Virus replication and competition in a cell culture: Application to the SARS-CoV-2 variants.

Viral replication in a cell culture is described by a delay reaction-diffusion system. It is shown that infection spreads in cell culture as a reaction-diffusion wave, for which the speed of propagation and viral load can be determined both analytically and numerically. Competition of two virus variants in the same cell culture is studied, and it is shown that the variant with larger individual wave speed out-competes another one, and eliminates it. This approach is applied to the Delta and Omicron variants of the SARS-CoV-2 infection in the cultures of human epithelial and lung cells, allowing characterization of infectivity and virulence of each variant, and their comparison.

[Surgical revascularization of the brain in patients with acute ischaemic stroke]. / Khirurgicheskaia revaskuliarizatsiia golovnogo mozga u patsientov s ostrym ishemicheskim insul'tom.

AIM: The study was aimed at developing a surgical policy for patients presenting with acute ischaemic stroke induced by lesions of the intra- and extracranial arteries. PATIENTS AND METHODS: The patients were enrolled into the study resulting from the current practice of the Department of Emergency Neurosurgery of the Research Institute of Emergency Medicine named after N.V. Sklifosovsky. Sampling of patients was carried out from 1st January, 2014 to 10th October, 2017, eventually comprising all those (n=160) operated on for verified pathology of brachiocephalic arteries and acute impairment of cerebral circulation. RESULTS: During the examination, 101 (63.1%) patients were found to have unilateral or bilateral stenosis of the internal carotid artery, 22 (10.7%) patients were diagnosed with occlusion of the internal carotid artery, and 36 (17.6%) had acute thrombosis of the internal carotid artery. All this was the cause of acute impairment of cerebral circulation. Also, 1 patient was found to have occlusion of the 1st segment of the vertebral artery. Depending on the type of the lesion to the internal carotid artery, the patients were subdivided into 3 groups: 1) patients with internal carotid artery stenosis who received carotid endarterectomy; 2) patients with occlusion of the internal carotid artery, subjected to creation of an extra-intracranial microanastomosis, and 3) those with thrombosis of the internal carotid artery, who depending on the degree of occlusion of the internal carotid artery and patency of the intracranial arteries underwent thrombintimectomy or an extra-intracranial microanastomosis. The indications for and contraindications to interventions were defined based on the existing symptomatic pathology of brachiocephalic arteries, the terms of acute impairment of cerebral circulation, perfusion of the brain, the risk for the development of malignant stroke and cerebral oedema, possible haemorrhagic transformation of the focus of ischaemia, the rehabilitational potential of the patient and the analysis of the current literature. The results of our work demonstrated that surgical treatment of patients in the cute period of ischaemic stroke makes it possible to improve the neurological outcomes in patients as compared with the preoperative status and is not accompanied by a high risk of surgical complications (with the haemorrhagic complication rate amounting to 0.6%). CONCLUSION: Surgical revascularization of the brain in the acute period of ischaemic stroke is a safe and effective method of treatment in patients with different haemodynamically significant damage of brachiocephalic arteries, once the patients were correctly selected for the operation.

Clustering of Thrombin Generation Test Data Using a Reduced Mathematical Model of Blood Coagulation.

Correct interpretation of the data from integral laboratory tests, including Thrombin Generation Test (TGT), requires biochemistry-based mathematical models of blood coagulation. The purpose of this study is to describe the experimental TGT data from healthy donors and hemophilia A (HA) and B (HB) patients. We derive a simplified ODE model and apply it to analyze the TGT data from healthy donors and HA/HB patients with in vitro added tissue factor pathway inhibitor (TFPI) antibody. This model allows the characterization of hemophilia patients in the space of three most important model parameters. The proposed approach may provide a new quantitative tool for the analysis of experimental TGT. Also, it gives a reduced model of coagulation verified against clinical data to be used in future theoretical large-scale modeling of thrombosis in flow.

[The role of transcranial electrostimulation in the treatment of psychosomatic disorders in industrial workers]. / Transkranial'naia élektrostimuliatsiia v lechenii psikhosomaticheskikh rasstroistv u rabotnikov promyshlennogo predpriiatiia.

BACKGROUND: The prevalence of psychosomatic disorders (PD) is steadily increasing from year to year. Suffice it to mention that the appealability for the primary health care during the recent period increased from 20% to 40-49%. Various psychological, pharmacological and physical methods for the treatment of somatoform disorders have been developed. However, the physiotherapeutic methods for the management of psychosomatic disorders remain virtually unexplored. At the same time, the stress-limiting effects of transcranial electrical stimulation (TES) have been convincingly demonstrated. AIM: The objective of the present study was to determine the effectiveness of the treatment of psychosomatic disorders based on the application of transcranial electrostimulation. MATERIAL AND METHODS: We evaluated during 4 months the general functional state (GFS) as well as the psychosomatic and vegetative status (VS) of the employees of the Scientific development and production enterprise 'Splav' including 14 women and 14 men who complained of deterioration of health. All the participants of the study were divided into groups depending on their functional disorders. The patients in one of the groups underwent transcranial electrostimulation. The age of the patients ranged from 36 to 66 years. The mean age of the men and women was 56±1.4 and 48±2.2 years respectively. The study was designed to estimate GFS and VS based on the integral characteristics obtained with the use of the 'Simona 111' hardware-software complex. The complaints of stressful conditions, deteriorated GFS, and psycho-somatic disorders were revealed in the relevant questionnaire studies. The participants of the study were divided into two groups: the main study group and the control group matched for the age and sex of their members. Those of the former group underwent relaxation procedures in the combination with transcranial electrostimulation whereas the control subjects were managed with the use of the relaxation procedures alone. RESULTS: The workers comprising the group with the impaired functional indices and psychosomatic disorders were initially characterized by the moderate or high stress level with the poor general functional state. These parameters as well as the indicators of the vegetative status were significantly improved after the course of transcranial electrical stimulation. Specifically, the activity of the parasympathetic nervous system increased in comparison with the baseline level, the activity of the sympathetic nervous system decreased, and the manifestations of psychosomatic disorders became less pronounced. The authors maintain that these effects were due to the selective activation of the stress-limiting mechanisms operating in the brain that caused normalization of the activity of the vegetative nervous system and the psycho-emotional state of the patients together with the improvement of their general functional condition. CONCLUSION: This study has demonstrated that psychosomatic disorders are especially common among the industrial workers with the poor general functional state of the organism. The clinical data give evidence of the effectiveness of transcranial electrical stimulation for the correction of the functional state and psychosomatic disorders.

[Assessing efficacy of carotid endarterectomy by means of intraoperative flowmetry]. / Otsenka éffektivnosti karotidnoi éndarteréktomii s pomoshch'iu intraoperatsionnoi floumetrii.

AIM: The study was aimed at determining volumetric blood flow velocity (VBFV) through the internal carotid artery (ICA) and assessing functional outcomes of treatment of patients, taking into consideration the dynamics of VBFV before and after the operation. PATIENTS AND METHODS: The study comprised a total of 53 patients subjected to examination of VBFV through the ICA before and after carotid endarterectomy (CEA) assessed by means of flowmetry. Neurological deficit was evaluated by means of the NIHS Scale (the M NIHSS prior to operation equalling 2.49), the degree of disability and functional independence of the patient after endured stroke was assessed by the modified Rankin Scale (the M mRS before operation amounting to 1.23) and the Rivermead Mobility Index, the severity of cognitive dysfunctions was determined by the Mini-Mental State Examination (the M MMSE before surgery equalling 24.85) and the Montreal Cognitive Assessment scale (the M MoCA before surgery scoring 23.3). RESULTS: At the first stage of examination we determined that the VBFV after the operation increased in all patients, but the highest values of the increment were observed in groups of patients with ICA stenoses of 80-89% and 90-99%. During the second stage, it was determined that with a higher value of the VBFV after carotid endarterectomy regression of neurological deficit occurred faster (the postoperative M NIHSS scoring 2.09, with the M mRS of 1.00), the patients demonstrated higher results while undergoing the tests for intellectual ability (the postoperative MMSE and MoCA equalling 25.4 and 23.91, respectively) compared with the patients showing low values of the increment of the VBFV after the operation. CONCLUSION: Intraoperative flowmetry is a simple and efficient method of evaluating the VBFV through the ICA, an increase in volumetric blood flow (VBF) contributes to restoration of normal perfusion of the brain and creates prerequisites for more complete and faster restoration of the lost functions after ischaemic stroke (IS) and chronic cerebral ischaemia.

[Surgical prevention of ischaemic strokes by means of an extraintracranial microanastomosis (EICMA) in the carotid basin]. / Khirurgicheskaya profilaktika ishemicheskikh insul'tov s ispol'zovaniem ekstra-intrakranial'nogo mikroanastomoza (EIKMA) v karotidnom basseine.

BACKGROUND: Not less than 50% of all ischaemic strokes appear to occur resulting from pathology of extracranial arteries. Occlusions and stenoses are more commonly encountered in carotid arteries, with the incidence of occlusion of the internal carotid artery (ICA) ranging from 5 to 10% within the structure of all lesions of brachiocephalic arteries (BCA). AIM: The study was aimed at assessing the results of a surgical procedure of extra-intracranial microanastomosis (EICMA) performed in patients presenting with occlusive and stenotic lesions of BCA at the Neurosurgical Department. PATIENTS AND METHODS: During the period between January 1st 2009 to September 30th 2015, specialists of the Neurosurgery Department of the Research Institute of Emergency Medical Care named after N.V. Sklifosovsky treated a total of 1,101 patients presenting with atherosclerotic lesions of BCA, with a total of 1,038 surgical interventions performed. Of these, there were 837 operations of carotid endarterectomy and 180 operations of EICMA in the carotid basin. The indications for performing EICMA in patients from the group with occlusive-stenosing lesions of the major arteries of the head and neck were as follows: the presence of occlusion of the internal carotid artery or M1 segment of the middle cerebral artery, verified by the findings of angiographic examinations; previously endured ischaemic-type cerebral circulation impairment in the basin on the side of occlusion; a decrease in the perfusion reserve of the brain on the side of occlusion by the findings of positron emission tomography (PET), single-photon emission computed tomography (SPECT) or CT-perfusion with loading tests. RESULTS: After performing EICMA, the neurological status in patients averagely improved by 1.2 points according to the NIH Stroke Scale; by 0.5 point according to the Rankin scale, and by 3.5 points according to the Rivermead Mobility Index (Table 1). The dynamics of functional outcomes of EICMA turned out to depend on the terms of the operation (R=2143, p<0.05) and the degree of neurological deficit prior to surgery (R=3422, p<0.05): the deeper the neurological deficiency was and the sooner after acute cerebral ischaemia (ACI) the operation was performed, the more significant restoration of the functions was at the moment of discharge from hospital. Major complications of EICMA in patients operated on at various terms of ischaemic stroke were as follows: recurrent cerebral circulatory impairments - in 6 (3.3%) patients, subdural and intracranial haemorrhage - in 2 (1.1%), temporal deepening of the neurological deficit on the background of reperfusion, confirmed by the findings of SPECT - in 2 (1.1%), pneumonia with a lethal outcome - in 1 (0.6%), fatal ischaemic stroke of the opposite hemisphere - in 1 (0.6%) subject. The lethality rate amounted to 1.1% (2 cases). The risk for the development of complications was higher in elderly and aged patients (75 years and over), in patients with pronounced neurological deficit and significant somatic disorders. CONCLUSION: The indications for surgical prevention of recurrent ischaemic stroke by means of EICMA should be based on the following factors: the presence of confirmed occlusion of the ICA, a history of endured acute cerebral ischaemia, and decreased cerebrovascular reserve. Performing a surgical intervention is not appropriate for patients presenting with pronounced neurological deficit and significant somatic pathology. Careful selection of patients, the use of modern methods of neuroimaging, and intraoperative control make it possible to achieve favourable functional outcomes and to decrease the postoperative complications rate.

[The use of extra-intracranial microanastomosis in the treatment of cerebral ischemia in patients with nontraumatic subarachnoid hemorrhage]. / Primenenie ekstra-intrakranial'nogo mikroanastomoza v lechenii ishemii golovnogo mozga u bol'nykh s netravmaticheskim subarakhnoidal'nym krovoizliyaniem.

OBJECTIVE: to evaluate the use of extra-intracranial microanastomosis (EICMA) in the treatment of brain ischemia in patients with nontraumatic subarachnoid hemorrhage (SAH). MATERIAL AND METHODS: In the period from 01.01.14 to 01.07.15, there were 229 surgeries for ruptured intracranial aneurysms performed in the urgent surgery unit. Nine patients with marked and widespread angiospasm, subcompensated and decompensated cerebral ischemia underwent the simultaneous clipping of ruptured intracranial aneurysms and EICMA. The age of patients varied from 32 to 52 years (mean 36 years). The severity of patient's state was assessed as III-IV grades on the Hunt and Hess scale before operation. The surgery was performed 1-2 days after admission to the hospital, 1-8 days after the development of SAH. RESULTS AND CONCLUSION: Excellent and good outcome was recorded in 4 patients, severe disability in 3 patients, fatal outcome in 2 patients. The fatal outcome was due to decompensated cerebral ischemia and progressive angiospasm with the high linear blood flow rate and the following reduction in perfusion in the affected hemisphere. The simultaneous clipping of ruptured intracranial aneurysms and EICMA in the acute stage of SAH of patients with subcompensated cerebral ischemia allow to improve treatment RESULTS: This technique is most applicable for patients with proximal angiospasm of M1- and M2-segments of the middle cerebral artery in the first 24 h of the development of a focal neurological deficit supported by the reduction in perfusion in the corresponding vascular area.

[Operations of revascularization in surgery of cerebrovascular aneurysms].

Surgical revascularization of the brain is one of the most important trends in the development of neurosurgery. Restoration of adequate blood flow through pre- and intracerebral arteries promotes prevention and treatment of ischaemic lesions of the brain in various pathology. The present work was aimed at analysing the experience gained in performing revascularizing operations in patients with cerebral aneurysms at the department of neurosurgery. The authors analysed therapeutic outcomes in a total of 45 patients presenting with giant and complex aneurysms of cerebral arteries and treated from 2009 to 2014. Of the 45 patients with giant and complex aneurysms of cerebral arteries, 31 (68.8%) patients underwent open microsurgical interventions (including 10 patients with the use of different variants of revascularizing operation) and 14 (31.2%) patients were subjected to endovascular exclusion of the aneurysm from the blood flow. It was shown that performing revascularizing operations in patients with complex and giant aneurysms of cerebral arteries makes it possible to compensate circulation in the interested arterial basin and to obtain good functional results.

ASSESSMENT OF CEREBRAL PERFUSION IN PATIENTS WITH HEMODYNAMIC ISCHEMIC STROKE UNDERGOING RECONSTRUCTIVE BRACHIOCEPHALIC ARTERY INTERVENTIONS.

The paper deals with the assessment of cerebral perfusion in patients in the acute period of acute cerebrovascular accident before and after revascularization surgery. It gives a clinical example of using contrast-free perfusion magnetic resonance imaging (MRI) in a patient with hemodynamic ischemic stroke. The use of this technique made it possible to determine indications for early carotid endarterectomy for the contralateral internal carotid artery and to evaluate positive postoperative changes in cerebral perfusion and the patient's neurological status. The authors analyzed the current literature on this problem with a particular emphasis on the possibilities of using dynamic susceptibility contrast-enhanced and arterial spin-labeling contrast-free perfusion MRI in this category of patients. Carotid endarterectomy in the acute period of acute cerebrovascular accident can improve cerebral hemodynamics and the patient's neurological status and prevent recurrent cerebral circulatory disorders. Indications for this surgery should be determined by taking into consideration the results of perfusion MRI techniques (single-photon computed tomography contrastenhanced and contrast-free perfusion MRI).

Modelling of platelet-fibrin clot formation in flow with a DPD-PDE method.

The paper is devoted to mathematical modelling of clot growth in blood flow. Great complexity of the hemostatic system dictates the need of usage of the mathematical models to understand its functioning in the normal and especially in pathological situations. In this work we investigate the interaction of blood flow, platelet aggregation and plasma coagulation. We develop a hybrid DPD-PDE model where dissipative particle dynamics (DPD) is used to model plasma flow and platelets, while the regulatory network of plasma coagulation is described by a system of partial differential equations. Modelling results confirm the potency of the scenario of clot growth where at the first stage of clot formation platelets form an aggregate due to weak inter-platelet connections and then due to their activation. This enables the formation of the fibrin net in the centre of the platelet aggregate where the flow velocity is significantly reduced. The fibrin net reinforces the clot and allows its further growth. When the clot becomes sufficiently large, it stops growing due to the narrowed vessel and the increase of flow shear rate at the surface of the clot. Its outer part is detached by the flow revealing the inner part covered by fibrin. This fibrin cap does not allow new platelets to attach at the high shear rate, and the clot stops growing. Dependence of the final clot size on wall shear rate and on other parameters is studied.

[CT perfusion in patients after EICMA in the postoperative period].

OBJECTIVE: To evaluate the influence of the EICMA on the circulation of the brain in patients with unilateral occlusion of the internal carotid artery (ICA) in the late postoperative period using CT perfusion. MATERIAL AND METHODS: Ten patients with unilateral internal carotid artery occlusion and stenosis of the opposite ICA 50-60%, with a history of a single ischemic stroke were examined. All patients underwent pre-and postoperative CT angiography of the brachiocephalic and intracranial arteries and CT perfusion. We also compared the results of neurological examinations preoperatively and during the year after the intervention. RESULTS: The neurological data during the first year after surgery demonstrated an improvement of neurological status and quality of life in all patients. Preoperative CT perfusion showed the patchy decrease in the cerebral blood flow (CBF) to 18 ml/100 g/min (average of 44-56 ml/100 g/min) and increase in the mean transit time (MTT) to 7.2 s (normally less 6c) in all cases on the side of occlusion. The most susceptible to chronic ischemic changes was the frontal region, temporal and parietooccipital regions were affected to a lesser extent. Due to stenosis of the opposite ICA, minimum CBF of the cortex in the opposite hemisphere was 24 ml/100 g/min and MTT was increased to 5.6 s. Six months after the applying of EICMA, the significant improvement of CT perfusion was noted on the side of the anastomosis in all patients: an increase in CBF (at least 44 ml/100 g/min) and MTT reduction (up to 6.1s in the frontal region), as well as the "synchronization" of CBF and CBV in similar areas of the cerebral cortex of the right and left hemisphere. CONCLUSION: CT perfusion in the late postoperative period after applying EICMA in patients with unilateral ICA occlusion demonstrates not only changes of the cerebral perfusion on the side of the occlusion, but also the increased collateral blood flow of the cortex in both hemispheres, which significantly improves brain blood flow generally within 6-12 months after surgery.

Modelling of thrombus growth in flow with a DPD-PDE method.

Hemostatic plug covering the injury site (or a thrombus in the pathological case) is formed due to the complex interaction of aggregating platelets with biochemical reactions in plasma that participate in blood coagulation. The mechanisms that control clot growth and which lead to growth arrest are not yet completely understood. We model them with numerical simulations based on a hybrid DPD-PDE model. Dissipative particle dynamics (DPD) is used to model plasma flow with platelets while fibrin concentration is described by a simplified reaction-diffusion-advection equation. The model takes into account consecutive stages of clot growth. First, a platelet is weakly connected to the clot and after some time this connection becomes stronger due to other surface receptors involved in platelet adhesion. At the same time, the fibrin mesh is formed inside the clot. This becomes possible because flow does not penetrate the clot and cannot wash out the reactants participating in blood coagulation. Platelets covered by the fibrin mesh cannot attach new platelets. Modelling shows that the growth of a hemostatic plug can stop as a result of its exterior part being removed by the flow thus exposing its non-adhesive core to the flow.

Thrombin activity propagates in space during blood coagulation as an excitation wave.

Injury-induced bleeding is stopped by a hemostatic plug formation that is controlled by a complex nonlinear and spatially heterogeneous biochemical network of proteolytic enzymes called blood coagulation. We studied spatial dynamics of thrombin, the central enzyme of this network, by developing a fluorogenic substrate-based method for time- and space-resolved imaging of thrombin enzymatic activity. Clotting stimulation by immobilized tissue factor induced localized thrombin activity impulse that propagated in space and possessed all characteristic traits of a traveling excitation wave: constant spatial velocity, constant amplitude, and insensitivity to the initial stimulation once it exceeded activation threshold. The parameters of this traveling wave were controlled by the availability of phospholipids or platelets, and the wave did not form in plasmas from hemophilia A or C patients who lack factors VIII and XI, which are mediators of the two principal positive feedbacks of coagulation. Stimulation of the negative feedback of the protein C pathway with thrombomodulin produced nonstationary patterns of wave formation followed by deceleration and annihilation. This indicates that blood can function as an excitable medium that conducts traveling waves of coagulation.

Finite platelet size could be responsible for platelet margination effect.

Blood flows through vessels as a segregated suspension. Erythrocytes distribute closer to the vessel axis, whereas platelets accumulate near vessel walls. Directed platelet migration to the vessel walls promotes their hemostatic function. The mechanisms underlying this migration remain poorly understood, although various hypotheses have been proposed to explain this phenomenon (e.g., the available volume model and the drift-flux model). To study this issue, we constructed a mathematical model that predicts the platelet distribution profile across the flow in the presence of erythrocytes. This model considers platelet and erythrocyte dimensions and assumes an even platelet distribution between erythrocytes. The model predictions agree with available experimental data for near-wall layer margination using platelets and platelet-modeling particles and the lateral migration rate for these particles. Our analysis shows that the strong expulsion of the platelets from the core to the periphery of the blood vessel may mainly arise from the finite size of the platelets, which impedes their positioning in between the densely packed erythrocytes in the core. This result provides what we believe is a new insight into the rheological control of platelet hemostasis by erythrocytes.

[Comparative characteristics of the physiometric parameters of schoolchildren' physical development].

The physiometric parameters (exercise performance, muscle strength, and lung capacity) were monitored in schoolchildren over the 15-year period. There has been a reduction in exercise performance, muscle strength and an increase in lung capacity in the past 15 years, which the pediatricians should take into account to detect physical development abnormalities in schoolchildren.

Platelet adhesion from shear blood flow is controlled by near-wall rebounding collisions with erythrocytes.

The efficacy of platelet adhesion in shear flow is known to be substantially modulated by the physical presence of red blood cells (RBCs). The mechanisms of this regulation remain obscure due to the complicated character of platelet interactions with RBCs and vascular walls. To investigate this problem, we have created a mathematical model that takes into account shear-induced transport of platelets across the flow, platelet expulsion by the RBCs from the near-wall layer of the flow onto the wall, and reversible capture of platelets by the wall and their firm adhesion to it. This model analysis allowed us to obtain, for the first time to our knowledge, an analytical determination of the platelet adhesion rate constant as a function of the wall shear rate, hematocrit, and average sizes of platelets and RBCs. This formula provided a quantitative description of the results of previous in vitro adhesion experiments in perfusion chambers. The results of the simulations suggest that under a wide range of shear rates and hematocrit values, the rate of platelet adhesion from the blood flow is mainly limited by the frequency of their near-wall rebounding collisions with RBCs. This finding reveals the mechanism by which erythrocytes physically control platelet hemostasis.

[The predictors of development of bronchial asthma in children].

The study of the predictors impacting the development of bronchial asthma in children was implemented. For each risk factor the chances ratio and its 95% confidence interval were calculated. The sample consisted of 280 children with bronchial asthma aged from 7 to 14 years and 210 healthy children of the same age. The risk factors having priority significance for the development of bronchial asthma in children are identified. The research data can be applied to calculate of the individual risks in population to forecast the development of bronchial asthma and its timely prevention.

Synthesis and evaluation of oligo-1,3-thiazolecarboxamide derivatives as HIV-1 reverse transcriptase inhibitors.

A set of oligo-1,3-thiazolecarboxamide derivatives able to interact with the minor groove of nucleic acids was synthesized. These oligopeptides contained different numbers of thiazole units presenting dimethylaminopropyl or EDTA moieties on the C-terminus, and aminohexanoyl or EDTA moieties on the N-terminus. The inhibition of such compounds on HIV-1 reverse transcriptase activity was evaluated using different model template primer duplexes: DNA x DNA, RNA x DNA, DNA x RNA and RNA x RNA. The biological properties of the thiazolecarboxamide derivatives were compared to those of distamycin, another minor groove binder which contains three pyrrole rings. Similar to distamycin, the thiazole containing oligopeptides were good inhibitors of the reverse transcription reaction in the presence of DNA x DNA. But in contrast to distamycin, the oligothiazolide derivatives were able to inhibit reverse transcription in the presence of RNA x DNA or DNA x RNA template primers. Both distamycin and oligothiazolecarboxamides had low affinity for RNA x RNA duplexes. The inhibition obtained with the newly synthesized thiazolecarboxamides showed that these compounds were more powerful and versatile inhibitors of the RT-dependent polymerization than the natural minor groove binder distamycin.