Charge-transfer excitons in DNA.

There have been a number of theoretical treatments of excitons in DNA, most neglecting both the intrachain and interchain wavefunction overlaps of the electron and hole, treating them as Frenkel excitons. Recently, the importance of the intrachain and interchain coupling has been highlighted. Experiments have shown that in (dA)n oligomers and in duplex (dA)n.(dT)n, to be abbreviated (A/T), where A is adenine and T is thymine, the exciton wavefunction is delocalized over several bases. In duplexes it is possible to have charge-transfer (CT) excitons. Theoretical calculations have suggested that CT excitons in DNA may have lower energy than single chain excitons. In all the calculations of excitons in DNA, the polarization of the surrounding water has been neglected. Calculations have shown, however, that polarization of the water by an excess electron or a hole in DNA lowers its energy by approximately 1/2 eV, causing it to become a polaron. It is therefore to be expected that polarization charge induced in the surrounding water has a significant effect on the properties of the exciton. In what follows, we present calculations of some properties CT excitons would have in an A/T duplex taking into account the wavefunction overlaps, the effect of the surrounding water, which results in the electron and hole becoming polarons, and the ions in the water. As expected, the CT exciton has lowest energy when the electron and hole polarons are directly opposite each other. By appropriate choice of the dielectric constant, we can obtain a CT exciton delocalized over the number of sites found in photoinduced absorption experiments. The absorption threshold that we then calculate for CT exciton creation in A/T is in reasonable agreement with the lowest singlet absorption deduced from available data.

Design of a privileging program for diagnostic imaging: costs and implications for a large insurer in Massachusetts.

PURPOSE: To evaluate the results of a privileging program aimed to ensure that health care providers and facilities adhere to standards of care. MATERIALS AND METHODS: Technical and professional privileging applications were mailed to more than 1,800 diagnostic imaging facilities and more than 6,000 professional providers of services to members of a health care plan. Site inspections were used to determine whether facilities met standards of care. Specialty providers were assigned a range of current procedural terminology codes for which they had privileges to bill the payer. RESULTS: Of 1,004 imaging sites inspected, 197 (20%) failed with the ability to remedy the violation, 106 (10%) failed with fundamental and serious deficiencies, and 701 (70%) passed. Site inspection pass rates varied substantially by specialty. Chiropractors and podiatrists were more likely to fail than medical and surgical specialists. A strong correlation was found between refusal to participate in the inspection and failure rate. Results suggest that reduction of the number of professional claims billed led to a 2% decline in total imaging expenditures, or a more than 10:1 return on the cost of implementing technical and professional privileging programs. CONCLUSION: Health care plans can positively influence costs and quality by developing, monitoring, and enforcing their own quality standards for diagnostic imaging.