In high-temperature superconductivity, the process that leads to the formation of Cooper pairs, the fundamental charge carriers in any superconductor, remains mysterious. We used a femtosecond laser pump pulse to perturb superconducting Bi(2)Sr(2)CaCu(2)O(8+δ) and studied subsequent dynamics using time- and angle-resolved photoemission and infrared reflectivity probes. Gap and quasiparticle population dynamics revealed marked dependencies on both excitation density and crystal momentum. Close to the d-wave nodes, the superconducting gap was sensitive to the pump intensity, and Cooper pairs recombined slowly. Far from the nodes, pumping affected the gap only weakly, and recombination processes were faster. These results demonstrate a new window into the dynamical processes that govern quasiparticle recombination and gap formation in cuprates.

We recently derived a CD4-independent virus from HIV-1͞IIIB, termed IIIBx, which interacts directly with the chemokine receptor CXCR4 to infect cells. To address the underlying mechanism, a cloned Env from the IIIBx swarm (8x) was used to produce soluble gp120. 8x gp120 bound directly to cells expressing only CXCR4, whereas binding of IIIB gp120 required soluble CD4. Using an optical biosensor, we found that CD4-induced (CD4i) epitopes recognized by mAbs 17b and 48d were more exposed on 8x than on IIIB gp120. The ability of 8x gp120 to bind directly to CXCR4 and to react with mAbs 17b and 48d in the absence of CD4 indicated that this gp120 exists in a partially triggered but stable state in which the conserved coreceptor-binding site in gp120, which overlaps with the 17b epitope, is exposed. Substitution of the 8x V3 loop with that from the R5 virus strain BaL resulted in an Env (8x-V3BaL) that mediated CD4-independent CCR5-dependent virus infection and a gp120 that bound to CCR5 in the absence of CD4. Thus, in a partially triggered Env protein, the V3 loop can change the specificity of coreceptor use but does not alter CD4 independence, indicating that these properties are dissociable. Finally, IIIBx was more sensitive to neutralization by HIVpositive human sera, a variety of anti-IIIB gp120 rabbit sera, and CD4i mAbs than was IIIB. The sensitivity of this virus to neutralization and the stable exposure of a highly conserved region of gp120 suggest new strategies for the development of antibodies and small molecule inhibitors to this functionally important domain.Binding of the HIV-1 envelope protein (Env) to CD4 induces structural alterations in the gp120 subunit that enable it to interact with an appropriate coreceptor (1-4). The major HIV-1 coreceptors are CCR5 and CXCR4, because all HIV-1 strains identified to date require one of these molecules for viral entry (5). The impressive resistance of individuals who lack CCR5 to virus infection demonstrates the importance of this receptor in vivo and indicates that viruses that use CCR5 (R5 virus strains) are largely responsible for transmission (6-9). Accrual of mutations in Env impart the ability to use other coreceptors, such as CXCR4. Sequences in the V3 loop of gp120 play a major role in governing coreceptor choice, though the V1͞V2 region also influences coreceptor use (10-15). The recently solved structure of a trimolecular complex consisting of a HIV-1 gp120 core fragment bound to soluble CD4 and the Fab derived from the CD4-induced (CD4i) mAb 17b has led to the identification of a highly conserved region in gp120 implicated in coreceptor binding (16)(17)(18). Because binding of antibodies to this region (including 17b) is markedly enhanced by CD4 binding as is the affinity of gp120 for CCR5 (1-3, 19, 20), it is likely that the coreceptorbinding site is at least partially sequestered in the native state. CD4 binding may lead to exposure and͞or formation of this region with subsequent coreceptor binding. Ultimately, conformational changes in Env th...