Photoluminescence decay dynamics in an InGaN/GaN/AlGaN single quantum well

X-ray diffraction results in magnetically oriented samples of aqueous solutions of the sodium salt of the dinucleoside phosphate G2 in the cholesteric N* phase are presented. The symmetry of the diffraction confirms a model of stacking of tetrameric disks forming cylindrical aggregates in the N* phase. A weak band at 7 A not previously observed gives further information on the inner structure of the cylindrical aggregates. Analysis of the diffraction peak position in low angle evidences a change in functional behaviour of the distance a between cylinder axes as a function of the volume concentration c, of G2. In the H phase, accc;'/' as it should for infinite cylinders; in the N* phase, ~c c c ; ' /~, typical of finite cylinders. The cross-over of functional behaviour occurs at the N*-H phase transition. There is therefore cylinder growth along the N* phase. IntroductionAmong the nucleosides commonly found in DNA and RNA, only guanosine (and some derivatives) is able to self-associate in water solution to form stable structures (reviewed in [l] and [2]): highly ordered gels, fibres and other macroaggregates have been in fact extensively investigated [3-61. The common structural unit has been found to be a planar disc-shaped aggregate formed by four guanosine residues, hydrogen bonded in a Hoogsteen mode [7]. Furthermore, NMR investigations of 5'-guanosine monophosphate aqueous solutions showed that the self-aggregation process can also take place in very dilute solutions, via stacking of planar tetramers [S-101. The biological relevance of this very special feature is not yet defined on the one hand this could have had prebiotic significance in the origin of the genetic code [9-lo]. On the other hand telomers and related guanine-rich sequences can form higher-order structures (quadriplexes) formed by guanine-quartet (see in particular [ 111): it has been then proposed, for example, that the quadriplex formation might provide a means for homologous chromosome alignment at meiosis [12] or provide a general mechanism for aligning and pairing two duplexes for recombination ([13], see also [l 13).Recently, the aqueous solution of the dinuceoside phosphate 2'-deoxyguanylyl-(3'-5')-2'-deoxyguanosine (d(GpG) or G2) has been studied [14,15]. In particular, the sodium salt of G2 forms cholesteric and hexagonal liquid-crystalline phases on decreasing water content. From the analysis of the measured X-ray diffraction peaks

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