In vitro and in vivo photosensitization by protoporphyrins possessing different lipophilicities and vertical localization in the membrane

Photodynamic therapy (PDT) is being evaluated in clinical trials for treatment of various oncologic and ophthalmic diseases. The main cause for cell inactivation and retardation of tumor growth after photoactivation of sensitizers is very short-lived singlet oxygen molecules that are produced and have limited diffusion distances. In this paper we show that the extent of biological damage can be modulated by using protoporphyrin, which was modified to increase its lipophilicity, and which also places the tetrapyrrole core deeper within the membrane by the carboxylate groups being anchored at the lipid:water interface. The uptake of the parent molecule (PPIX) and its diheptanoic acid analogue (PPIXC6) by WiDR and CT26 cells was investigated by fluorescence microscopy and by fluorescence intensity from the cells. The uptake of PPIXC6 increased almost linearly with incubation length for over 24 h, whereas for PPIX only 1 h was needed to reach maximal intracellular concentration. Fluorescence microscopy of both cell lines indicated that both drugs were distributed diffusely in the plasma membrane and cytoplasm, but remained outside the nucleus. The efficiency of in vitro inactivation of WiDr and CT26 cells increased with the length of the alkylcarboxylic chain. Tumors in mice that were treated with PPIX-PDT grew more slowly than control tumors. However, tumors that were given PPIXC6 followed by light exposure showed a significant delay in their growth.     

Hydrolysis of alpha-alkyl-alpha-(methylthio)methylene Meldrum's acids. A kinetic and computational investigation of steric effects

The rates of hydrolysis of alpha-R-alpha-(methylthio)methylene Meldrum's acids (8-R with R = H, Me, Et, s-Bu, and t-Bu) were determined in basic and acidic solution in 50% DMSO-50% water (v/v) at 20 degrees C. In basic solution (KOH), nucleophilic attack to form a tetrahedral intermediate (T(OH)-) is rate limiting for all substrates (k1(OH)). In acidic solution (HCl) and at intermediate pH values (acetate buffers), water attack (k1(H2O) is rate limiting for 8-Me, 8-Et, and 8-s-Bu; the same is presumably the case for 8-t-Bu, but rates were too slow for accurate measurements at low pH. For 8-H, water attack is rate limiting at intermediate pH but at pH < 4.5 MeS- departure from the tetrahedral intermediate becomes rate limiting. Our interpretation of these results is based on a reaction scheme that involves three pathways for the conversion of T(OH)- to products, two of which being unique to hydrolysis reactions and taking advantage of the acidic nature of the OH group in T(OH)-. This scheme provides an explanation why even at high [KOH] T(OH)- does not accumulate to detectable levels even though the equilibrium for OH- addition to 8-R is expected to favor T(OH)-, and why at low pH water attack is rate limiting for R = Me, Et, s-Bu, and t-Bu but leaving group departure becomes rate limiting with the sterically small R = H. The trend in the k1(OH) and k1(H2O) indicates increasing steric crowding at the transition state with increasing size of R, but this effect is partially offset by a sterically induced twisting of the C=C double bond in 8-R which leads to its elongation and makes the substrate less stable and hence more reactive. Our computational results suggest that this effect becomes particularly pronounced for R = t-Bu and explains why k1(OH) for 8-t-Bu is somewhat higher than for the less crowded 8-s-Bu.     

Two novel 1,2,4,5-tetrazines that participate in inverse electron demand Diels-Alder reactions with an unexpected regioselectivity

[reaction: see text] Two new unsymmetrical 1,2,4,5-tetrazines, 3-methylsulfinyl-6-methylthio-1,2,4,5-tetrazine (4) and 3-(benzyloxycarbonyl)amino-6-methylsulfinyl-1,2,4,5-tetrazine (5), were prepared, and the scope of their participation in intermolecular inverse electron demand Diels-Alder reactions was defined. As anticipated, sulfoxides 4 and 5 (4 > 5) display a reactivity that is substantially greater than that of their corresponding sulfides (2 and 3), being derived from their enhanced electron-deficient character and resulting in a wider range of potential dienophile choices or the use of milder reaction conditions. The cycloaddition reactions were expectedly regioselective, typically producing a single cycloadduct, ensuring their synthetic utility, but both were found to proceed with a regioselectivity opposite what would be anticipated and complementary to that observed with 2 and 3.     

Substituted 2-methylene-1,3-oxazolidines, -1,3-thiazolidines, -1,3-benzothiazines, -1,3-oxazines, and substituted imidazopyrimidinediones from Cl(CH2)nNCO and Cl(CH2)nNCS and active methylene compounds

The reaction of omega-chloroalkyl isocyanates Cl(CH2)nNCO (n = 2 (2), 3 (4)) and isothiocyanate Cl(CH2)2NCS (3) with active methylene compounds CH2YY' 1 in the presence of Et3N or Na give 2-YY'-methylene-1,3-oxazolidines, (E,Z)-1,3-thiazolidines, and 1,3-oxazines from 2, 3, and 4, respectively. 2-(Chloromethyl)phenyl isocyanate 8 gives with 1 the corresponding benzo-oxazines. Ethyl 2-isothiocyanatobenzoate 10 gives the corresponding benzothiazolinone, whereas the analogous isocyanate 12 gives noncyclic enols. Ethoxycarbonyl isothiocyanate 14 gives an open-chain thioenol or an enol-thioamide. The cyanoamides CH2(CN)CONHR, R = H, Me, CHPh2, give with Et3N and 2 the bicyclic imidazopyrimidinediones 16, derived from two molecules of 2, but with their preformed Na salt they give the 1,3-oxazolidines. Reaction of cyanoacetamide with 3 in the presence of Na gave a tricyclic triaza(thia)indacene, derived from two molecules of 3. A reaction mechanism involving an initial attack of the anion 1- on the N=C=X (X = O, S) moiety gives an anion 18, which cyclizes intramolecularly and after tautomerization gives the mono-ring heterocycle. With the cyanoamides, the N- site of the ambident ion 18 attacks another molecule of 2 giving the anion 20, which by intramolecular attack on the CN, followed by expulsion of the Cl- gives the bicyclic 16 after tautomerization.     

Total synthesis of [Ψ[C(═S)NH]Tpg4]vancomycin aglycon, [Ψ[C(═NH)NH]Tpg4]vancomycin aglycon, and related key compounds: reengineering vancomycin for dual D-Ala-D-Ala and D-Ala-D-Lac binding

The total synthesis of [Ψ[C(═S)NH]Tpg(4)]vancomycin aglycon (8) and its unique AgOAc-promoted single-step conversion to [Ψ[C(═NH)NH]Tpg(4)]vancomycin aglycon (7), conducted on a fully deprotected substrate, are disclosed. The synthetic approach not only permits access to 7, but it also allows late-stage access to related residue 4 derivatives, alternative access to [Ψ[CH(2)NH]Tpg(4)]vancomycin aglycon (6) from a common late-stage intermediate, and provides authentic residue 4 thioamide and amidine derivatives of the vancomycin aglycon that will facilitate ongoing efforts on their semisynthetic preparation. In addition to early stage residue 4 thioamide introduction, allowing differentiation of one of seven amide bonds central to the vancomycin core structure, the approach relied on two aromatic nucleophilic substitution reactions for formation of the 16-membered diaryl ethers in the CD/DE ring systems, an effective macrolactamization for closure of the 12-membered biaryl AB ring system, and the defined order of CD, AB, and DE ring closures. This order of ring closures follows their increasing ease of thermal atropisomer equilibration, permitting the recycling of any newly generated unnatural atropisomer under progressively milder thermal conditions where the atropoisomer stereochemistry already set is not impacted. Full details of the evaluation of 7 and 8 along with several related key synthetic compounds containing the core residue 4 amidine and thioamide modifications are reported. The binding affinity of compounds containing the residue 4 amidine with the model D-Ala-D-Ala ligand 2 was found to be only 2-3 times less than the vancomycin aglycon (5), and this binding affinity is maintained with the model d-Ala-d-Lac ligand 4, representing a nearly 600-fold increase in affinity relative to the vancomycin aglycon. Importantly, the amidines display effective dual, balanced binding affinity for both ligands (K(a)2/4 = 0.9-1.05), and they exhibit potent antimicrobial activity against VanA resistant bacteria ( E. faecalis , VanA VRE) at a level accurately reflecting these binding characteristics (MIC = 0.3-0.6 μg/mL), charting a rational approach forward in the development of antibiotics for the treatment of vancomycin-resistant bacterial infections. In sharp contrast, 8 and related residue 4 thioamides failed to bind either 2 or 4 to any appreciable extent, do not exhibit antimicrobial activity, and serve to further underscore the remarkable behavior of the residue 4 amidines.     

On the coexistence of the “reactivity-selectivity” and the “constant selectivity” relationships in reactions of carbonium ions with nucleophiles

It is noted that the “reactivity-selectivity” and the “constant selectivity” relationships for the R⁺ + Nu reactions can coexist if diffusion-controlled rates for one nucleophile govern the behaviour at the reactivity-selectivity region.     

Phase shifts in stress-strain relationships of viscoelastic materials

The classical relationships between the elastic constants are extended to apply to viscoelastic materials. It is shown that when such materials are strained in simple tension or compression, volume changes are out of phase with longitudinal and lateral deformations.