Synthesis and Physicochemical Properties of Metallobacteriochlorins

Access to metallobacteriochlorins is essential for investigation of a wide variety of fundamental photochemical processes, yet relatively few synthetic metallobacteriochlorins have been prepared. Members of a set of synthetic bacteriochlorins bearing 0–4 carbonyl groups (1, 2, or 4 carboethoxy substituents, or an annulated imide moiety) were examined under two conditions: (i) standard conditions for zincation of porphyrins [Zn­(OAc)₂·2H₂O in <i>N</i>,<i>N</i>-dimethylformamide (DMF) at 60–80 °C], and (ii) treatment in tetrahydrofuran (THF) with a strong base [e.g., NaH or lithium diisopropylamide (LDA)] followed by a metal reagent MX_<i>n</i>. Zincation of bacteriochlorins that bear 2–4 carbonyl groups proceeded under the former method whereas those with 0–2 carbonyl groups proceeded with NaH or LDA/THF followed by Zn­(OTf)₂. The scope of metalation (via NaH or LDA in THF) is as follows: (a) for bacteriochlorins that bear two electron-releasing aryl groups, M = Cu, Zn, Pd, and InCl (but not Mg, Al, Ni, Sn, or Au); (b) for bacteriochlorins that bear two carboethoxy groups, M = Ni, Cu, Zn, Pd, Cd, InCl, and Sn (but not Mg, Al, or Au); and (c) a bacteriochlorin with four carboethoxy groups was metalated with Mg (other metals were not examined). Altogether, 15 metallobacteriochlorins were isolated and characterized. Single-crystal X-ray analysis of 8,8,18,18-tetramethylbacteriochlorin reveals the core geometry provided by the four nitrogen atoms is rectangular; the difference in length of the two sides is ∼0.08 Å. Electronic characteristics of (metal-free) bacteriochlorins were probed through electrochemical measurements along with density functional theory calculation of the energies of the frontier molecular orbitals. The photophysical properties (fluorescence yields, triplet yields, singlet and triplet excited-state lifetimes) of the zinc bacteriochlorins are generally similar to those of the metal-free analogues, and to those of the native chromophores bacteriochlorophyll <i>a</i> and bacteriopheophytin <i>a</i>. The availability of diverse metallobacteriochlorins should prove useful in a variety of fundamental photochemical studies and applications

A Maltose-Binding Protein Fusion Construct Yields a Robust Crystallography Platform for MCL1

<div><p>Crystallization of a maltose-binding protein MCL1 fusion has yielded a robust crystallography platform that generated the first apo MCL1 crystal structure, as well as five ligand-bound structures. The ability to obtain fragment-bound structures advances structure-based drug design efforts that, despite considerable effort, had previously been intractable by crystallography. In the ligand-independent crystal form we identify inhibitor binding modes not observed in earlier crystallographic systems. This MBP-MCL1 construct dramatically improves the structural understanding of well-validated MCL1 ligands, and will likely catalyze the structure-based optimization of high affinity MCL1 inhibitors.</p></div

Comparison of PDB 4HW3 and MBP-MCL1 with fragment 4.

<p>The structure of MBP-MCL1 with fragment <b>4</b> (yellow) determined to 2.4 Å (blue) overlaid with the structure of MCL1 171–323 determined at 2.4 Å (PDB ID 4HW3, gray). The carboxylic acid of 4HW3 adopts multiple conformations depending on the chain; only chain A is shown for clarity.</p

The structure of Apo MBP-MCL1 determined at 1.90 Å.

<p>(A) The MBP domain (red) is connected by a short GS linker (orange) to MCL1 173–321 (blue). A portion of alpha helix four is not ordered in the structure (red dashed-line). Maltose ligand is shown in yellow. (B) The MCL1 domain is structurally very similar to the NMR structure of Apo-MCL1 (gray).</p

The structure of MBP-MCL1 bound to fragment 5 determined at 1.9 Å.

<p>Fragment <b>5</b> (yellow) binds similarly in comparison to the elaborated ligand from PDB ID 4OQ6 (gray).</p

The conformational flexibility of the binding pocket of MCL1.

<p>Surface representations are shown as side views and ligands are shown as yellow sticks. (A and B) Fragment 4 maps onto L78 of NoxaB from PDB ID 2NLA, with only minor structural perturbation of the BH3-binding groove of MCL1. In contrast, binding of fragment 6 creates a significant pocket (C) which is further expanded upon binding of ligand 1 (D).</p

Structure of MBP-MCL1 bound to fragment 6 determined at 2.0 Å.

<p>(A) The surface side-view shows that fragment <b>6</b> shifts and makes water mediated hydrogen bond contacts with the peptide backbone of R263. (B) The elaborated ligand of fragment <b>6</b> (PDB ID 4OQ5) shifts to allow the methyl-naphthalene to bind in the hydrophobic pocket, requiring the carboxylic acid to make a single hydrogen bond with the sidechain of R263. (C) Overlay of crystallized fragment <b>6</b> and the elaborated ligand in PDB ID 4OQ5 reveals a distinct binding pose for <b>6</b>.</p

MCL1 ligands used in co-crystallization experiments.

<p>MCL1 ligands used in co-crystallization experiments.</p

Crystal packing of MCL1 173–321 is mediated by zinc and pyrophosphate.

<p>(A) The structure of MCL1 173–321 was determined to 1.70 Å. (B) In the ligand-bound MCL1 173–321 structure, the imidazole group of <b>1</b> coordinates with zinc along with H224 and pyrophosphate.</p

Comparison of PDB 4HW2 and MBP-MCL1 with ligand 2.

<p>The structure of MBP-MCL1 with ligand <b>2</b> (yellow) determined to 1.55 Å (blue) overlaid with the structure of MCL1 171–323 determined at 2.4 Å (PDB ID 4HW2, gray).</p