Velautham Sivakumar

FTIR Difference Spectroscopy in Combination with Isotope Labeling for Identification of the Carbonyl Modes of P700 and P700+ in Photosystem I

Room temperature, light induced (P700(+)-P700) Fourier transform infrared (FTIR) difference spectra have been obtained using photosystem I (PS I) particles from Synechocystis sp. PCC 6803 that are unlabeled, uniformly (2)H labeled, and uniformly (15)N labeled. Spectra were also obtained for PS I particles that had been extensively washed and incubated in D(2)O. Previously, we have found that extensive washing and incubation of PS I samples in D(2)O does not alter the (P700(+)-P700) FTIR difference spectrum, even with approximately 50% proton exchange. This indicates that the P700 binding site is inaccessible to solvent water. Upon uniform (2)H labeling of PS I, however, the (P700(+)-P700) FTIR difference spectra are considerably altered. From spectra obtained using PS I particles grown in D(2)O and H(2)O, a ((1)H-(2)H) isotope edited double difference spectrum was constructed, and it is shown that all difference bands associated with ester/keto carbonyl modes of the chlorophylls of P700 and P700(+) downshift 4-5/1-3 cm(-1) upon (2)H labeling, respectively. It is also shown that the ester and keto carbonyl modes of the chlorophylls of P700 need not be heterogeneously distributed in frequency. Finally, we find no evidence for the presence of a cysteine mode in our difference spectra. The spectrum obtained using (2)H labeled PS I particles indicates that a negative difference band at 1698 cm(-1) is associated with at least two species. The observed (15)N and (2)H induced band shifts strongly support the idea that the two species are the 13(1) keto carbonyl modes of both chlorophylls of P700. We also show that a negative difference band at approximately 1639 cm(-1) is somewhat modified in intensity, but unaltered in frequency, upon (2)H labeling. This indicates that this band is not associated with a strongly hydrogen bonded keto carbonyl mode of one of the chlorophylls of P700.

Photoinduced electron transfer interaction of anthraquinones with aniline quenchers: Influence of methyl substitution in aniline donors

Photoinduced electron transfer between triplet state of 9,10-anthraquinone (AQ) and its two derivatives: 2-chloro-9,10-anthraquinone (CAQ) and sodium anthraquinone-2-sulfonate (AQS) and ground state aniline (AN) and its dimethyl substitutions: 2,3-dimethylaniline (2,3-DMA), 2,6-dimethylaniline (2,6-DMA), 3,5-dimethylaniline (3,5-DMA) and N,N-dimethylaniline (N,N-DMA) is studied using nanosecond laser flash photolysis at room temperature. Detection of radical bands of quinone anions and aniline cations along with their formation and/or decay kinetics are used to confirm the electron transfer (ET) process. In MeCN medium, AN quenches the triplet state of CAQ (CAQT) but not the triplets AQT or AQST. However in aqueous medium, AN quenches AQST and forms radical ion pair. All the DMAs can react through ET with all the triplet quinones at different degrees of efficiency in MeCN medium. Noticeably, the ring substituted DMAs are less efficient in electron donation to AQT or AQST while the N,N-DMA shows high efficiency in donating electron to all triplet quinones in MeCN medium. Charge distribution of donor molecules, in MeCN medium is calculated using density functional theory (DFT), and shows an enhancement of electron density of the ring of N,N-DMA, making it an ideal electron donor for ET studies compared to other DMAs. This systematic selection and usage of anilines with electrochemically tunable quinones can be viewed as a working model of donor-acceptor system that can be utilized in photoinduced ET applications.