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Disulfide bonds play a key role in stabilizing protein structures, with disruption strongly associated with loss of protein function and activity. Previous data have suggested that disulfides show only modest reactivity with oxidants. In the current study, we report kinetic data indicating that selected disulfides react extremely rapidly, with a variation of 104 in rate constants. Five-membered ring disulfides are particularly reactive compared with acyclic (linear) disulfides or six-membered rings. Particular disulfides in proteins also show enhanced reactivity. This variation occurs with multiple oxidants and is shown to arise from favorable electrostatic stabilization of the incipient positive charge on the sulfur reaction center by remote groups, or by the neighboring sulfur for conformations in which the orbitals are suitably aligned. Controlling these factors should allow the design of efficient scavengers and high-stability proteins. These data are consistent with selective oxidative damage to particular disulfides, including those in some proteins.

The macrophage scavenger receptor CD36 plays a key role in the initiation of atherosclerosis through its ability to bind to and internalize oxidized low-density lipoproteins (oxLDL). Prompted by recent findings that the CD36 receptor also recognizes amyloid fibrils formed by beta-amyloid and apolipoprotein C-II, we investigated whether the oxidation of low-density lipoproteins (LDL) generates characteristic amyloid-like structures and whether these structures serve as CD36 ligands. Our studies demonstrate that LDL oxidized by copper ions, 2,2-azobis(2-amidinopropane) dihydrochloride (AAPH), or ozone react with the diagnostic amyloid dyes thioflavin T and Congo Red and bind to serum amyloid P component (SAP), a universal constituent of physiological amyloid deposits. X-ray powder diffraction patterns for native LDL show a diffuse powder diffraction ring with maximum intensity corresponding to an atomic spacing of approximately 4.7 A, consistent with the spacing between beta-strands in a beta-sheet. Ozone treatment of LDL generates an additional diffuse powder diffraction ring with maximum intensity indicating a spacing of approximately 9.8 A. This distance is consistent with the presence of cross-beta-structure, a defining characteristic of amyloid. Evidence that these cross-beta-amyloid structures in oxLDL are recognized by macrophages is provided by the observation that SAP strongly inhibits the association and internalization of (125)I-labeled copper-oxidized LDL by peritoneal macrophages. The ability of SAP to bind to amyloid-like structures in oxLDL and prevent lipid uptake by macrophages highlights the potential importance of these structures and suggests an important preventative role for SAP in foam cell formation and early-stage atherosclerosis.

Free-radical chemistry has come a long way in a relatively short period of time. The synthetic practitioner takes for granted the wealth of mechanistic and rate constant data now available and can apply free-radical techniques to the synthesis of many different classes of target molecule with confidence. Despite this, there are still mechanistic anomalies that need to be addressed. This Account highlights recent work involving nucleophilic radicals with low-lying unoccupied orbitals, such as acyl, oxyacyl, silyl, stannyl, and germyl radicals. Through interesting singly occupied molecular orbital (SOMO)-pi* and highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) interactions during these reactions, the radicals involved are able to mask as electrophiles, providing high levels of regiocontrol and efficient methods for the synthesis of important heterocycles.

Alkyl (aryltelluro)formates are effective precursors of oxyacyl, methyl, and primary and secondary alkyl radicals. At room temperature, irradiation of a benzene solution of methyl (aryltelluro)formates 10−12, 2-methylpropyl (aryltelluro)formates 14 and 15, octyl (phenyltelluro)formate (17), cyclohexyl (aryltelluro)formates 19 and 20, 3β-cholestanyl (aryltelluro)formates 22 and 23, cholesteryl (phenyltelluro)formate (24) and benzyl (phenyltelluro)formate (27) with a 250-W low-pressure mercury lamp leads to the formation of oxyacyl radicals (34), which can be trapped by diphenyl diselenide to give the corresponding alkyl (phenylseleno)formates 13, 16, 18, 21, 24, 26, and 28 with excellent overall conversions. Thermolysis of these telluroformates at 160 °C in the dark leads to the formation of methyl and primary and secondary alkyl aryl tellurides 36−43 in excellent yields. Presumably, these transformations involve oxyacyl radicals, which undergo subsequent decarboxylation at the elevated temperature to afford alkyl radicals, which become involved in further radical chemistry. When 1-(benzylseleno)-5-hexyl (phenyltelluro)formate (44) was thermolysed under these conditions, 2-methylselenane (45) was observed as the sole selenium-containing product, demonstrating the synthetic utility of (aryltelluro)formates as alkyl radical precursors.

Ab initio molecular orbital calculations using the 6-311G**, cc-pVDZ and aug-cc-pVDZ basis sets, with (MP2, QCISD, CCSD(T)) and without (HF) the inclusion of electron correlation indicate that decarboxylation reactions of alkoxyacyl (alkoxycarbonyl) radicals are significantly exothermic. Transition states (16) for these decarboxylation reactions are calculated to have C TS -O TS separations in the range: 1.813-1.892 Å; these distances appear to be affected somewhat by steric compression. At the CCSD(T)/6-311G**//MP2/6-311G** level of theory, energy barriers of 75.9, 72.8, 67.0 and 60.3 kJ mol Ϫ1 are calculated for the decarboxylation reactions involving the methoxyacyl, ethoxyacyl, isopropoxyacyl and tert-butoxyacyl radicals (2) respectively, while the reverse reactions are calculated to require energies in excess of 130.9 kJ mol Ϫ1 . By comparison, the decarbonylation reaction of the acetyl radical (8) is predicted to be significantly endothermic; methyl radicals are calculated to prefer to add to carbon monoxide with an energy barrier of only 24.0 kJ mol Ϫ1 at the CCSD(T)/aug-cc-pVDZ//MP2/aug-cc-pVDZ level of theory, in good agreement with available experimental data.Similar calculations for reactions involving (methoxy)thioacyl, (methylthio)acyl and (methylthio)thioacyl radicals (12-14, R = Me) suggest that only (alkoxy)thioacyl radicals (12) provide synthetically useful β-fragmentation reactions, the remaining systems (13, 14) are unlikely to be useful as alkyl radical precursors in synthesis; the reverse reactions are calculated to be competitive with the β-fragmentation process in these cases.

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