate turn formation in the early events of the folding process.
Asunto(s)
Asparagina/química , Péptidos/química , Amidas/química , Gases/química , Enlace de Hidrógeno , Estructura Secundaria de Proteína , Teoría Cuántica , Espectrofotometría InfrarrojaRESUMEN
High resolution electronic spectra of 2-phenylindole (PI) and N-phenylcarbazole (PC) have been recorded in the collision-free environment of a molecular beam. Inertial defects determined from fits of the spectra were used to determine the twist angles between the two chromophores and their attached benzene rings in the ground (S0) and excited (S1) electronic states. PI was found to be significantly more planar than PC, especially in the S1 state. Stark-effect measurements of the permanent electric dipole moments of both molecules in both states show that significantly more charge is transferred from the phenyl group to the chromophore in PI (0.13e) than in PC (0.076e) when the photon is absorbed. Thereby demonstrated for the first time is a direct connection between photo-induced geometry change and charge transfer on excitation of an isolated molecule by light.