Kenneth Karlin

Kenneth Karlin

Ira Remsen Professor

Contact Information

Research Interests: Inorganic/Bioinorganic Chemistry

Education: PhD, Columbia University

Department of Chemistry > People > Kenneth Karlin

Kenneth D. Karlin is the Ira Remsen Professor of Chemistry and the previous department Chair at Johns Hopkins University in Baltimore, Maryland, USA. Educated at Stanford University (B.S. 1970) and at Columbia University, New York (Ph.D. 1975), he was a N.A.T.O. postdoctoral fellow at Cambridge University in England before being appointed Assistant Professor of Chemistry at SUNY Albany (Albany, New York, USA) in 1977. He moved to the Johns Hopkins University as professor in 1990. Dr. Karlin is Editor-in-Chief for Progress in Inorganic Chemistry(John Wiley & Sons) and holds or has held advisory or administrative positions with the Society for Biological Inorganic Chemistry (SBIC), the Petroleum Research Fund (PRF) (of the American Chemical Society (ACS)) and the Division of Inorganic Chemistry (DIC) of the ACS, most recently as 2013 DIC Chair (elected). He is also a fellow of the American Association for the Advancement of Science, and winner of a 2009 ACS National Award, the F. Albert Cotton Award in Synthetic Inorganic Chemistry. He has been organizer/chair of a number of international meetings on copper and/or bioinorganic chemistry, the 1998 Metals in Biology Gordon Research Conference and the 1989 International Conference on Bioinorganic Chemistry (ICBIC-4). Dr. Karlin’s bioinorganic research focuses on coordination chemistry relevant to biological and environmental processes, involving copper and/or heme (porphyrin-iron) complexes and their chemistry with molecular oxygen, its reduced derivatives, and nitrogen oxide compounds.

Dr. Karlin's bioinorganic research focuses on coordination chemistry relevant to biological and environmental processes, involving copper or heme (porphyrin-iron) complexes. Of interest are the reactivity with dioxygen (O2), nitrogen oxides (NOx) and organohalide (e.g., R-Cl) pollutants, and metal/ O2 chemistries with organic substrates, DNA and proteins. Essential copper or heme-containing enzymes function in O2-transport, electron transfer, O2-reduction (oxidase activity), biological substrate oxygenation (i.e., oxygenase activity), and reduction of NO2¯, NO or N2O A variety of novel metal sites are observed in proteins, including those with 1 Cu ion plus cofactor, 2 Cu's, a cluster of 3 or 4 Cu's, or porphyrinate-iron/Cu(Fe) centers.

Dr. Karlin's main research activities involves synthetic modeling, i.e. biomimetic chemistry. Research approaches utilized include (a) the rational design and syntheses of ligands allowing formation of appropriate Cu or Fe complexes, (b) synthesis and purification of such compound, (c) elucidation of structure and physical properties using IR, UV-Vis, resonance Raman and EPR spectroscopies, X-ray crystallography, stopped-flow kinetics, magnetic susceptibility and cyclic voltammetry, and (d) reactivity and mechanistic investigations such as in reversible O2-binding by copper complexes, and the oxidation of substrates. The main objective is to establish relevant coordination chemistry of copper or heme-Cu(Fe) systems, to provide a sound basis for deducing biological active site structures, the nature of reactive intermediates, and mechanism. These studies may also provide a rationale for the design of practical O2-carriers, organic oxidation and NOx reduction catalysts, or agents which can dehalogenate R-X pollutants.

Professor Ken Karlin is also part of the recently awarded Collaborative Research in Environmental Molecular Science (CRAEMS) at Johns Hopkins University.

Biomimetic Studies of Copper-Dioxygen Adducts and Chemistry

Copper is a biologically essential trace element, which functions at the active sites of enzymes involved in electron transfer, processing of dioxygen, and in manipulation of certain nitrogen oxides. My group’s interest has focused on chemistry related to the latter two functions. Reversible copper(I)/dioxygen interactions occur in hemocyanin proteins, which are blood dioxygen carriers in molluscs and arthropods. Monooxygenase enzymes “activate” O2, promoting oxygen atom incorporation into biological substrates. Study of these enzymes or models for their action are also of interest in developing practical reagents or catalysts for oxidative transformations. Oxidases couple the reduction of O2 (to either hydrogen peroxide or water), while effecting one or two-electron substrate (e.g., alchohols, amines, other) oxidations. Copper ions also react with reduced dioxygen species such as superoxide (O2-), in Cu-Zn superoxide dismutases, and may also be involved in copper-mediate oxdiative damage in biological media, including possibly in Alzheimer’s disease.

A primary focus in recent years has been the study of reversible dioxygen binding with copper complexes, to elucidate possible structures of Cu-O2species, their associated spectroscopy (UV-Vis, resonance Raman, etc.), and subsequent reactivity. To illustrate both the type of approach we use in our research, and some of the results obtained, it is instructive to provide some details of our work on a compound which provided the first x-ray crystallographic description for a copper-dioxygen adduct, synthetic or protein. [{(TMPA)CuII}2(O2-)]2+ (1), a Cu/O2 2:1 adduct, with PF6- or ClO4- counter-anions, forms by reversible oxygenation of the copper(I) precursor complex [(TMPA)CuI(RCN)]+ (1a, R = Me, Et, TMPA = tris[2-pyridylmethyl]amine). Product (1) is only stable at -80 C in solution or as a solid.
Physical Properties of {[(TMPA)CuII}2(O22-)]2+ (1)

We have shown that the binding of O2 is reversible, and this can be illustrated by the spectroscopic experiments shown, wherein we can remove the bound O2 by application of a vacuum, and cycle back and forth between unoxygenated copper(I) complex (1a; lmax = 340 nm and (1; lmax = 525 nm, 600 nm) .

Models for Heme-Copper Oxidase and Related Enzymes

Heme-copper oxidases, including cytochrome c oxidase (CcO), catalyze the four-electron, four-proton reduction of dioxygen to water, while also ‘pumping’ four protons (per O2 molecule reduced) across the cell membrane. The electrochemical potential gradient generated by this process ultimately provides the driving force for ATP synthesis. The key chemistry for O2-binding, reduction, and coupled proton translocation occurs at the binuclear heme a3- CuB active-site. Recent protein x-ray structures show the copper ion ligated to three imidazole (from histidine) N-donor groups, with the high spin heme a3 being bound to an imidazole from histidine which is distal to copper (Figure).

The goals of our model studies for heme-copper oxidases are to develop chemistry relevant to (i) the O2-binding and reduction chemistry, (ii) the effects of protonation and/or addition of electrons (as in the enzyme), and (iii) the numerous studies carried out on oxidized resting state of the enzyme, which may give Fe-X-Cu species (X = oxide, hydroxide, cyanide, azide, carboxylate).

In our initial attempts to delve into this type of chemistry, we chose to use TMPA-copper complexes, with known O2-chemistry (see above). When (3) and (1a) react with O2 (1/2 equivalent), [(F8-TPP)FeIII-(O2-)-CuII(TMPA)]+ (4) is isolated. This chemistry already represents a crude functional model for CcO, since O2 has been reduced to the oxidation state level of water; isotope labeling studies indicate the bridging oxo group in (4) comes from O2. Interestingly, the acid-base reaction of equimolar amounts of (5), (6), and Et3N gives the same complex, (4).

The x-ray structure of (7) reveals a linear Fe(III)-oxo-Cu(II) coordination (angle of Fe-O-Cu = 178.2?, with very short Fe-O and Cu-O bond lengths. The complex has also been characterized by UV-Vis, IR, Mssbauer, and NMR spectroscopies, as well as magnetic susceptibility measurements. The 1H-NMR spectrum is very rich, with resonances ranging from +65 ppm downfield to -104 ppm upfield. A detailed NMR study has been published (ref. # 146) and the unusual behavior is ascribed to the S = 2 ground state with strong antiferromagnetic coupling between Fe(III) and Cu(II) ions.

Protonation of (4) by addition of one equivalent of triflic acid produces the hydroxo-bridged complex, [(F8-TPP)Fe-(OH)-Cu(TMPA)]2+ (7). Structural insights from x-ray absorption spectroscopy reveal elongated Fe-OH and Cu-OH bonds and a bending such that angle of Fe-O-Cu at 157? Complex (7) is a plausible model for the resting (‘as-isolated) oxidized enzyme, with its supposed Fe-X-Cu antiferromagnetic coupling. Since protonation of enzyme turnover intermediates leads to product water and/or translocated protons, it is of interest that the oxo moiety in (4) is basic; Protonation experiments suggest that pKa ~ 8 ?2.5 in water.

While the dioxygen chemistry and mechanism of formation of (4) from (1a) and (3) is of interest, we have chosen to emphasize related studies on binucleating ligands, where a tetradentate or tridentate donor for copper (or another metal ion) is synthetically tethered to the periphery of a porphyrin. Recent work with the 6L (see Figure) indicates analogous chemistry affords a di-iron species with 6L (ref. 136) and [(6L)FeIII-OH], with empty tether. Reaction of [(6L)FeIII-OH] with a Cu(II) salt in presence of a base results in the formation of the new oxo-bridged complex [(6L)FeIII-O-CuII]+, which has been characterized by UV-Vis and 1H-NMR spectroscopies, mass spectrometry, and X-ray diffraction (see below). Preliminary observations show that a reduced species [(6L)FeII-CuI]+ can be generated, and that its subsequent reaction with O2 leads to [(6L)FeIII-O-CuII]+. [(6L)FeIII-O-CuI]+ is prepared by reduction of [(6L)FeIII-OH] with dithionite followed by addition of a copper(I) salt. It is characterized by lambda(max) = 424 nm, and by 1H-NMR properties consistent with high-spin Fe(II) in tetrahydrofuran (THF) as solvent. Interestingly, when the oxygenation of this reduced compound is carried out at -80 �C in THF, a new spectroscopically distinct species is observed; this cleanly converts to [(6L)FeIII-O-CuII]+ after warming to room-temperature. These observations bode well for future investigations where we wish to probe the possibility of forming Fe-peroxo-Cu or other interesting intermediates which may have relevance to the processes of O2-binding, reduction, O-O cleavage and proton translocation by the Fe-Cu center in cytochrome c oxidases.

Other Research Projects or Activities

A number of other projects have been or are being studied in our laboratories. In some cases, these have arisen as natural extensions of chemistry uncovered from the investigations described above.

Nitrogen-Oxide Cu or Fe Chemistry

We have been off-and-on interested in the chemistry of nitrogen oxides (e.g., NO, N2O, NO2-) (see publications #77, #85, #135), since their chemistry is of environmental significance, nitrite (NO2-) and nitrous oxide (N2O) are natural substrates for copper enzymes (for example in detrifying bacteria), and (as mentioned above), NO(g) is a substrate for a heme/non-heme diiron active site in NO Reductase. Nitric oxide has also been used to probe the active site chemistry of iron and copper enzymes, since NO(g) is considered an O2 surrogate. We have found that NO(g) readily reacts with our copper(I) complexes, with evolution of N2O, and these reactions are being studied mechanistically. Structural, spectrsocopic and rectivity models for NO Reductase are also being investigated. We are also trying to design compounds which may be reactive enough to deoxygenate nitrous oxide, which is kinetically highly inert, but which is known to be reduced at a dicopper enzyme active center in Nitrous Oxide Reductase.

Hydrolysis

The actives sites of a variety of hydrolases, for example enzymes which carry out ester (e.g. phosphate esters) or amide (peptide) hydrolysis reactions, often contain binuclear or trinuclear metal active sites. The metal ion involved may be zinc, manganese, nickel, iron, magnesium, or even copper as a substitute has activity. A key feature of the chemistry appears to be a metal-hydroxide or M-(OH)-M moiety. In our copper-dioxygen chemistry studies, binuclear or trinuclear copper-hydroxide complexes have been generated and characterized. As a consequence, we have studies several examples of dicopper complex mediated hydrolysis, and have found our compounds are capable of hydrolysis of unactived amides (publication #113), esters or even nitriles (publication #144), and carbon dioxide (publication # 108). A novel case occurs when hydrolysis of amides can be effected through direct copper-dioxygen chemistry (#113 ). Continued study of hydrolysis chemistry with synthetically designed multimetal (Zn, Cu, Mn) complexes is planned.

Reactions of Copper Complexes with DNA/Proteins

The study of metal complexes which are capbable of oxidative chemistry and/or hydrolytic activity is of interest with respect to interactions with DNA or proteins. Metal complexes may be utilized as ‘footprinting’ agents to help probe DNA or protein structure, and/or DNA/protein complexes which are important in gene regulation. Copper complexes, especially a Cu-phenanthroline species, has been found to be of considerable biochemical or biotechnological interest. Thus, with the many types of redox and hydrolytically active compounds studied in our group, we have a small but active ongoing effort to survey our complexes in this regard. Initial studies (see publication #138) show that a trinuclear complex we have synthesized is very active in oxidative cleavage of DNA plasmids. Further studies are in progress, aimed at a better mechanistic understanding, discovering possible hydrolytic processes, and extending the investigations to peptides or proteins.

Paramagnetic NMR

Thorough characterization of complexes generated in our group inlucdes characterization of NMR-spectroscopy. Interestingly, a number of dicopper(II) complexes we have studies show very sharp, but paramagnetically shifted proton NMR signals. Ligand-based NMR signals for copper(II) complexes is unusual because of unfavorable relaxation times, but the binuclear nature of the complexes is seen to be responsible (see publication #140). Further investigation will seek to broaden the scope of complex type exhibiting this kind of NMR spectroscopic behavior, and the application to characterization paramagnetic multinuclear copper complexes involved in O2-binding or oxidative chemistry. We have also studied (publication #146) and are continuing to study in some detail the highly interesting NMR-spectroscopic behavior of antiferromagnetically coupled Fe(III)-X-Cu(II) (S = 2) systems, relevant to our heme-copper model program (see above).

Selected Journal Articles

See a full list of publications on Google Scholar.

  • Roman Davydov, Austin E. Herzog, Richard J. Jodts, Kenneth D. Karlin, and Brian M. Hoffman, “End-On Copper(I)-Superoxo and Cu(II)-Peroxo and Hydroperoxo Complexes Generated byCryoreduction/Annealing and Characterized by EPR/ENDOR Spectroscopies” J. Am. Chem. Soc. 2022, 144, 377-389. doi.org/10.1021/jacs.1c10252
  • Sarmistha Bhunia, Atanu Rana, Shabnam Hematian, Kenneth D. Karlin and Abhishek Dey, “Proton Relay in Iron Porphyrins for Hydrogen Evolution Reaction”, Inorg. Chem. 2021, 60, 13876−13887. https://dx.doi.org/10.1021/acs.inorgchem.1c01079. PMCID: PMC8485179
  • Mayukh Bhadra, Wesley J. Transue, Hyeongtaek Lim, Ryan E. Cowley, Jung Yoon C. Lee, Maxime A. Siegler, Patrick Josephs, Gerald Henkel, Markus Lerch, Siegfried Schindler, Adam Neuba, Keith O. Hodgson, Britt Hedman, Edward I. Solomon and Kenneth D. Karlin “A Thioether-Ligated Cupric Superoxide Model with Hydrogen Atom Abstraction Reactivity” J. Am. Chem. Soc. 2021, 143, 3707-3713. https://dx.doi.org/10.1021/jacs.1c00260. PMCID: PMC8023764
  • Bhadra, Mayukh; Karlin, K. D. “Copper Enzymes Involved in Multi-Electron Processes.” In Comprehensive Coordination Chemistry III; Constable, E. C., Parkin, G., Que Jr, L., Eds., Vol. 8, Elsevier, 2021; pp 524–540. https://dx.doi.org/10.1016/B978-0-12-409547-2.14821-8.
  • Kim, Hyun, Rogler, Patrick J.; Sharma, Savita; Schaefer, Andrew, W.; Solomon, Edward I.; Karlin, Kenneth D. “Ferric Heme Superoxide Reductive Transformations to Ferric Heme (Hydro)Peroxide Species: Spectroscopic Characterization and Thermodynamic Implications for H-atom Transfer (HAT)” Angew. Chem. Intl. Ed. 2021, 60, 5907-5912, https://dx.doi.org/10.1002/anie.202013791. PMCID: PMC7920932

Books Edited

  • “Copper Coordination Chemistry: Biochemical and Inorganic Perspectives” Karlin, K. D.; Zubieta, J., Eds.; Adenine Press: Guilderland, N.Y., 1983.
  • “Biological & Inorganic Copper Chemistry” Karlin, K. D.; Zubieta, J., Eds.; Adenine Press: Guilderland, N.Y., 1986; Vol. 1-2.
  • “Bioinorganic Chemistry of Copper”  Karlin, K. D., Tyeklár, Z., Eds., Chapman & Hall:  New York, 1993
  • “Copper-Oxygen Chemistry”, (Eds.: S. Itoh, K. D. Karlin), Wiley-VCH, Hoboken, 2011, 11 chapters, 462 pages. Wiley Series on Reactive Intermediates in Chemistry and Biology, Vol. 5 (Series Ed.: S. E. Rokita).
  • Progress in Inorganic Chemistry, Karlin, K. D., Editor-in-Chief, John Wiley & Sons: N.Y, N.Y.
    • Volume 41, 1994 (9 chapters, 848 pp); Volume 42, 1995 (5 chapters, 606 pp)
    • Volume 43, 1996 (6 chapters, 621 pp); Volume 44, 1996 (7 chapters, 421 pp)
    • Volume 45, 1997 (6 chapters, 510 pp); Volume 46, 1997 (4 chapters, 488 pp)
    • Volume 47, 1998 (8 chapters, 978 pp): Volume 48, 1999 (6 chapters, 603 pp)
    • Volume 49, 2001 (6 chapters, 700 pp): Volume 50, 2001 (8 chapters, 631 pp)
    • Volume 51, 2003 (5 chapters, 640 pp); Volume 52, 2004 (11 chapters, 738 pp)
    • Volume 53, 2005 (2 chapters, 602 pp): Volume 54, 2005, (7 chapters, 535 pp)
    • Volume 55, 2007 (9 chapters, 759 pp): Volume 56, 2009 (6 chapters, 568 pp);
    • Volume 57, 2011 (9 chapters, 615 pp); Volume 58, 2014 (6 chapters, 517 pp);
    • Volume 59, 2014 (7 chapters, 583 pp)
  • Jeff’s article on the first example of direct rRaman characterization of copper peroxynitrito complex is online in Angewandte Chemie! Congratulations Dr Jeffrey J. Liu and our collaborator Dr Pierre Moenne-Loccoz @OHSU!
  • Read our latest article in JACS on new isoporphyrin system as model for cofactor biogenesis of CcO! Congratulations to Dr Melanie Ehudin and the Ivanovic group!
  • Congratulations to Dr Patrick Rogler for accepting a position at Intel Corporation!
  • Congratulations to Dr Melanie Ehudin and the whole team of collaborators for a detailed analysis of heme-oxo models in presence of lewis acids and axial bases! Read the fantastic article in JACS!
  • Read our latest article in JACS on temperature and H-bonding dependent spin crossover in heme-copper peroxo systems, in collaboration with the Solomon lab! Congratulations Melanie and Andy!
  • Congratulations to Dr Daniel Diaz for accepting a postdoctoral position at the Walton lab!
  • Best wishes for Dr Jeffrey J. Liu for starting his postdoctoral work at the Marinescu lab!

Notable achievements in our labs include:

  1. the detailed chemical (e.g., spectroscopy, kinetics of formation) and structural determination of a several types of copper-dioxygen adducts, including the trans µ-1,2-peroxodicopper(II) complex shown.
  2. the generation of model systems for cytochrome c oxidase enzyme chemistry, i.e., reactions of molecular oxygen with reduced porphryin Fe(II)-copper(I); the products are adducts formulated as iron(III)-peroxide-copper(II) species. The dynamics (kinetics-thermodynamics of O2 reaction, structures and detailed spectroscopic properties have or are being deduced. Recent efforts also include taking such heme-peroxo-copper complexes and modeling the next step of the enzyme reaction, reductive O-O cleavage.
  3. the demonstration of multi-copper complex (i.e., those with two or three proximate copper ions) site-specific oxidative cleavage near the junction of double and single-stranded DNA (diagrams).   The specificity and type of DNA oxidation (i.e., sugar moiety versus nuclobase cleavage) varies as the nature of the copper complex.  Thus copper-dioxygen derived complexes (e.g., peroxo-dicopper(II), hydroperoxo-dicopper(II), others (?)) are able to effect oxidation of substrates which come in close contact with these reactive species. A range of organic substrates can be oxidized and mechanistic studies have or are being carried out.

Recent activities center on:

  1. studies of copper-dioxygen chemistry in order to elucidate characteristics of peroxo-dicopper(II) and bis-µ-oxo dicopper(III) complexes, the nature of the equilibrium interconverting them, searching for possible differential reactivity, and examining and probing for new highly reactive higher-valent copper-oxo  species.
  2. developing new copper ion peptide chemistry, using amino acids and peptide sequences which are relevant to copper protein active sites; specific structural motifs will be examined. The research is also aimed to study peptides which bind copper and which have been implicated to be toxic (and effect biological oxidative damage) in Alzheimer’s disease states.
  3. reactions of copper ion complexes with elemental sulfur, to generate new copper-sulfide species. A reduced tetracopper(I)-sulfide complex facilitates reduction of nitrous oxide in Nature. Newly synthesized dicopper(II)-disulfide complexes have been characterized and (e.g., see diagram) and are being used as synthons in the generation of new compounds with copper-sulfide moieties for N2O reactivity studies.
  4. the further reactions of O²-adducts in heme-copper environments, i.e., iron(III)-peroxo-copper(II) complexes see above), with electron/proton sources. The key step in the cytochrome c oxidase reaction with molecular oxygen is the reductive O-O scission reaction; this step is fundamental to all forms of ‘oxygen-activation’ wherein molecular oxygen is made to be more reactive than it would be otherwise (i.e., towards substrate oxidation) by interaction with reduced metal ions. We have recently achieved O-O cleavage in heme copper chemistry (diagram) and are probing the mechanisms of such reactions.
  5. Nitrogen monoxide (NO; previously usually referred to as nitric oxide) is an important molecule, synthesized in Nature as a signaling agent whose action is initiated by its binding to a metalloenzyme active site. It also is processed as one of a series of nitrogen oxide electron acceptors in dentrifying bacteria. The enzyme NO reductase (NOR) takes two molecules of NO and reduces them to nitrous oxide N2O; the active site for this enzyme is evolutionarily related to cytochrome c oxidase, and possesses a non-heme iron (instead of copper) in close proximity to a porphyrin-iron moiety. As such, we are carrying out studies on NO-reactions with new heme/non-heme diiron complexes, to provide fundamental information concerning NO-binding in such an environment. A major goal is to develop systems which can couple NO molecules and study the mechanisms of such biomimetic reactions. Peroxynitrite (ONOO¯) is a very toxic highly oxidizing and nitrating species which forms readily from the reaction of NO and O2¯ (superoxide). It is seen to have a critical and deleterious role in biological processes. We are therefore studying its possible formation via metal ion mediated chemistry, i.e., M-NO + O2 ––> peroxynitrite-M, or conversely, M-O2 + NO  ––> peroxynitrite-M, processes which may occur in biological systems. Very recently, with either copper or iron complexes, we have been able to demonstrate the viability of such reactions; metal-peroxynitrite complexes we have generated are capable of either oxidation (e.g., hydroxylation) or nitration reactions (diagram). The scope of such chemistry and mechanisms involved are being investigated.