Deducing the conformational space for an octa-proline helix.

A molecular dyad, PY-P-PER, comprising a proline octamer sandwiched between pyrene and perylene terminals has been synthesized in order to address the dynamics of electronic energy transfer (EET) along the oligo-proline chain. A simple pyrene-based control compound equipped with a bis-proline attachment serves as a reference for spectroscopic studies. The N-H NMR signal at the terminal pyrene allows distinction between and amides and, although the crystal structure for the control has the conformation, temperature-dependent NMR studies provide clear evidence for / isomerisation in D-DMSO. Polar solvents tend to stabilise the structure for the pyrene amide group, even for longer oligo-proline units. Circular dichroism shows that the proline spacer for PY-P-PER exists mainly in the all- geometry in methanol. Preferential excitation of the pyrene chromophore is possible at wavelengths in the 320-350 nm range and, for the dyad, is followed by efficacious EET to the perylene emitter. The probability for intramolecular EET, obtained from analysis of steady-state spectroscopic data, is 80-90% in solvents of disparate polarity. Comparison with the Förster critical distance suggests the terminals are 18 Å apart. Time-resolved fluorescence spectroscopy, in conjunction with DFT calculations, indicates the dyad exists as a handful of conformers displaying a narrow range of EET rates. Optimisation of a distributive model allows accurate simulation of the EET dynamics in terms of reasonable structures based on isomerisation of certain amide groups.