Nitroxide-controlled free radical polymerization of a sugar-carrying acryloyl monomer

Controlled free radical polymerization of a sugar-carrying acrylate, 3-O-acryloyl-1,2 : 5,6-di-O-isopropylidene-α-D -glucofuranoside (AIpGlc), was achieved in p-xylene at 100°C by using a di-tert-butyl nitroxide (DBN)-based alkoxyamine as an initiator and dicumyl peroxide (DCP) as an accelerator. The polymerization gave low-polydispersity (1.2 < M_w/M_n < 1.6) polymers with predicted molecular weights. The same approach with a DBN-capped polystyrene (PS-DBN) as an initiator afforded block copolymers of the type PS-b-PAIpGlc. The acidolysis of the homopolymers and block copolymers gave well-defined glucose-carrying polymers PAGlc and PS-b-PAGlc, respectively. These amphiphilic PS-b-PAGlc block copolymers were observed to exhibit microdomain surface morphologies that differ for different copolymer compositions. This success opens up a new, simple route to the synthesis of well-defined sugar-carrying polymers of various architectures.     

Crystallization kinetics of atomic crystals revealed by a single-shot and single-particle X-ray diffraction experiment

Crystallization is a fundamental natural phenomenon and the ubiquitous physical process in materials science for the design of new materials. So far, experimental observations of the structural dynamics in crystallization have been mostly restricted to slow dynamics. We present here an exclusive way to explore the dynamics of crystallization in highly controlled conditions (i.e., in the absence of impurities acting as seeds of the crystallites) as it occurs in vacuum. We have measured the early formation stage of solid Xe nanoparticles nucleated in an expanding supercooled Xe jet by means of an X-ray diffraction experiment with 10-fs X-ray free-electron laser (XFEL) pulses. We found that the structure of Xe nanoparticles is not pure face-centered cubic (fcc), the expected stable phase, but a mixture of fcc and randomly stacked hexagonal close-packed (rhcp) structures. Furthermore, we identified the instantaneous coexistence of the comparably sized fcc and rhcp domains in single Xe nanoparticles. The observations are explained by the scenario of structural aging, in which the nanoparticles initially crystallize in the highly stacking-disordered rhcp phase and the structure later forms the stable fcc phase. The results are reminiscent of analogous observations in hard-sphere systems, indicating the universal role of the stacking-disordered phase in nucleation.

Crystallization is one of the most ubiquitous physical phenomena in nature, yet its microscopic and mesoscopic mechanisms are not fully understood. Crystallization is generally assumed to start from nucleation, a nanometer-sized nucleus formation, which is followed by the nucleus growth. The initially grown phase structure has been a long-standing subject of controversy. Classical nucleation theory (CNT) (1, 2) assumes that nucleation occurs at a spherical crystallite having the stable lattice structure of the bulk. More than a century ago, Ostwald (3) pointed out the role of metastable phases in nucleation and advocated his step rule, which states that a metastable phase is initially formed and that it may change to the stable phase later. Alexander and McTague (4) argued that the initial phase would be body-centered cubic regardless of the stable phase structure. Meanwhile, a recent numerical simulation of ice nucleation (5) suggests that ice nucleation occurs in a stacking-disordered phase, thereby increasing the nucleation rate by several orders of magnitude as compared with the CNT prediction. Understanding the structural dynamics upon crystallization is crucial for the creation of various new materials, as the structures and properties of the final product are crucially influenced by the kinetic pathway of the phase transition. To date, the nonequilibrium dynamics upon crystallization have been extensively studied numerically, whereas experimental observations have been mostly restricted to slow dynamics, such as crystallization of colloids (6) and proteins (7), or more recently, to the crystallization of supercooled liquids (8).The crystallization dynamics in atomic systems is challenging to investigate experimentally. First, sufficiently high temporal resolution and atomic spatial resolutions are required to resolve transient structures emerging upon crystallization. Second, orientation and ensemble averaging of the structural information can lead to significant structural detail loss in individual crystallites; nevertheless, the averaged information is helpful in identifying the phase in the early stages of crystallization. Third, growth in the presence of a substrate would influence the nucleation and growth process, potentially causing disagreement with theories. Recently, ultrashort and intense X-ray pulses from X-ray free-electron lasers (XFELs) (9, 10) became available, providing novel opportunities for the structural determination of fragile samples, such as aerosol particles (11), superfluid nanodroplets (12), and live biospecimens (13). In single-shot diffraction experiments using XFELs, instantaneous structures of single free-flying nanoparticles can be captured with snapshot exposures of femtosecond X-ray pulses. Therefore, single-shot diffraction has the potential to overcome the above-mentioned technical challenges. Although the structural determination of single small nuclei at the critical stage (typically consisting of several hundred atoms) is not achievable with the current fluence of XFEL beams, the structure of the initially grown phase can be inferred by examining larger crystals immediately after the growth.In this study, we demonstrate the uniqueness of the single-shot diffraction to conclusively elucidate the structural dynamics upon crystallization. We investigate the structure of Xe nanoparticles crystallized in a supercooled Xe gas jet by single-shot and single-particle diffraction using XFEL pulses. Rare-gas systems are suitable atomic model systems for investigating the structural dynamics upon crystallization as the atomic interactions are well approximated by the simple Lennard–Jones interaction. In addition, the absence of interaction between the sample and a supporting substrate provided us with an ideal experimental environment for the investigation of the homogeneous nucleation process. The comparison of the experimental diffraction data with numerical simulation results revealed the coexistence of the face-centered cubic (fcc) and randomly stacked hexagonal close-packed (rhcp) phases in single Xe nanoparticles. Based on our observations, we discuss the crystallization kinetics of rare-gas nanoparticles in connection with the literature on crystallization of other systems, such as hard-sphere systems and water.     

Tumor hypoxia in pelvic recurrences of cervical cancer

We have previously demonstrated in primary cancer of the uterine cervix that tumor hypoxia, as determined polarographically, is strongly associated with clinical malignant progression of the disease. Having applied a similar methodological approach to investigate loco-regional relapses, we found a pronounced shift to more hypoxic oxygenation profiles in the recurrent tumors than in the primary tumors. Median pO₂ values in 53 pelvic recurrences were significantly lower than the median pO₂ values of 117 primary tumors of comparable sizes (7.1 ± 1.1 mmHg vs. 12.1 ± 1.0 mmHg, p = 0.0013). The differences in tumor oxygenation between primary and recurrent tumors mirrored the differences in the patients' 5-year survival probabilities. In the cohort of patients with pelvic relapses, median tumor pO₂ < 4 mmHg indicated a significantly shorter median survival time as compared to median tumor pO₂ ≥ 4 mmHg. Our results further support our thesis that in cervical cancer, tumor hypoxia and clinical aggressiveness in terms of resistance to therapy and tumor dissemination, are interrelated. Int. J. Cancer (Pred. Oncol.) 79:365–369, 1998. © 1998 Wiley-Liss, Inc.     

Stent placement for recurrent vasospastic angina resistant to medical treatment

The successful stent placement for treatment of recurrent vasospastic angina in a patient with nonstenotic coronary arteries is described. Use of the Palmaz-Schatz stent resulted in successful vasodilation that completely prevented anginal attacks. This procedure represents an alternative treatment for patients with vasospastic angina refractory to aggressive medical therapy. Cathet. Cardiovasc. Diagn. 42:440–443, 1997. © 1997 Wiley-Liss, Inc.     

p53 gene mutations in oral carcinomas from India

In this study, we analyzed 53 oral squamous-cell carcinomas among Indians for the presence of alterations in the tumor-suppressor gene p53 by PCR-SSCP and sequencing methods. Our results showed that 21% (II/53) of oral carcinomas analyzed carried mutations within the exons 5–8 of the p53 gene. We have identified II single-base pair substitutions consisting of 10 mis-sense mutations and one at the splice acceptor site, and one deletion mutation involving 4 consecutive bases. The majority of the base substitutions were transitions (5 TA to CG and 5 GC to AT), while only one transversion (TA to GC) was observed. Probable hot-spots for the mutation induction were identified at codons 149 and 274, which have not been observed before in head-and-neck cancers. The mutational spectrum might have originated from base alkylations at guanine and thymine residues, caused by some alkylating agents. The present results are thus consistent with the involvement of tobacco-related nitrosoamines in the etiology of oral squamous-cell carcinoma. © 1996 Wiley-Liss, Inc.