1,4-Naphthoquinone (NQ) and its monosubstituted derivatives, 2-methyl, 2-ethyl, and 2-butyl-1,4-naphthoquinone, have been incorporated into the A1 binding site of photosystem I (PS I) after organic solvent extraction of the native phylloquinone. The charge separated state P700 +NQ- has been studied by multifrequency transient EPR. The Q-band (35 GHz) spectra of fully deuterated 2-methyl-1,4-NQ-d 8 and 2-ethyl-1,4-NQ-d 10 show sufficient spectral resolution for the orientation of the quinone g tensor and thus the headgroup of the molecule to be determined. All orientation parameters of the substituted NQs are found to be the same as those established for native phylloquinone in PS I. However, for 2-ethyl-1,4-NQ and 2-butyl-1,4-NQ the X- and Q-band spectra exhibit a well resolved 1:2:1 hyperfine splitting. From the fact that it is absent when the first methylene group of the side chain is selectively deuterated, the splitting is assigned to the hyperfine coupling of the methylene protons. The principal values of the axially symmetric hyperfine coupling tensor are determined to be |A xx | = 12.2 MHz, |A yy | = 16.8 MHz, |A zz | = 12.2 MHz, and a iso = 13.7 MHz. The large methylene proton hyperfine coupling arises from a high spin density on the ring carbon atom to which the alkyl tail is attached. This in turn suggests that only one of the carbonyl groups of 2-alkyl-1,4-NQ is H-bonded to the protein and that the alkyl tail must be in the ortho position relative to the carbonyl group with the H bond. This implies that the alkyl side chain of the substituted NQs resides in the space normally occupied by the methyl group of phylloquinone and not that of the phytyl tail, which is meta to the H-bonded carbonyl group according to the X-ray structure. In addition, the hyperfine tensor indicates that the first two C−C bonds of the alkyl tail must be coplanar with the aromatic ring. However, the X-ray structure of PS I shows, for the native phylloquinone, that the phytyl tail is bent out of the quinone plane with the second C−C bond.

Gas–surface interactions are some of the most important yet complex chemical processes to occur, as they intrinsically involve many-body phenomena across a wide spectrum of energies and length scales. To understand these complicated interfacial interactions, we often use model systems such as thiolate self-assembled monolayers (SAMs) to study phenomena like reactivity and passivation, as these systems afford fine control over the surface parameters governing the events in question. In this study, we examine the effect of chain length on the reactivity of alkanethiolate SAMs with atomic hydrogen by monitoring morphological surface evolution throughout the reaction. These spatiotemporal data were obtained using ultrahigh vacuum scanning tunneling microscopy (UHV-STM) with directed in situ atomic hydrogen dosing. For a series of alkanethiolate SAMs 8- to 11-carbons long, we find that small increases in chain length cause disproportionately large decreases in reactivity. These reaction trends led us to develop a kinetic model characterized by two rate constants: a slow rate for hydrogen reactivity with close-packed domains, which is chain-length dependent, and a fast rate for reactivity with low-density regions, which is the same for all samples examined. In addition to reaction rates, we also tracked chain-length-dependent changes in surface morphology, notably how the size and shape of the SAMs’ etch pits evolved following hydrogen exposure. Few differences were observed in the 10C and 11C samples, while there was a significant increase in the mean etch pit area of the 8C and 9C SAMs. Overall, this study provides important quantitative insights into how surface packing and dynamic disorder of organic thin films can influence their passivation capabilities.

In photosystem I (PS I), phylloquinone (PhQ) acts as a low potential electron acceptor during light-induced electron transfer (ET). The origin of the very low midpoint potential of the quinone is investigated by introducing anthraquinone (AQ) into PS I in the presence and absence of the iron-sulfur clusters. Solvent extraction and reincubation is used to obtain PS I particles containing AQ and the iron-sulfur clusters, whereas incubation of the menB rubA double mutant yields PS I with AQ in the PhQ site but no iron-sulfur clusters. Transient electron paramagnetic resonance spectroscopy is used to investigate the orientation of AQ in the binding site and the ET kinetics. The low temperature spectra suggest that the orientation of AQ in all samples is the same as that of PhQ in native PS I. In PS I containing the iron sulfur clusters, (i) the rate of forward electron transfer from the AQ ⅐ ؊ to F X is found to be faster than from PhQ ⅐ ؊ to F X , and (ii) the spin polarization patterns provide indirect evidence that the preceding ET step from A 0 ⅐؊ to quinone is slower than in the native system. The changes in the kinetics are in accordance with the more negative reduction midpoint potential of AQ. Moreover, a comparison of the spectra in the presence and absence of the iron-sulfur clusters suggests that the midpoint potential of AQ is more negative in the presence of F X . The electron transfer from the AQ ؊ to F X is found to be thermally activated with a lower apparent activation energy than for PhQ in native PS I. The spin polarization patterns show that the triplet character in the initial state of P 700 ⅐؉ AQ ⅐ ؊ increases with temperature. This behavior is rationalized in terms of a model involving a distribution of lifetimes/redox potentials for A 0 and related competition between charge recombination and forward electron transfer from the radical pair P 700 ⅐؉ A 0 ⅐؊ .In oxygenic photosynthesis, photosystem I (PS I) 1 and photosystem II (PS II) act in tandem to oxidize water and to reduce NADP ϩ to NADPH on the luminal and stromal sides of the thylakoid membrane, respectively. Both photosystems use light to drive the transfer of an electron from a chlorophyll dimer as donor on the luminal side of the complex via an intermediate acceptor to a quinone on the stromal side. In PS I, the chlorophyll dimer is referred to as P 700 , the intermediate is a chlorophyll monomer, called A 0 , and the quinone, A 1 is phylloquinone (PhQ). In PS II, the chlorophyll dimer is P 680 , the intermediate is pheophytin, and the quinone, Q A is plastoquinone-9. The x-ray structures of the two complexes (1-4) show that the structural arrangement of these co-factors is very similar with two nearly symmetric branches of acceptors extending across the membrane from the chlorophyll dimer. The structures also reveal an accessory chlorophyll monomer located between P 680 and pheophytin in PS II and between P 700 and A 0 in PS I. The function of these accessory chlorophylls is uncertain at present, but it is likely that they are invol...

This study examines the impact of surface temperature on alkanethiolate self-assembled monolayer (SAM) reactivity with atomic hydrogen (H) as well as how the combined effects of temperature and alkanethiol chain length alter the reaction outcome. This is achieved using ultrahigh vacuum scanning tunneling microscopy (UHV-STM) to monitor the spatiotemporal evolution of the monolayer throughout the reaction. We find that with decreasing temperature, the reaction rate of alkanethiol SAMs with atomic H decreases monotonically. Furthermore, the kinetic profile of the low-temperature reaction differs from that at room temperature, indicating structural and dynamical fluctuations within the monolayer that influence reactivity. Chain length is also seen to significantly affect reactivity at reduced substrate temperature, with longer alkanethiols reacting more slowly than shorter ones. Finally, we observe a unique surface rearrangement of the SAM upon exposure to atomic H, including changes in the organization of close-packed thiol domains and the evolution of gold adatom islands not observed at elevated temperatures. Overall, this work provides both quantitative and nanoscopic insight into how substrate temperature influences the structural dynamics of thiolate monolayers and how these fluctuations influence chemical reactivity.