THE PHOTOCHEMISTRY OF RIBOFLAVIN—II. POLAROGRAPHIC STUDIES ON THE HYDROXYETHYL AND FORMYLMETHYL ANALOGS OF RIBOFLAVIN*

Abstract— The anaerobic photobleaching of 6, 7-dimethyl-9-(2′-hydroxyethyl)-isoalloxazine, 9-(2′-hydroxyethyl)-isoalloxazine and 6,7-dimethyl-9-(formylmethyl)-isoalloxazine have been compared. Polarographic and spectrophotometric techniques were used for rate studies. The methyl groups on the aromatic ring at the 6 and 7 positions affects the product distributtion in these simple flavins. Formylmethylflavin is postulated to undergo a bimolecular photoredox reaction which is novel to flavin photochemistry. It was demonstrated that there is a common type of leucoflavin formed during anaerobic photobleaching of riboflavin, formylmethylflavin and hydroxyethylflavin. This leucocompound also formed during the photoreduction of formylmethylflavin but not during the photoreduction of the other flavins.     

Enantioselective DNA threading dynamics by phenazine-linked.

The interactions between the stereoisomers of the chiral bis-intercalator [mu-C4(cpdppz)(2)-(phen)(4)Ru(2)](4+) and DNA reveal interesting dynamic discrimination properties. The two enantiomers Delta-Delta and Lambda-Lambda both form very strong complexes with calf thymus DNA with similar thermodynamic affinities. By contrast, they display considerable variations in their binding kinetics. The Delta-Delta enantiomer has higher affinity for calf thymus DNA than for [poly(dA-dT)](2), and the association kinetics of the dimer to DNA, as well as to polynucleotides, requires a multiexponential fitting function. The dissociation reaction, on the other hand, could be described by a single exponential for [poly(dA-dT)](2), whereas two exponentials were required for mixed-sequence DNA. To understand the key mechanistic steps of the reaction, the kinetics was studied at varied salt concentration for different choices of DNA and chirality of the threading complex. The enantiomers were found to have markedly different dissociation rates, the Lambda-Lambda enantiomer dissociating about an order of magnitude faster than the Delta-Delta enantiomer. Also, the salt dependence of the dissociation rate constants differed between the enantiomers, being stronger for the Lambda-Lambda enantiomer than for the Delta-Delta enantiomer. Since the dissociation reaction requires unthreading of bulky parts of the bis-intercalator through the DNA helix, a considerable conformational change of the DNA must be involved, possibly defining the rate-limiting step.     

Chaotic spin-wave solitons in magnetic film feedback rings

Chaotic spin-wave solitons in magnetic film active feedback rings were observed for the first time. At some ring gain level, one observes the self-generation of a single spin-wave soliton pulse in the ring. When the pulse circulates in the ring, its amplitude varies chaotically with time. Numerical simulations based on a gain-loss nonlinear Schr?dinger equation reproduce the observed responses.     

Observation of the h(c)(1P) Using e+ e- collisions above the DD threshold

Using 586 pb(-1) of e+ e- collision data at E(c.m.) = 4170 MeV, produced at the Cornell Electron Storage Ring collider and collected with the CLEO-c detector, we observe the process e+ e- → π+ π- h(c)(1P). We measure its cross section to be 15.6±2.3±1.9±3.0 pb, where the third error is due to the external uncertainty on the branching fraction of ψ(2S) → π0 h(c)(1P), which we use for normalization. We also find evidence for e+ e- → ηh(c)(1P) at 4170 MeV at the 3σ level and see hints of a rise in the e+ e- → π+ π- h(c)(1P) cross section at 4260 MeV.     

Thread Insertion of a Bis(dipyridophenazine) Diruthenium Complex into the DNA Double Helix by the Extrusion of AT Base Pairs and Cross‐Linking of DNA Duplexes

The crystal structure of the Δ,Δ enantiomer of the binuclear “light‐switch” ruthenium complex [μ‐(11,11′‐bidppz)(1,10‐phenanthroline)₄ Ru₂]⁴⁺ bound to the oligonucleotide d(CGTACG) shows that one dppz moiety of the dumbbell‐like compound inserts into the DNA stack through the extrusion of an AT base pair. The second dppz moiety recruits a neighboring DNA molecule, and the complex thus cross‐links two adjacent duplexes by bridging their major grooves.     

DNA‐Binding Properties of Amidine‐Substituted Spiropyran Photoswitches

Two amidine‐substituted spiropyran derivatives have been characterized with respect to the DNA‐binding properties over a broad pH interval. The two derivatives differ in the number of positive charges. By varying the pH, the protonation state of the derivatives is also changed, allowing for additional variations in the charge distribution. We show that the closed spiro isomer does not bind for either of the two derivatives, whereas the open merocyanine forms bind both in the protonated and in the nonprotonated state, but with dramatically different binding constants. Flow‐oriented linear dichroism (LD) measurements also show that there are differences in the binding modes between the various forms. We rationalize these differences in terms of structure and charge distribution.     

Different varieties of massive Dirac neutrinos

The concept of Majorana and Dirac massive neutrinos is discussed for the case in which there are several types, or flavors, of leptons. Different varieties of Dirac neutrinos are possible, distinguised by their magnetic moments and anomalous interactions with probabilities proportional to (m/E)².     

Mechanically manipulating the DNA threading intercalation rate

The dumbbell shaped binuclear ruthenium complex DeltaDelta-P requires transiently melted DNA in order to thread through the DNA bases and intercalate DNA. Because such fluctuations are rare at room temperature, the binding rates are extremely low in bulk experiments. Here, single DNA molecule stretching is used to lower the barrier to DNA melting, resulting in direct mechanical manipulation of the barrier to DNA binding by the ligand. The rate of DNA threading depends exponentially on force, consistent with theoretical predictions. From the observed force dependence of the binding rate, we demonstrate that only one base pair must be transiently melted for DNA threading to occur.     

Complex DNA binding kinetics resolved by combined circular dichroism and luminescence analysis

We recently reported that ruthenium complexes, with general structure [mu-bidppz(bipy)4Ru2](4+) (B) or [mu-bidppz(phen)4Ru2](4+) (P) (bidppz=11,11'-bi(dipyrido[3,2- a:2',3'-c]phenazinyl)), show extreme kinetic selectivity for long AT tracts over mixed-sequence calf thymus DNA (ct-DNA), a selectivity that also varies markedly with the size (between B and P) and sense of chirality of the complex. Earlier studies, exploiting the great increase in luminescence intensity when the compound intercalates, have yielded complex kinetics indicating the presence of both first- and second-order processes. Even with a homogeneous DNA sequence, such as poly(dAdT)2, the luminescence kinetics generally requires more than a single exponential for a satisfactory fit. We here reveal that at least part of the complexity is a result of the extreme sensitivity of the effective quantum yield of the complexes, so that the luminescence trajectories also reflect subtle variations in the environment and binding geometry that the complex is sampling on its path to its final binding site. By monitoring the rearrangement process using circular dichroism (CD), we show that threading of both enantiomers of B and P into poly(dAdT)2 is effectively a monoexponential process, as expected if the compounds are not affecting each other during the intercalation process. Thus, the complex luminescence trajectories may be explained by slow relaxations in the binding geometry (DNA conformation) and associated changes in the environment of the entering complexes. To further disentangle the intriguing features of the threading intercalation kinetics, and how they may depend on the flexibility and size of the ruthenium complexes, we have also designed and studied two new ruthenium complexes, [mu-dtpf(phen)4Ru2](4+) (F) (dtpf=4,5,9,12,16,17,21,25-octaaza-23 H-ditriphenyleno[2,3-b:2,3-h]fluorene), in which the bridging ligand is made totally rigid, and [mu-bidppz([12]aneS4) 2Ru2](4+) (S), which has less bulky, nonaromatic ancillary ligands. The threading of F into poly(dAdT)2, also found to be a monoexponential process, is about 3 times slower than for P, indicating that the flexibility of the bridging ligand is an important factor for the intercalation rate. Surprisingly, in contrast to all other compounds, S requires two exponentials to fit its binding kinetics as monitored by CD. Also surprisingly, in view of the smaller steric bulk, even the fastest phase is roughly 2 times slower for S than for B and P. Thus, not only the size of the ancillary ligand but also other properties that can influence the energy landscape of the threading path are rate-determining factors. With mixed-sequence ct-DNA, threading of B and that of P are both multiphasic processes when monitored with CD as well as with luminescence. The rate constants for threading into ct-DNA show much larger variations between complexes than for poly(dAdT)2, confirming earlier results based on luminescence data.