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T M Cotton

Publications and source records attributed to T M Cotton.

29 records · Page 2Linked to original sources
PubMed1988

Surface-enhanced resonance Raman scattering spectroscopy of bacterial photosynthetic membranes. The carotenoid of Rhodospirillum rubrum.

Resonance Raman scattering by the carotenoid, spirilloxanthin (Spx), in a suspension of chromatophores (cytoplasmic side out) isolated from the photosynthetic bacterium, Rhodospirillum rubrum, is greatly enhanced when the membranes are adsorbed onto the surface of an anodized Ag electrode. The phenomenon is the basis for surface-enhanced resonance Raman scattering (SERRS) spectroscopy. The Spx SERRS peaks observed were at 1505-1510, 1150-1155, and 1000-1005 cm-1 with laser excitation wavelengths ranging between 457.9 and 568.2 nm. Similar peaks were not observed with spheroplasts (periplasmic side out) isolated from the same species. The difference in signal detected in chromatophores and spheroplasts is not due to differences in membrane surface charge, presence of residual cell wall on the spheroplast surface, lack of adhesion of spheroplasts to metals, or large differences in pigment content per unit membrane area. Instead, the results indicate an asymmetric distribution of Spx in vivo across the membrane (i.e., it is located on the cytoplasmic side of the membrane). The results also demonstrate that the SERRS effect is extremely distance sensitive, and the thickness of a single bacterial membrane (separating the Ag electrode from the carotenoid) is sufficient to prevent detection of Spx spectra. Studies of chromatophores from the F24 strain (a reaction centerless mutant) have pin-pointed B880 antenna complex as the source of the Spx SERRS spectra, and a schematic model of the minimal structural unit of B880 is presented. This work demonstrates the potential of the SERRS technique as a probe for surface topology of pigmented membranes.

Carotenoids↗
PubMed1985

A comparative study of the resonance Raman spectra of bacterial cytochromes.

Resonance Raman spectra were obtained for two newly isolated bacterial cytochromes, Alcaligenes faecalis (ATCC 8750) c554 and Alcaligenes faecalis c556. Their spectra were compared with those of mammalian cytochrome c and two other bacterial cytochromes, Paracoccus denitrificans c550 and Pseudomonas aeruginosa c551. The positions of the Raman bands indicated that, while Al. c554 and Al. c556 were c-type cytochromes with two thioether linkages, several common features found in their Raman spectra were anomalous. These features suggest that the electronic charge density of the porphyrin macrocycle of Al. c554 and Al. c556 is more asymmetric than that of other bacterial and mammalian c-type cytochromes. The Raman evidence indicates that the electronic properties of the heme are controlled by the protein in these two Alcaligenes cytochromes.

Alcaligenes↗
PubMed1976

New proposal for structure of special-pair chlorophyll.

A new model is proposed for the structure of the special pair of chlorophyll a molecules believed to correspond to the P700 species in green plants and algae. The proposed model, although admittedly speculative, is based upon exciton-theoretical considerations and on in vitro infrared and visible absorption spectra of a 700 nm absorbing ethanol adduct of chlorophyll a. In the new model, two chlorophyll a molecules are held together by (a) two strong ring V keto C[unk]O[unk]H[unk]O (or keto C[unk]O[unk]H[unk]N, or keto C[unk]O[unk]H[unk]S) hydrogen bonds and by (b) pi-pi van der Waals stacking interactions between the two chlorophyll a macrocycles. Macrocycle stacking provides the intermolecular pi-pi overlap necessary to promote the (experimentally observed) delocalization of the unpaired electron of the in vivo radical over two chlorophyll pi systems. The new model provides an explicit role for the participation of protein in the formation of the chlorophyll special pair. The in vitro system yields an electron spin resonance signal indistinguishable from that of oxidized P700 in Chlorella vulgaris.

Journal Article↗
PubMedDate not supplied

Metalloprotein complexes for the study of electron-transfer reactions. Characterization of diprotein complexes obtained by covalent cross-linking of cytochrome c and plastocyanin with a carbodiimide.

Cytochrome c (cyt) and zinc cytochrome c (Zncyt) are separately cross-linked to plastocyanin (pc) by the carbodiimide EDC according to a published method. The changes in the protein reduction potentials indicate the presence of approximately two amide cross-links. Chromatography of the diprotein complexes cyt/pc and Zncyt/pc on CM-52 resin yields multiple fractions, whose numbers depend on the eluent. UV-vis, EPR, CD, MCD, resonance Raman, and surface-enhanced resonance Raman spectra show that cross-linking does not significantly perturb the heme and blue copper active sites. Degrees of heme exposure show that plastocyanin covers most of the accessible heme edge in cytochrome c. Impossibility of cross-linking cytochrome c to a plastocyanin derivative whose acidic patch had been blocked by chemical modification shows that it is the acidic patch that abuts the heme edge in the covalent complex. The chromatographic fractions of the covalent diprotein complex are structurally similar to one another and to the electrostatic diprotein complex. Isoelectric points show that the fractions differ from one another in the number and distribution of N-acylurea groups, byproducts of the reaction with the carbodiimide. Cytochrome c and plastocyanin are also tethered to each other via lysine residues by N-hydroxysuccinimide diesters. Tethers, unlike direct amide bonds, allow mobility of the cross-linked molecules. Laser-flash-photolysis experiments show that, nonetheless, the intracomplex electron-transfer reaction cyt(II)/pc(II)----cyt(III)/pc(I) is undetectable in complexes of either type. Only the electrostatic diprotein complex, in which protein rearrangement from the docking configuration to the reactive configuration is unrestricted, undergoes this intracomplex reaction at a measurable rate.

Amides↗