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Controlled ensemble formation of protein-surfactant systems provides a fundamental concept for the realization of nanoscale devices with self-organizing capability. In this context, spectroscopic monitoring of pigment-containing proteins yields detailed structural information. Here we have studied the association behavior of the bacterial light-harvesting protein LH2 from Rhodobacter spheroides in an n,n-dimethyldodecylamine-n-oxide/water environment. Time-resolved studies of the excitation annihilation yielded information about aggregate sizes and packing of the protein complexes therein. The results are compared to transmission electron microscopy images of instantaneously frozen samples. Our data indicate the manifestation of different phases, which are discussed with respect to the thermodynamic equilibrium in ternary protein-surfactant-water systems. Accordingly, by varying the concentration the formation of different types of aggregates can be controlled. Conditions for the appearance of isolated LH2 complexes are defined.

Femtosecond transient absorption measurements were performed on native and a series of reconstituted LH2 complexes from Rhodopseudomonas acidophila 10050 at room temperature. The reconstituted complexes contain chemically modified tetrapyrrole pigments in place of the native bacteriochlorophyll a -B800 molecules. The spectral characteristics of the modified pigments vary significantly, such that within the B800 binding sites the B800 Q y absorption maximum can be shifted incrementally from 800 to 670nm. As the spectral overlap between the B800 and B850 Q y bands decreases, the rate of energy transfer (as determined by the time-dependent bleaching of the B850 absorption band) also decreases; the measured time constants range from 0.9ps (bacteriochlorophyll a in the B800 sites, Q y absorption maximum at 800nm) to 8.3ps (chlorophyll a in the B800 sites, Q y absorption maximum at 670nm). This correlation between energy transfer rate and spectral blue-shift of the B800 absorption band is in qualitative agreement with the trend predicted from Forster spectral overlap calculations, although the experimentally determined rates are ∼5 times faster than those predicted by simulations. This discrepancy is attributed to an underestimation of the electronic coupling between the B800 and B850 molecules.

We present a simple example of temporal control of the evolving dynamics in a reactive system — a unimolecular reaction with two channels. Using three femtosecond pulses, a wave packet is prepared, probed, and intercepted in the transition-state region. The femtosecond control pulse intercepts the predissociative system at a fixed time and internuclear separation, both determined by the ‘window’ of the laser pulse. Experiments are performed on the NaI system which is a prototype for studies of elementary nuclear motion and reaction dynamics along two potentials, the covalent (Na + I) and the ionic (Na^+ + I^−) channels.