[In vivo dosimetry study of semi-conductors EPD-20 in total body irradiation technique]

PURPOSE

The objective of this work was the study of in vivo dosimetry performed in a series of 54 patients receiving total body irradiation (TBI) at the Salah-Azaiz Institute of Tunis since 2004. In vivo dosimetry measurements were compared to analytically calculated doses from monitor units delivered.

PATIENTS AND METHOD

The irradiation was conducted by a linear accelerator (Clinac 1800, Varian, Palo Alto, USA) using nominal X-rays energies of 6 MV and 18 MV, depending on the thickness of the patient at the abdomen. The dose was measured by semi-conductors p-type EPD-20. These diodes were calibrated in advance with an ionization chamber "PTW Farmer" type of 0.6cm(3) and were placed on the surface of plexiglas phantom in the same TBI conditions. A study of dosimetric characteristics of semi-conductors EPD-20 was carried out as a function of beam direction and temperature. Afterwards, we conducted a comparative analysis of doses measured using these detectors during irradiation to those calculated retrospectively from monitor units delivered to each patient conditioned by TBI.

RESULTS

Experience showed that semi-conductors are sensitive to the angle of beam radiation (0-90 degrees ) and the temperature (22-40 degrees C). The maximum variation is respectively 5 and 7%, but in our irradiation conditions these correction factors are less than 1%. The analysis of the results of the in vivo dosimetry had shown that the ratio of the average measured doses and analytically calculated doses at the abdomen, mediastina, right lung and head are 1.005, 1.007, 1.0135 and 1.008 with a standard deviation "type A" respectively of 3.04, 2.37, 7.09 et 4.15%.

CONCLUSION

In vivo dosimetry by semi-conductors is in perfect agreement with dosimetry by calculation. However, in vivo dosimetry using semiconductors is the only technique that can reflect the dose actually received instantly by the patient during TBI given the many factors that calculation can not take into account: patient and organs motions and the heterogeneity of the targets.

Minimal surgery for pilonidal disease using trephines: description of a new technique and long-term outcomes in 1,358 patients

PURPOSE

This study describes and reports the results of a new, minimally-invasive surgical technique for pilonidal disease.

METHODS

From March 1993 to January 2003, 1,358 patients (out of a total of 1,435 patients) with symptomatic pilonidal disease underwent treatment in a military surgical clinic dedicated for pilonidal disease. Patients were operated on under local anesthesia, utilizing trephines to excise pilonidal pits and to débride underlying cavities and tracts.

RESULTS

One thousand three hundred fifty-eight symptomatic patients participated in the study and were mostly male (84.3 percent) and the mean age 20.9 +/- 3.6 years. Rates of postoperative infection, secondary bleeding, and early failure were 1.5, 0.2, and 4.4 percent, respectively. In patients with full postoperative clinical attendance, complete healing was observed within 3.4 +/- 1.9 weeks. Phone interview included 1,165 patients (85.8 percent) with a mean follow-up interval of 6.9 +/- 1.8 years. Recurrence rates after 1 year was 6.5 percent, 5 years was 13.2 percent, and 10 years was 16.2 percent. Mean time to recurrence was 2.7 +/- 2.6 years postoperatively. The disease-free probability estimate was 93.5 percent at one year and 86.5 percent at 5 years.

CONCLUSIONS

Compared with frequently used pilonidal operations, the trephine technique is associated with a lower recurrence rate and a low postoperative morbidity rate.

Cost-effectiveness analysis of laparoscopic gastric bypass, adjustable gastric banding, and nonoperative weight loss interventions

BACKGROUND

Laparoscopic adjustable gastric banding (LAGB) and laparoscopic Roux-en-Y gastric bypass (LRYGB) are the two most commonly performed bariatric procedures. Although both procedures likely reduce healthcare expenditures related to the resolution of co-morbid conditions, they have different rates of perioperative risks and different rates of associated weight loss. We designed a model to evaluate the incremental cost-effectiveness of these procedures compared with nonoperative weight loss interventions and with each other.

METHODS

We used a deterministic, payer-perspective model comparing the lifetime expected costs and outcomes of LAGB, LRYGB, and nonoperative treatment. The major endpoints were survival, health-related quality of life, and weight loss. Life expectancy and lifetime medical costs were calculated across age, gender, and body mass index (BMI) strata using previously published data.

RESULTS

For both men and women, LRYGB and LAGB were cost-effective at <$25,000/quality-adjusted life-year (QALY) even when evaluating the full range of baseline BMI and estimates of adverse outcomes, weight loss, and costs. For base-case scenarios in men (age 35 y, BMI 40 kg/m(2)), the incremental cost-effectiveness was $11,604/QALY for LAGB compared with $18,543/QALY for LRYGB. For base-case scenarios in women (age 35 y, BMI 40 kg/m(2)), the incremental cost-effectiveness was $8878/QALY for LAGB compared with $14,680/QALY for LRYGB.

CONCLUSION

The modeled cost-effectiveness analysis showed that both operative interventions for morbid obesity, LAGB and RYGB, were cost-effective at <$25,000 and that LAGB was more cost-effective than RYGB for all base-case scenarios.

Early mortality among Medicare beneficiaries undergoing bariatric surgical procedures

CONTEXT

Case series demonstrate that bariatric surgery can be performed with a low rate of perioperative mortality (0.5%), but the rate among high-risk patients and the community at large is unknown.

OBJECTIVES

To evaluate the risk of early mortality among Medicare beneficiaries and to determine the relative risk of death among older patients.

DESIGN

Retrospective cohort study.

SETTING AND PATIENTS

All fee-for-service Medicare beneficiaries, 1997-2002.

MAIN OUTCOME MEASURES

Thirty-day, 90-day, and 1-year postsurgical all-cause mortality among patients undergoing bariatric procedures.

RESULTS

A total of 16 155 patients underwent bariatric procedures (mean age, 47.7 years [SD, 11.3 years]; 75.8% women). The rates of 30-day, 90-day, and 1-year mortality were 2.0%, 2.8%, and 4.6%, respectively. Men had higher rates of early death than women (3.7% vs 1.5%, 4.8% vs 2.1%, and 7.5% vs 3.7% at 30 days, 90 days, and 1 year, respectively; P<.001). Mortality rates were greater for those aged 65 years or older compared with younger patients (4.8% vs 1.7% at 30 days, 6.9% vs 2.3% at 90 days, and 11.1% vs 3.9% at 1 year; P<.001). After adjustment for sex and comorbidity index, the odds of death within 90 days were 5-fold greater for older Medicare beneficiaries (aged > or =75 years; n = 136) than for those aged 65 to 74 years (n = 1381; odds ratio, 5.0; 95% confidence interval, 3.1-8.0). The odds of death at 90 days were 1.6 times higher (95% confidence interval, 1.3-2.0) for patients of surgeons with less than the median surgical volume of bariatric procedures (among Medicare beneficiaries during the study period) after adjusting for age, sex, and comorbidity index.

CONCLUSIONS

Among Medicare beneficiaries, the risk of early death after bariatric surgery is considerably higher than previously suggested and associated with advancing age, male sex, and lower surgeon volume of bariatric procedures. Patients aged 65 years or older had a substantially higher risk of death within the early postoperative period than younger patients.

A systematic review of whether oral contrast is necessary for the computed tomography diagnosis of appendicitis in adults

BACKGROUND

There are several methods of contrast administration when performing computed tomography (CT) scanning for suspected appendicitis. In this systematic review we evaluated the diagnostic performance of CT with and without contrast material.

METHODS

Twenty-three reports were identified using a Medline search.

RESULTS

The aggregated diagnostic performance characteristics of all modes of CT scanning were excellent with a range of sensitivity (83--97%), specificity (93--98%), positive predictive value (86--98%), negative predictive value (94--99%), and accuracy (92--97%). The diagnostic performance of CT without oral contrast was similar (sensitivity, 95% vs. 92% [not statistically significant]; negative predictive value, 96% for both protocols) or surprisingly better (specificity, 97% vs. 94%; positive predictive value, 97% vs. 89%; accuracy, 96% vs. 92%; P<.0001) than with oral contrast.

CONCLUSIONS

Noncontrast CT techniques to diagnose appendicitis showed equivalent or better diagnostic performance compared with CT scanning with oral contrast. A prospective comparative trial of CT with and without oral contrast for appendicitis should be performed to assess the adequacy of this modality.

Temporal changes in the management of diverticulitis

PURPOSE

This study was designed to evaluate temporal trends in the use and type of operative and non-operative interventions in the management of diverticulitis.

METHODS

A retrospective cohort using a statewide administrative database was used to identify all patients hospitalized for diverticulitis in the state of Washington (1987-2001). Poisson and logistic regression were used to calculate changes in the frequency of hospitalization, operative and percutaneous interventions, and colostomy over time.

RESULTS

Of the 25,058 patients hospitalized non-electively with diverticulitis (mean age 69 +/- 16, 60% female) there were only minimal changes in the frequency of admissions over time (0.006% increase per year-IRR 1.00006 95% CI 1.00004, 1.00008). The odds of an emergency colectomy at initial hospitalization decreased by 2% each year (OR 0.98 95% CI 0.98, 0.99) whereas the odds of percutaneous abscess drainage increased 7% per year (OR 1.07 95% CI 1.05, 1.1). Among patients undergoing percutaneous drainage, the odds of operative interventions decreased by 9% compared to patients who did not have a percutaneous intervention (OR 0.91 95% CI 0.87, 0.94). The proportion of patients undergoing colostomy during emergency operations remained essentially stable over time (range 49-61%), as did the proportion of patients undergoing prophylactic colectomy after initial non-surgical management (approximately 10%).

CONCLUSIONS

There was a minimal increase in the frequency of diverticulitis admissions over time. A rise in percutaneous drainage procedures was associated with a decrease in emergency operative interventions. The proportion of patients undergoing colostomy remained stable, and there does not seem to be a significant increase in the use of one-stage procedures for diverticulitis.

Computed tomography diagnosis of ischemia and complete obstruction in small bowel obstruction: a systematic review

This review was designed to describe the diagnostic performance of computed tomography (CT) in assessing bowel ischemia and complete obstruction in small bowel obstruction (SBO). A MEDLINE search (1966-2004) identified 15 studies dealing with the CT diagnosis of ischemia and complete obstruction in SBO. Ischemia was defined by operative findings, and complete obstruction was defined by enteroclysis or operative findings. Aggregated sensitivity, specificity, and positive and negative predictive values (PPV and NPV) were calculated. Eleven of 15 studies reported on the CT diagnosis of ischemia in SBO based on 743 patients. The aggregated performance characteristics of CT for ischemia in SBO were sensitivity of 83% (range, 63-100%), specificity of 92% (range, 61-100%), PPV of 79% (range, 69-100%), and NPV of 93% (range, 33.3-100%). Seven of 15 studies evaluated the CT classification of complete obstruction based on 408 patients. The aggregated performance characteristics of CT for complete obstruction were sensitivity of 92% (range, 81-100%), specificity of 93% (range, 68-100%), PPV of 91% (range, 84-100%), and NPV of 93% (range, 76-100%). This review demonstrates the high sensitivity of CT for ischemia in the setting of SBO and suggests that a CT scan finding of partial SBO is likely to reflect a clinical condition that will resolve without surgical intervention.

Hartmann's colectomy and reversal in diverticulitis: a population-level assessment

PURPOSE

This study was designed to assess the costs and outcomes of colostomy and colostomy reversal in patients with diverticulitis and examine the impact of such procedures on the health care system.

METHODS

We employed a retrospective design and used a Washington State administrative database to identify patients undergoing operations with colostomy (1987-2002) who were followed over time. Descriptive and comparative analysis was performed, focusing on patients with diverticulitis.

RESULTS

There were 16,556 patients who underwent colostomy and 5,420 (32.7 percent) were for diverticulitis and its related complications (mean age, 64.8 +/- 15.1 years; 53.2 percent female). In patients with diverticulitis, the rate of colostomy reversal was 56.3 percent (80 percent in patients less than 50 years, and 30 percent in patients over 77 years). The in-hospital mortality rate after colostomy reversal was 0.36 percent, and was 2.6 percent in those over 77 years of age. After colostomy reversal a second stoma was used in 3.4 percent, reoperation was required for bleeding complications in 0.6 percent, and infectious complications were noted in 2 percent. The length of time from colostomy to its reversal was approximately five months (138.1 +/- 164 days; interquartile range, 72-156). The relationship between the length of time from colostomy to reversal was evaluated and the adjusted odds of a second stoma being used at the time of colostomy reversal were 45 percent higher (odds ratio, 1.45; 95 percent confidence interval, 1.22, 1.73) for each increase in time interval (<3, 6-9, 9-12, >12 months).

CONCLUSIONS

One-third of all colostomies were related to diverticulitis and only 56 percent were reversed. We identified a higher than expected mortality rate among older patients undergoing colostomy reversal. The impact of colostomy and reversal operations on both patients and the health care system is significant.

The timing of elective colectomy in diverticulitis: a decision analysis

BACKGROUND

Determining the optimal strategy for elective colectomy in patients with diverticular disease involves a balance of the morbidity, mortality, costs, and quality of life associated with both elective and expectant management. We used decision and cost analysis to simulate the clinical and economic outcomes after recovery from an episode of nonsurgically treated diverticulitis to determine the preferable management strategy.

STUDY DESIGN

A Markov model was constructed to evaluate lifetime risks of death and colostomy, care costs, and quality of life associated with elective colectomy after subsequent episodes of diverticulitis. The analysis was from the payer's perspective, using hypothetical cohorts of 35- and 50-year-old patients who recovered from a nonsurgically treated diverticulitis episode. Probabilities of clinical events and costs for the base-case analysis were derived from a large cohort using a statewide administrative database and published estimates.

RESULTS

Performing colectomy after the fourth rather than the second episode in patients older than 50 years resulted in 0.5% fewer deaths, 0.7% fewer colostomies, and saved US 1,035 dollars per patient. In younger patients, performing colectomy after the fourth episode compared with the first episode resulted in 0.1% fewer deaths, 2% fewer colostomies, and saved US 5,429 dollars per patient. Expectant management through three recurrent episodes with elective colectomy after the fourth episode was the dominant strategy across the full range of the variables tested in the sensitivity analysis compared with earlier intervention.

CONCLUSIONS

This study suggests that expectant management is associated with lower rates of death and colostomy and is cost-saving for both younger and older patients.

Primary anastomosis or Hartmann's procedure for patients with diverticular peritonitis? A systematic review

PURPOSE

This systematic literature review was designed to summarize and compare the reported outcomes of one-stage and two-stage operations for the treatment of perforated diverticulitis with peritonitis.

METHODS

This review identified 98 published studies (1957-2003) dealing with the surgical management of perforated diverticulitis with peritonitis, either with primary resection and anastomosis or with the Hartmann's procedure. Aggregated results of adverse outcomes were calculated but statistical comparisons were not appropriate because of data and design heterogeneity.

RESULTS

Operative mortality data from patients with diverticular peritonitis undergoing Hartmann's procedure (n = 1,051) were derived from 54 studies. Considering the Hartmann's procedure and its reversal procedures together, the mortality rate was 19.6 percent (18.8 percent for the Hartmann's procedure and 0.8 percent for its reversal), the wound infection rate was 29.1 percent (24.2 percent for the Hartmann's procedure and 4.9 percent for its reversal), and stoma complications and anastomotic leaks (in the reversal operation) occurred in 10.3 and 4.3 percent, respectively. Of 569 reported cases of primary anastomosis from 50 studies, the aggregated mortality rate was 9.9 (range, 0-75) percent with an anastomotic leak rate of 13.9 (range, 0-60) percent and a wound infection rate of 9.6 (range, 0-26) percent.

CONCLUSIONS

Reported mortality and morbidity in patients with diverticular peritonitis who underwent primary anastomosis were not higher than those in patients undergoing Hartmann's procedure were. This suggests that primary anastomosis is a safe operative alternative in certain patients with peritonitis. Despite inclusion of only patients with peritonitis in this analysis, selection bias may have been a limitation and a prospective, randomized trial is recommended.

Impact of Surgical Clipping on Survival in Unruptured and Ruptured Cerebral Aneurysms

Background and Purpose— The management of aneurysms is controversial because little is known about the impact of clipping on long-term outcome. This study was designed to evaluate long-term survival of patients with aneurysms undergoing clipping in a statewide population.

Methods— We used a retrospective design using an administrative database to identify patients hospitalized with aneurysms (1987 to 2001). Time-to-event analysis was used to determine the risk of death from all causes and from neurological causes.

Results— 4619 patients (mean age 54.7±15.3, 66.3% female) were hospitalized with cerebral aneurysms. Survival among patients with ruptures was significantly lower compared with patients with unruptured aneurysm ( P <0.001) with adjusted hazard ratio (HR) of death after clipping 40% higher (HR: 1.4; 95% CI: 1.2, 1.7) in patients with rupture compared with those that were unruptured. Survival estimates for unruptured patients who underwent clipping were significantly higher than among those unruptured patients who did not undergo clipping ( P <0.001), with adjusted HR of death 30% higher in patients with unruptured aneurysm that were not clipped compared with unruptured patients who were clipped (HR: 1.3; 95% CI: 1.1, 1.6). Patients with unruptured aneurysm who underwent clipping and survived beyond the 30-day postoperative period were less likely to die from neurologically related causes (5.6 versus 2.3%, P <0.001). Patients with ruptures and aneurysms who underwent clipping have a higher rate of death compared with the general population in the long-term.

Conclusions— Short-term and long-term mortality after clipping of cerebral aneurysms is higher than previously reported. Patients with unruptured aneurysms who undergo clipping have improved survival compared with those who do not undergo clipping. This study supports the use of early intervention in the management of patients with unruptured aneurysms.

Development of epidural abscess following surgical drainage of perianal abscess: report of a case

PURPOSE

A case of epidural abscess originating from a perianal abscess is reported.

METHODS

The history of the patient, erythrocyte sedimentation rate, magnetic resonance imaging, and bacteriological tests were used to reach a diagnosis and the possible mechanism.

RESULTS

Epidural abscess was suspected because the patient had a fever and intense low back pain following drainage of a perianal abscess. Magnetic resonance imaging was used to correctly diagnose the epidural abscess and bacteriologic studies disclosed the pathophysiologic mechanism.

CONCLUSIONS

Epidural abscess is an extremely rare complication of perianal abscess. It should always be suspected in a patient with acute onset of back pain, fever, history of recent infection, and an elevated erythrocyte sedimentation rate, because delay in diagnosis can cause neurologic compromise and even death.

Acute abdominal pain. Four classifications can guide assessment and management

Abdominal pain is a common occurrence in older persons and a frequent catalyst for office and emergency room visits. Complaints must be investigated thoroughly because they often indicate serious underlying pathology such as Infection, mechanical obstruction, malignancy, biliary disease, cardiac problems, and GI ischemia. One means of overcoming a sprawling differential diagnosis is to determine whether the problem falls into one of four general categories: peritonitis, bowel obstruction, vascular catastrophe, or nonspecific abdominal pain. A comprehensive history, careful physical examination, and use of abdominal imaging studies facilitate effective assessment. As atypical presentations are frequently encountered in older persons, liberal use of ultrasound and contrast CT and early surgical consultation are recommended.

Coordination of the initiation of recombination and the reductional division in meiosis in Saccharomyces cerevisiae

Early exchange (EE) genes are required for the initiation of meiotic recombination in Saccharomyces cerevisiae. Cells with mutations in several EE genes undergo an earlier reductional division (MI), which suggests that the initiation of meiotic recombination is involved in determining proper timing of the division. The different effects of null mutations on the timing of reductional division allow EE genes to be assorted into three classes: mutations in RAD50 or REC102 that confer a very early reductional division; mutations in REC104 or REC114 that confer a division earlier than that of wild-type (WT) cells, but later than that of mutants of the first class; and mutations in MEI4 that do not significantly alter the timing of MI. The very early mutations are epistatic to mutations in the other two classes. We propose a model that accounts for the epistatic relationships and the communication between recombination initiation and the first division. Data in this article indicate that double-strand breaks (DSBs) are not the signal for the normal delay of reductional division; these experiments also confirm that MEI4 is required for the formation of meiotic DSBs. Finally, if a DSB is provided by the HO endonuclease, recombination can occur in the absence of MEI4 and REC104.

Suppressor analysis of the Saccharomyces cerevisiae gene REC104 reveals a genetic interaction with REC102

REC104 is a gene required for the initiation of meiotic recombination in Saccharomyces cerevisiae. To better understand the role of REC104 in meiosis, we used an in vitro mutagenesis technique to create a set of temperature-conditional mutations in REC104 and used one ts allele (rec104-8) in a screen for high-copy suppressors. An increased dosage of the early exchange gene REC102 was found to suppress the conditional recombinational reduction in rec104-8 as well as in several other conditional rec104 alleles. However, no suppression was observed for a null allele of REC104, indicating that the suppression by REC102 is not "bypass" suppression. Overexpression of the early meiotic genes REC114, RAD50, HOP1, and RED1 fails to suppress any of the rec104 conditional alleles, indicating that the suppression might be specific to REC102.

The effects of nanoliter ejections of lidocaine into the pontomedullary reticular formation on cortically induced rhythmical jaw movements in the guinea pig

In the ketamine/urethane anesthetized guinea pig, electromyographic (EMG) responses of the anterior digastric muscle were studied when loci within the lower brainstem were microejected with lidocaine (2%) during rhythmical jaw movements (RJMs) evoked by repetitive electrical stimulation of the masticatory area of the cortex. The area investigated was between the trigeminal motor nucleus (Mot V) and the rostral pole of the inferior olive. Microejections of lidocaine, contralateral to the cortical stimulus site, into the ventral-medial portion of Mot V where digastric motoneurons are known to be located, resulted in reduction or complete abolishment of the digastric EMG activity ipsilateral to the ejection with no effective change in mean cycle duration (CD) or mean percent normalized integrated amplitude of the contralateral digastric EMG. Microejections of lidocaine, contralateral to the cortical stimulus site, into the ponto-medullary reticular formation in areas that included portions of the caudal nucleus pontis caudalis (PnC), nucleus gigantocellularis (GC), medial nucleus parvocellularis (PCRt), and dorsal paragigantocellularis (dPGC), in most cases produced a bilateral reduction in the mean normalized integrated amplitude and a bilateral increase in the mean cycle duration. In these sites, the bilateral increase in mean cycle duration of digastric EMG bursts was also associated with a significant increase of coefficient of variation in CD. In many cases, microejection of lidocaine completely abolished rhythmical digastric activity, bilaterally. HRP injections into Mot V were performed to determine the locations of trigeminal premotoneurons and their relationship to effective lidocaine sites for rhythmical jaw movement suppression. Retrogradely labeled cells were found mainly in the mesencephalic nucleus of V; trigeminal principal and spinal V sensory nuclei, bilaterally; and within the intermediate and lateral regions of reticular formation, bilaterally. No labeling was found in the medial reticular formation, including the nucleus gigantocellularis and dorsal paragigantocellularis.(ABSTRACT TRUNCATED AT 400 WORDS)

The influence of prior wakefulness on REM sleep

Data from studies of naps and of shifted sleep were used to determine the relationship between two measures of rapid eye movement (REM) sleep (percentage of REM in the first 2 hr of sleep and REM latency) and prior wakefulness. For each sample, we calculated the difference between the observed value and that predicted by a cosine function that estimated the circadian rhythm of REM sleep propensity. The difference values were found to correlate reliably with hours and log hours of prior wakefulness. We conclude that while REM sleep is regulated in part by an endogenous circadian oscillator, it is also influenced by the duration of prior wakefulness.

Slow-wave sleep in daytime and nocturnal sleep: an estimate of the time course of "Process S"

Daan et al. (1984) have proposed that sleep and wakefulness are regulated, in part, by a "Process S" that increases during wakefulness and declines during sleep. Data derived from several studies were taken to determine the time course of Process S during both wakefulness and sleep. As required by the model, slow-wave-sleep (SWS; an index of Process S) was found to increase exponentially as a function of prior wake time (equation 1) and to decline exponentially as a function of time asleep (equation 2). The equations accounted for 91% and 96% of the variance, respectively. In addition, equation 1 accurately predicted the amount the amount of SWS in the first hour of nocturnal sleep.

Theory of photochemical reactions

Although the great number of electronic states available to an excited molecule might seem to preclude a coherent picture of photochemical reaction mechanisms, it is possible to bring out some basic features common to a great many reactions. The electronic states of the primary diradical intermediates, surface crossings, topicity, and avoided surface crossings have been shown to be essential components of the electronic theory of photochemical reactions. Diradicals have four important electronic states. Knowing these states, and making a simple electron count, it is possible to draw state correlation diagrams. Some diagrams show a typical surface crossing of the ground singlet state with the lowest (singlet, triplet) pair of excited states, with clear-cut consequences of quantum yields under various conditions. In other reactions the surfaces stay apart. The critical discriminating feature that determines the type of correlation diagram is the topicity. Photochemical reactions can be classified according to topicity, which is useful in interpreting their mechanisms (53). Avoided surface crossings can also be classified into different types. Figure 7, which illustrates the interplay of a covalent and an ionic surface responsible for photochemical electron transfer, is a typical multidimensional representation of a photochemical reaction. The chemical behavior of the excited zwitterionic states of common intermediates, such as twisted ethylene or diallyl, reflects the quantum mechanical nature of photochemical processes. In these states, for perfectly symmetric systems, charge oscillates back and forth between two symmetry-equivalent sites. Slight geometric perturbations can create a sudden polarization of the excited molecule, with localization of almost a full charge at one end of the molecule. A photon is transformed into an electrical signal thanks to an appropriate molecular distortion. Nature may have used this simple process in the N-retinylidene visual chromophore to trigger an electrical response to vision.

The Energy of Substituted Ethanes. Asymmetry Orbitals

The leading terms in the energy of a general substituted ethane are derived in explicit form as a function of the torsional angle θ , the substituent electronegativities, and their mutual overlaps. The energy is found to be the sum of all four overlaps between pairs of asymmetry orbitals, and satisfies the requisite symmetry properties.

Potential Energy- Sheets for the nπ* and ππ* Triplet States of Acrolein

The energy of the nπ* and ππ* triplet states of acrolein as a function of the twist of the CH2 terminal groups and relaxation of the three skeletal σ bonds has been investigated in an "ab initio" self-consistent field formalism. The potential energy sheets of these triplets show that the cis photoadducts of cyclic enones may arise from "planar, relaxed" triplet states whereas the trans photoadducts are given by "twisted, unrelaxed" triplet states.

THE ROLE OF LONG-RANGE FORCES IN THE COHESION OF LIPOPROTEINS

The various long-range forces which are effective between molecules in their electronic ground states are examined. Orders of magnitude are given for those forces which should occur in the interaction of lipide and protein chains. It is found that electrostatic forces should be responsible for bringing and holding together protein and lipide components, but London – Van der Waals dispersion forces are probably of paramount importance in maintaining the lipide chains together in micelles or double layers.Special attention is drawn to the dispersion forces and to the conditions under which these forces are locally additive; one can calculate accurate values of the dispersion energy of interaction between saturated hydrocarbon chains at short distances (a few angstroms apart) by adding all the bond–bond interactions. A general expression is given for the dispersion energy between two parallel and opposed chains built out of identical units, and numerical values are given for the case of closely packed hydrocarbon chains.The total attraction energy is extremely sensitive to the intermolecular distance. The role of this "distance-specificity" in interactions involving unsaturated fatty acid chains and its contribution to the stability of lipoproteins is briefly examined.