Envelope structure of deeply embedded young stellar objects in the Serpens Molecular Cloud

Aperture synthesis and single-dish (sub) millimeter molecular lines and continuum observations reveal in great detail the envelope structure of deeply embedded young stellar objects (SMM1, SMM2, SMM3, SMM4) in the densely star-forming Serpens Molecular Cloud. Resolved millimeter continuum emission constrains the density structure to a radial power law with index -2.0 +/- 0.5, and envelope masses of 8.7, 3.0, and 5.3 M_sol for SMM1, SMM3, and SMM4. The core SMM2 does not seem to have a central condensation and may not have formed a star yet. The molecular line observations can be described by the same envelope model, if an additional, small amount of warm (100 K) material is included. This probably corresponds to the inner few hundred AU of the envelope were the temperature is high. In the interferometer beam, the molecular lines reveal the inner regions of the envelopes, as well as interaction of the outflow with the surrounding envelope. Bright HCO+ and HCN emission outlines the cavities, while SiO and SO trace the direct impact of the outflow on ambient gas. Taken together, these observations provide a first comprehensive view of the physical and chemical structure of the envelopes of deeply embedded young stellar objects in a clustered environment on scales between 1000 and 10,000 AU.Comment: 46 pages, incl. 12 postscript figures, uses ApJ latex and psfig macro

Inter-pigment interactions in the peridinin chlorophyll protein studied by global and target analysis of time resolved absorption spectra

Inter-pigment interactions define the functioning of light-harvesting protein complexes. To describe the particularly complex molecular dynamics and interactions of peridinin and chlorophyll in the peridinin chlorophyll protein of Amphidinium carterae, we applied global and target analysis to a series of ultrafast transient absorption experiments. We have created and validated a model that consistently describes and characterizes the interactions and evolution of excited and ground-state populations after excitation in all different experiments. The series of energy transfer steps that follow excitation are described by our model of cascading populations and numerous rate constants that correspond to intra-molecular thermal relaxation, fast and slow peridinin-to-chlorophyll energy transfer steps, and chlorophyll excited-state annihilation. By analyzing the spectral response of ground-state peridinins to excited chlorophylls we have identified which specific peridinin molecule is most closely coupled to the chlorophylls. No evidence was found that the intra-molecular charge transfer (ICT) state of peridinin, identified in studies of peridinin in solution, is a separate entity in the protein. The peridinin that exhibited slow peridinin-to-chlorophyll energy transfer was characterized by a difference spectrum free from ICT features, consistent with the importance of coupled ICT and S1 states for energy transfer.9 page(s

Low-Intensity Pump-Probe Measurements on the B800 Band of Rhodospirillum molischianum

We have measured low-intensity, polarized one-color pump-probe traces in the B800 band of the light-harvesting complex LH2 of Rhodospirillum molischianum at 77 K. The excitation/detection wavelength was tuned through the B800 band. A single-wavelength and a global target analysis of the data were performed with a model that accounts for excitation energy transfer among the B800 molecules and from B800 to B850. By including the anisotropy of the signals into the fitting procedure, both transfer processes could be separated. It was estimated in the global target analysis that the intra-B800 energy transfer, i.e., the hopping of the excitation from one B800 to another B800 molecule, takes ∼0.5 ps at 77 K. This transfer time increases with the excitation/detection wavelength from 0.3 ps on the blue side of the B800 band to ∼0.8 ps on the red side. The residual B800 anisotropy shows a wavelength dependence as expected for energy transfer within an inhomogeneously broadened cluster of weakly coupled pigments. In the global target analysis, the transfer time from B800 to B850 was determined to be ∼1.7 ps at 77 K. In the single-wavelength analysis, a speeding-up of the B800 → B850 energy transfer rate toward the blue edge of the B800 band was found. This nicely correlates with the proposed position of the suggested high-exciton component of the B850 band acting as an additional decay channel for B800 excitations

Energy Transfer in Light-Harvesting Complexes LHCII and CP29 of Spinach Studied with Three Pulse Echo Peak Shift and Transient Grating

Three pulse echo peak shift and transient grating (TG) measurements on the plant light-harvesting complexes LHCII and CP29 are reported. The LHCII complex is by far the most abundant light-harvesting complex in higher plants and fulfills several important physiological functions such as light-harvesting and photoprotection. Our study is focused on the light-harvesting function of LHCII and the very similar CP29 complex and reveals hitherto unresolved excitation energy transfer processes. All measurements were performed at room temperature using detergent isolated complexes from spinach leaves. Both complexes were excited in their Chl b band at 650 nm and in the blue shoulder of the Chl a band at 670 nm. Exponential fits to the TG and three pulse echo peak shift decay curves were used to estimate the timescales of the observed energy transfer processes. At 650 nm, the TG decay can be described with time constants of 130 fs and 2.2 ps for CP29, and 300 fs and 2.8 ps for LHCII. At 670 nm, the TG shows decay components of 230 fs and 6 ps for LHCII, and 300 fs and 5 ps for CP29. These time constants correspond to well-known energy transfer processes, from Chl b to Chl a for the 650 nm TG and from blue (670 nm) Chl a to red (680 nm) Chl a for the 670 nm TG. The peak shift decay times are entirely different. At 650 nm we find times of 150 fs and 0.5–1 ps for LHCII, and 360 fs and 3 ps for CP29, which we can associate mainly with Chl b ↔ Chl b energy transfer. At 670 nm we find times of 140 fs and 3 ps for LHCII, and 3 ps for CP29, which we can associate with fast (only in LHCII) and slow transfer between relatively blue Chls a or Chl a states. From the occurrence of both fast Chl b ↔ Chl b and fast Chl b → Chl a transfer in CP29, we conclude that at least two mixed binding sites are present in this complex. A detailed comparison of our observed rates with exciton calculations on both CP29 and LHCII provides us with more insight in the location of these mixed sites. Most importantly, for CP29, we find that a Chl b pair must be present in some, but not all, complexes, on sites A(3) and B(3). For LHCII, the observed rates can best be understood if the same pair, A(3) and B(3), is involved in both fast Chl b ↔ Chl b and fast Chl a ↔ Chl a transfer. Hence, it is likely that mixed sites also occur in the native LHCII complex. Such flexibility in chlorophyll binding would agree with the general flexibility in aggregation form and xanthophyll binding of the LHCII complex and could be of use for optimizing the role of LHCII under specific circumstances, for example under high-light conditions. Our study is the first to provide spectroscopic evidence for mixed binding sites, as well as the first to show their existence in native complexes