Long range 1,4 and 1,6-interstrand cross-links formed by a trinuclear platinum complex. Minor groove preassociation affects kinetics and mechanism of cross-link formation as well as adduct structure

Reported here is a comparison of the kinetics of the stepwise formation of 1,4- and 1,6-GG interstrand cross-links by the trinuclear platinum anticancer compound 15N-[{trans-PtCl(NH3)2}2{μ-trans-Pt(NH3)2(H2N(CH2)6NH2)2}]4+, (1,0,1/t,t,t (1) or BBR3464). The reactions of 15N-1 with the self-complementary 12-mer duplexes 5‘-{d(ATATGTACATAT)2} (I) and 5‘-{d(TATGTATACATA)2} (II) have been studied at 298 K, pH 5.3 by [1H,15N] HSQC 2D NMR spectroscopy. The kinetic profiles for the two reactions are similar. For both sequences initial electrostatic interactions with the DNA are observed for 1 and the monoaqua monochloro species (2) and changes in the chemical shifts of certain DNA 1H resonances are consistent with binding of the central charged {PtN4} linker unit in the minor groove. The pseudo first-order rate constants for the aquation of 1 to 2 in the presence of duplex I (3.94 ± 0.03 × 10-5 s-1), or II (4.17 ± 0.03 × 10-5 s-1) are ca. 40% of the value obtained for aquation of 1 under similar conditions in the absence of DNA. Monofunctional binding to the guanine N7 of the duplex occurs with rate constants of 0.25 ± 0.02 M-1 s-1 (I) and 0.34 ± 0.02 M-1 s-1 (II), respectively. Closure to form the 1,4- or 1,6-interstrand cross-links (5) was treated as direct from 3 with similar rate constants of 4.21 ± 0.06 × 10-5 s-1 (I) and 4.32 ± 0.04 × 10-5 s-1 (II), respectively. Whereas there is only one predominant conformer of the 1,6 cross-link, evidence from both the 1H and [1H,15N] NMR spectra show formation of two distinct conformers of the 1,4 cross-link, which are not interconvertible. Closure to give the major conformer occurs 2.5-fold faster than for the minor conformer. The differences are attributed to the initial preassociation of the central linker of 1 in the minor groove and subsequently during formation of both the monofunctional and bifunctional adducts. For duplex I, molecular models indicate two distinct pathways for the terminal {PtN3Cl} groups to approach and bind the guanine N7 in the major groove with the central linker anchored in the minor groove. To achieve platination of the guanine residues in duplex II the central linker remains in the minor groove but 1 must diffuse off the DNA for covalent binding to occur. Clear evidence for movement of the linker group is seen at the monofunctional binding step from changes of chemical shifts of certain CH2 linker protons as well as the Pt−NH3 and Pt−NH2 groups. Consideration of the 1H and 15N shifts of peaks in the Pt−NH2 region show that for both the 1,4 and 1,6 interstrand cross-links there is a gradual and irreversible transformation from an initially formed conformer(s) to product conformer(s) in which the amine protons of the two bound {PtN3} groups exist in a number of different environments. The behavior is similar to that observed for the 1,4-interstrand cross-link of the dinuclear 1,1/t,t compound. The potential significance of preassociation in determining kinetics of formation and structure of the adducts is discussed. The conformational flexibility of the cross-links is discussed in relation to their biological processing, especially protein recognition and repair, which are critical determinants of the cytotoxicity of these unique DNA-binding agents

Seen but unheard: navigating turbulent waters as Māori and Pacific postgraduate students in STEM

The experiences of Māori and Pacific postgraduate students in STEM (Science, Technology, Engineering and Mathematics) offer insights into how universities, particularly science faculties, currently underserve Māori and Pacific people. This article shares the experiences of 43 current or past postgraduate students at New Zealand universities. Collectively, our stories offer insight into how representation, the white imprint, space invaders/stranger making, and institutional habits, specifically operate to exclude and devalue Māori and Pacific postgraduates in STEM. We provide new understandings of the white imprint (rewarding and incentivising white behaviour), where Māori and Pacific postgraduates were prevented from being their authentic selves. Importantly, this research documents how Māori and Pacific postgraduates experience excess labour because of institutional habits. This research also provides insight into how the science funding system results in superficial and unethical inclusion of Māori and Pacific postgraduates. Our stories provide persuasive evidence that the under-representation of Māori and Pacific in STEM will not be addressed by simply bolstering university enrolments. Instead, our stories highlight the urgent requirement for universities to change the STEM learning environment which continues to be violent and culturally unsafe for Māori and Pacific postgraduates.fals

Designed Compounds for Recognition of 10 Base Pairs of DNA with Two AT Binding Sites

Short AT base pair sequences that are separated by a small number of GCs are common in eukaryotic parasite genomes. Cell-permeable compounds that bind effectively and selectively to such sequences present an attractive therapeutic approach. Compounds with linked, one or two amidine-benzimidazole-phenyl (ABP) motifs were designed, synthesized and evaluated for binding to adjacent AT sites by biosensor-surface plasmon resonance (SPR). A surprising feature of the linked ABP motifs is that a set of six similar compounds has three different minor groove binding modes with the target sequences. Compounds with one ABP bind independently to two separated AT sites. Unexpectedly, compounds with two ABP motifs can bind strongly either as monomers or as cooperative dimers to the full site. The results are supported by mass spectrometry and circular dichroism, and models to explain the different binding modes are presented