Design and engineering of water-soluble light-harvesting protein maquettes.

Natural selection in photosynthesis has engineered tetrapyrrole based, nanometer scale, light harvesting and energy capture in light-induced charge separation. By designing and creating nanometer scale artificial light harvesting and charge separating proteins, we have the opportunity to reengineer and overcome the limitations of natural selection to extend energy capture to new wavelengths and to tailor efficient systems that better meet human as opposed to cellular energetic needs. While tetrapyrrole cofactor incorporation in natural proteins is complex and often assisted by accessory proteins for cofactor transport and insertion, artificial protein functionalization relies on a practical understanding of the basic physical chemistry of protein and cofactors that drive nanometer scale self-assembly. Patterning and balancing of hydrophobic and hydrophilic tetrapyrrole substituents is critical to avoid natural or synthetic porphyrin and chlorin aggregation in aqueous media and speed cofactor partitioning into the non-polar core of a man-made water soluble protein designed according to elementary first principles of protein folding. This partitioning is followed by site-specific anchoring of tetrapyrroles to histidine ligands strategically placed for design control of rates and efficiencies of light energy and electron transfer while orienting at least one polar group towards the aqueous phase.

Synthesis and photophysical properties of chlorins bearing 0-4 distinct meso-substituents.

The presence of substituents at designated sites about the chlorin macrocycle can alter the spectral properties, a phenomenon that can be probed through synthesis. Prior syntheses have provided access to chlorins bearing distinct aryl substituents (individually or collectively) at the 5, 10, and 15-positions, but not the 20-position. A new Western half (5-phenyl-2,3,4,5-tetrahydro-1,3,3-trimethyldipyrrin) has been employed in condensation with an Eastern half (9-bromodipyrromethane-1-carboxaldehyde) followed by oxidative cyclization to give (5% yield) the zinc(II) 20-phenylchlorin. Condensation of the same Western half and a diaryl-substituted Eastern half provided (11% yield) the zinc(II) 5,10,20-triarylchlorin; demetalation with TFA followed by 15-bromination and Suzuki coupling gave the free base 5,10,15,20-tetraarylchlorin. Altogether, 10 new synthetic chlorins have been prepared. The near-UV (B) absorption band of the free base chlorins shifts bathochromically from 389 to 429 nm and that for the zinc chlorins from 398 to 420 nm as the number of meso-aryl rings is increased stepwise from 0-4. The long-wavelength (Q(y)) absorption band undergoes a bathochromic and hypochromic shift upon increase in number of meso-aryl groups. Regardless of the number and positions of the meso-aryl substituents (including "walking a phenyl group around the ring"), the respective fluorescence quantum yields (0.17 to 0.27) and singlet excited-state lifetimes (9.4 to 13.1 ns) are comparable among the free base chlorins and the same is true for the zinc chelates (0.057 to 0.080; 1.2 to 1.6 ns). Density functional theory calculations show that of the frontier molecular orbitals of the chlorin, the energy of the HOMO-1 is the most affected by meso-aryl substituents, undergoing progressive destabilization as the number of meso-aryl groups is increased. The availability of chlorins with 0-4 distinct meso-aryl substituents provides the individual stepping-stones to bridge the known unsubstituted chlorin and the meso-tetraarylchlorins.

Effects of linker torsional constraints on the rate of ground-state hole transfer in porphyrin dyads.

Understanding hole/electron-transfer processes among interacting constituents of multicomponent molecular architectures is central to the fields of artificial photosynthesis and molecular electronics. Herein, we utilize a recently demonstrated (203)Tl/(205)Tl hyperfine "clocking" strategy to probe the rate of hole/electron transfer in the monocations of a series of three thallium-chelated porphyrin dyads, designated Tl(2)-U, Tl(2)-M, and Tl(2)-B, that are linked via diarylethynes wherein the number of ortho-dimethyl substituents on the aryl group of the linker systematically increases (none, one, and two, respectively). Variable-temperature (160-340 K) EPR studies on the monocations of the three dyads were used to examine the thermal activation behavior of the hole/electron-transfer process and reveal the following: (1) Hole/electron transfer at room temperature (295 K) slows as torsional constraints are added to the diarylethyne linker [k(Tl(2)-U) > k(Tl(2)-M) > k(Tl(2)-B)], with rate constants that correspond to time constants in the 2-5 ns regime. (2) As the temperature decreases, the hole/electron-transfer rates for the monocations of the three types of dyads converge and then cross over. At the lowest temperatures examined (160-170 K), the trend in the hole/electron-transfer rates is essentially reversed [k(Tl(2)-B) > k(Tl(2)-M) ~ k(Tl(2)-U)]. The trends in the temperature dependence of hole/electron-transfer among the three dyads are consistent with torsional motions of the aryl rings of the linker providing for thermal activation of the process at higher temperatures in the case of the less torsionally constrained dyads, Tl(2)-U and Tl(2)-M. In the case of the most torsionally constrained dyad, Tl(2)-B, the hole/electron-transfer process is activationless at all temperatures studied. The reversal in rates of hole/electron transfer among the three dyads at low temperature is qualitatively explained by the results of density functional theory calculations, which predict that static electronic factors could dominate the hole/electron-transfer process when torsional dynamics are thermally diminished.

The Baylis-Hillman acetates as a valuable source for one-pot multistep synthesis: a facile synthesis of functionalized tri-/tetracyclic frameworks containing azocine moiety.

The Baylis-Hillman acetates have been conveniently transformed into tri-/tetracyclic heterocyclic frameworks containing an important azocine moiety via one-pot multistep protocol involving alkylation, reduction, and cyclization sequence.