PubMed · 4706285
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J M Rosenberg, N C Seeman, J J Kim, F L Suddath, H B Nicholas, A Rich. 1973-05-18. Double helix at atomic resolution.. https://doi.org/10.1038/243150a0
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Information transfer from activated heterotrimeric guanine nucleotide-binding proteins (G proteins) to downstream effectors occurs through noncovalent protein-protein interactions. Such interactions involve multiple regions of contact between the G protein and the effector. Some of these regions mediate information transfer, as defined by their ability to change the activity of their downstream binding partners, whereas other interactions appear to contribute solely to binding affinity. Such modular configurations occur in functionally diverse proteins such as myosin and a regulator of the double-stranded DNA stimulated protein kinase (PKR) called PACT. In most cases, it appears that both charge complementarity and the architecture of the interacting surfaces provide the appropriate balance between specificity of interactions and their reversibility. Information transfer regions appear to display conformational flexibility in interactions. Such flexible interactions may be essential for the local conformational changes necessary to induce change in activity by an induced fit-type mechanism. Thus, a general mechanism for information transfer by protein-protein interactions could use specific regions that induce conformation changes in the downstream partner. Other binding regions may be arranged within the protein to impart specificity of recognition and thereby maintain overall contact between the partners during the conformational dynamics that occur in the signal-transfer process.
BACKGROUND AND PURPOSE: To evaluate the combined effect of increased photon transmission, reduced photon scatter, increased secondary electron range and loss of electronic equilibrium for narrow 6-MV beams in and around a simulated air channel. MATERIALS AND METHODS: A measuring method was developed in-house for relative dose measurements near simulated air-like/soft-tissue interfaces in an automated water phantom. A Styrofoam cylinder (density 0.03 g/cm3) of 2-cm diameter was submersed in the water phantom and irradiated with small rectangular radiation fields. The field length was fixed at 10 cm and the field widths ranged from 1 to 4 cm. The axis of the foam cylinder and the long side of the field were parallel. A water layer of 2 cm was realised upstream of the cylinder. Relative depth dose and profiles behind the foam cavity were assessed using a diamond detector with a sensitive crystal thickness of 0.21 mm located at 1 mm from the top of the encapsulation. RESULTS: The dose at central axis 1.1 mm behind the cavity was found to be 92 and 74% for a field size of 10 x 2 and 10 x 1 cm2, respectively. The highly convex dose profile of the 10 x 1-cm2 field, characterising the homogeneous case, is flattened. CONCLUSIONS: The diamond detector is an excellent choice as a detector in small photon fields with high-dose gradients as they occur near air channels, provided the orientation of the detector is appropriate. Doses near air channels are subject to significant local variations as a function of small changes of field width, and local underdosing may occur in particular cases.
A linear diode array was interfaced to a conventional water scanner for the measurement of dose distribution of dynamically wedged treatment fields. The system was validated by comparing the dose distribution of an open field measured with the system and with a scanning ionization chamber. The system was also compared with a commercial ion chamber array scanner for the measurement of dose distributions of dynamic wedges. The dose distributions of enhanced dynamic wedge fields measured with the two dosimetry systems agree well beyond the depth of maximum dose. However, in the buildup region, the measurements made with the linear diode array system differ by more than 10% from that of the ion chamber array scanner.