Dynamics of unfolded polypeptide chains as model for the earliest steps in protein folding

The rate of formation of intramolecular interactions in unfolded proteins determines how fast conformational space can be explored during folding. Characterization of the dynamics of unfolded proteins is therefore essential for the understanding of the earliest steps in protein folding. We used triplet-triplet energy transfer to measure formation of intrachain contacts in different unfolded polypeptide chains. The time constants (1/k) for contact formation over short distances are almost independent of chain length, with a maximum value of about 5ns for flexible glycine-rich chains and of 12ns for stiffer chains. The rates of contact formation over longer distances decrease with increasing chain length, indicating different rate-limiting steps for motions over short and long chain segments. The effect of the amino acid sequence on local chain dynamics was probed by using a series of host-guest peptides. Formation of local contacts is only sixfold slower around the stiffest amino acid (proline) compared to the most flexible amino acid (glycine). Good solvents for polypeptide chains like EtOH, GdmCl and urea were found to slow intrachain diffusion and to decrease chain stiffness. These data allow us to determine the time constants for formation of the earliest intrachain contacts during protein folding. © 2003 Elsevier Ltd. All rights reserved

The structure and first ¹H NMR spectral assignment of piperazine-C₆₀ adducts

The 1H NMR spectrum of the piperazine-C60 monoadduct has been assigned for the first time almost 10 years after it was synthesised. The preparation and characterisation of the first (C2-substituted piperazine)-C60 monoadduct are also described, revealing the C2-substituent as occupying an exo position on the addend

Stability of odd versus even electron gas-phase (quasi)molecular ions derived from pyridine-substituted N-heterotriangulenes

Electrospray ionisation of N-heterotriangulenes (i.e. dimethylmethylene-bridged triphenylamines) with up to three pyridyl groups at their periphery, produces the true radical cation ([M]+*) and the protonated molecule ([M+H]+) simultaneously. These ions are studied as model systems to illustrate the stability alternation of odd- versus even-electron ions in energy-dependent collision-induced dissociation (CID) experiments. All ions show the same fragmentation pattern, the consecutive loss of three methyl radicals (*CH3) from the dimethylmethylene bridges of the central triangulene core. [M]+* ions dissociate at much lower collision energies than their [M+H]+ counterparts. The radical cation forms a singlet fragment with an extended aromatic system that is energetically favoured. Ab initio and density functional theory calculations support this interpretation and allow the assignment of the electronic structure of the fragment ions. Consecutive collision-induced dissociations provide a better match with theory when studied with an ion trap, rather than a linear quadrupole. This is attributed to the resonant nature of the excitation of intermediate ions