Magnetic field and temperature dependencies shed light on the recombination kinetics of a transition metal donor/acceptor system.

The radical pair recombination of an intramolecular electron-transfer system containing a transition metal moiety has been addressed by femtosecond spectroscopy. The radical pair is formed by ultrafast electron transfer (90 fs) from a ferrocene residue to a photoexcited Nile blue moiety. Its recombination proceeds on the picosecond time scale in a multiexponential fashion. The kinetic pattern is a manifestation of spin processes competing with electron transfer. Magnetic field effects on these kinetics allow one to disentangle the two contributions. Their temperature dependencies yield the activation parameters of the two processes. The discussion focuses on the mechanism of electron spin relaxation. Strong evidence for the Orbach/Kivelson mechanism will be given.

Femtosecond time-resolved guanine oxidation in acridine modified alanyl peptide nucleic acids.

Alanyl peptide nucleic acids have been designed to generate linear and rigid pairing complexes. Femtosecond time resolved electron transfer dynamics studies of alanyl-PNA double strands where both strands contain an intercalated 9-amino-6-chloro-2-methoxy-acridine in its protonated state reveal a strong similarity to nearest neighbor interstrand/intrastrand guanine oxidation in the corresponding B-DNA fragment. This observation implies that the combined influence of electronic couplings and energetic parameters, driving force and reorganization energy, on electron transfer dynamics is similar in both structures. With respect to the alanyl-PNA structure, this result is consistent with the notion of stacking distances in the nucleobase staple similar to the one in B-DNA and thus provides additional structural evidence for nucleobase stacking in alanyl-PNA double strands.

Picosecond time-resolved FRET in the fluorescent protein from Discosoma Red (wt-DsRed).

Ultrafast, intra-oligomer fluorescence resonance energy transfer (FRET) between an immature green-emitting GFP-like chromophore to the mature red-emitting chromophore is found in the novel red fluorescent protein wt-DsRed (the picture shows the steady-state absorption (solid line) and emission (dotted) spectra). Since FRET is by its very nature a short range process, it represents a highly suitable method to probe oligomerization. This work describes a method preferentially applicable to the efficient screening of protein variants with mutagenetically altered surface docking sites.