The qualitative solvent- and temperature-dependent conformational behavior of
a peptide in the proximity of solid substrates with different adsorption
properties is investigated by means of a simple lattice model. The resulting
pseudophase diagrams exhibit a complex structure, which can be understood by
analysing the minima of the free-energy landscape in dependence of appropriate
system parameters.Comment: 3 pages, 1 figur

Bachmann, Michael

Janke, Wolfhard

English

arXiv

Bachmann, M.

Janke, W.

Juelich Shared Electronic Resources

John von Neumann Institute for ComputingAdsorption Phenomena at Organic-InorganicInterfacesMichael Bachmann, Wolfhard Jankepublished inNIC Workshop 2006,From Computational Biophysics to Systems Biology,Jan Meinke, Olav Zimmermann,Sandipan Mohanty, Ulrich H.E. Hansmann (Editors)John von Neumann Institute for Computing, Ju¨lich,NIC Series, Vol. 34, ISBN-10: 3-9810843-0-6,ISBN-13: 978-3-9810843-0-6, pp. 95-98 , 2006.c© 2006 by John von Neumann Institute for ComputingPermission to make digital or hard copies of portions of this work forpersonal or classroom use is granted provided that the copies are notmade or distributed for profit or commercial advantage and that copiesbear this notice and the full citation on the first page. To copy otherwiserequires prior specific permission by the publisher mentioned above.http://www.fz-juelich.de/nic-series/volume34Adsorption Phenomena at Organic-Inorganic InterfacesMichael Bachmann and Wolfhard JankeInstitut fu¨r Theoretische Physik and Centre for Theoretical Sciences (NTZ),Universita¨t Leipzig, 04109 Leipzig, GermanyE-mail: {bachmann, janke}@itp.uni-leipzig.deThe qualitative solvent- and temperature-dependent conformational behavior of a peptide inthe proximity of solid substrates with different adsorption properties is investigated by meansof a simple lattice model. The resulting pseudophase diagrams exhibit a complex structure,which can be understood by analysing the minima of the free-energy landscape in dependenceof appropriate system parameters.1 IntroductionThe interest in understanding adsorption of polymers and peptides to inorganic materialhas grown recently with the development of nanotechnological equipment that enablespresent and future applications ranging from nanoelectronic devices [as, for example, thecelebrated organic light-emitting diodes (OLEDs)] to pattern-recognising nanosensors inbiomedicine. In recent experiments, evidence for substrate specificity of synthetic peptideadsorption to semiconductor substrates has been found.1 The investigation of organic-inorganic interfaces is connected with the substantial problem of setting up an effectivemicroscopic model for the adsorption of organic to inorganic matter in addition to theintrinsic interactions governing the folding properties of the organic substance. Van derWaals based interaction models2 were employed, e.g., in studies of gold-binding pep-tides.3 Here, we study a minimalistic lattice model, where the folding part is governedby the hydrophobic-polar (HP) model,4 and the interaction with the substrate is restrictedto monomer-type dependent surface-layer attraction.5, 62 Conformational Transitions in a Simple Hybrid Interface ModelBased on the assumption that peptide folding is mainly due to the hydrophobic effect, wedistinguish only hydrophobic (H) and polar residues (P ). According to the minimalisticHP model4, the intramolecular energy of a conformation is related to the number of hy-drophobic nearest-neighbor contacts nHH. For the study of specificity of surface–binding,we investigate three attractive substrate models: (a) type-independent attractive, (b) hy-drophobic, and (c) polar substrate. In case (a), the energy of the system is proportional tothe total number of monomer–surface contacts, nH+Ps , while in the cases (b) and (c) the re-spective hydrophobic (nHs ) and polar (nPs ) surface contacts are energetically favored. Thegeneric model can be defined by E(ns, nHH) = −ns − snHH, where, depending on thesubstrate model, ns = nH+Ps , nHs , or nPs . The parameter s controls the solvent quality (thelarger the value of s, the worse the solvent). The contact numbers ns and nHH are naturalsystem parameters that allow the discrimination of conformational pseudophases. In ourinvestigations of this system, we employed the contact-density chain-growth method which95Figure 1. Contact-number map of all free-energy minima for an exemplified peptide with 103 monomers5 and asubstrate which is equally attractive to all monomers. Circles correspond to minima of the contact free energyFT,s(nH+Ps , nHH) in the parameter space T ∈ [0, 10], s ∈ [−2, 10]. Lines illustrate how the contact freeenergy changes with the temperature at constant values of the solvent parameter s.has successfully been applied in adsorption studies of polymers.8 This method, which is asuitably generalized variant of the multicanonical chain-growth algorithm,7 allows us to es-timate the contact density g(ns, nHH), which simply counts the number of conformationsfor a given pair of contact numbers ns and nHH. For fixed temperature T and solubilitys, we define the contact free energy FT,s(ns, nHH) ∼ −T ln g(ns, nHH) exp(−Es/T ).Assuming that the minimum of the free-energy landscape FT,s(n(0)s , n(0)HH) is related to theclass of macrostates with n(0)s surface and n(0)HH hydrophobic contacts, this class dominatesthe pseudophase the system resides in. For this reason, it is instructive to calculate all min-ima of the contact free energy and to determine the associated contact numbers in a widerange of values for the external parameters. This can easily be done with the knowledge ofg(ns, nHH) by simple reweighting.In Fig. 1, the map of all possible free-energy minima (circles) in the range of exter-nal parameters T ∈ [0, 10] and s ∈ [−2, 10] is shown for a peptide with 103 monomers(37 hydrophobic, 66 polar) in the vicinity of a substrate that is equally attractive for bothhydrophobic and polar monomers.5, 6 There are a few regions of circles which can be asso-ciated with pseudophases. Clearly, where the number of surface contacts nH+Ps is zero, theheteropolymer is desorbed and in the bulk regime two main pseudophases can be identified:desorbed extended (DE) and desorbed collapsed (DC) hydrophobic-core conformations.In case of adsorption, we distinguish also extended (AE) and compact (AC1-3) conforma-tions, and an intermediate globular pseudophase (AG), where conformations are globallycompact, but do not exhibit pronounced hydrophobic domains. The discrimination of thethree AC subphases is due to layering: In dependence of the solvent quality, hydrophobicdomains form layers: single-layer domains in AC1 (good solvent, substrate contact favoredprior formation of intrinsic hydrophobic contacts), and respective hydrophobic double- andtriple-layer cores in AC2 and AC3. Solid lines in Fig. 1 visualize “paths” through the96free-energy landscape when changing temperature, but leaving the solvent parameter un-changed (s = const). Recalling that larger s values correspond to worse quality of thesolvent, it is obvious that the s = 6 path at low temperatures starts in the highly compactAC3 subphase. Increasing the temperature, the monomers reorder and the system entersfor entropic reasons the globular pseudophase AG. Approaching T ≈ 2.3 a noticeablefirst-order-like unbinding transition occurs towards pseudophase DC, before the compactstructure decays (DE). Under very good solvent conditions as, e.g., for the trajectory ofs = 1, the favored conformations at low temperatures are film-like. Increasing the temper-ature slightly to T ≈ 0.2, the system crosses over to the AE pseudophase. This subphase isvery stable under these solvent conditions, but close to T ≈ 2.3 the heteropolymer desorbsinto the dissolved bulk phase DE.For the other two substrates, the pseudophase diagrams look quantitatively differentbut most of the main pseudophases are also present in these cases.53 Concluding RemarksBeside the expected adsorbed and desorbed phases, hybrid organic-inorganic systems ex-hibit a rich subphase structure in the adsorbed phases with compact hydrophobic domains.In this region, the binding behavior is strongly substrate-specific and depends in detailon the quality of the solvent. Since current experimental equipment can reveal molecularstructures at the nanometer scale, it should be possible to investigate the grafted structuresin dependence of the solvent quality. Such experiments would be essential for studies ofbinding forces that are strong enough to refold peptides or proteins.AcknowledgmentsThis work is partially supported by the DFG (German Science Foundation) under GrantNo. JA 483/24-1 and the computer time Grant No. hlz11 of the John von Neumann Institutefor Computing (NIC), Forschungszentrum Ju¨lich.References1. S. R. Whaley, D. S. English, E. L. Hu, P. F. Barbara, and A. M. Belcher, Nature(London) 405, 665 (2000); K. Goede, P. Busch, and M. Grundmann, Nano Lett. 4,2115 (2004).2. R. Hentschke, Macromol. Theory Simul. 6, 287 (1997).3. R. Braun, M. Sarikaya, and K. Schulten, J. Biomater. Sci. Polym. Ed. 13, 747 (2002).4. K. A. Dill, Biochemistry 24, 1501 (1985).5. M. Bachmann and W. Janke, Phys. Rev. E 73, 020901(R) (2006);6. M. Bachmann and W. Janke, Chain-growth simulations of lattice-peptide adsorptionto attractive substrates, in: Proceedings of the NIC Symposium 2006, John von Neu-mann Institute for Computing, Ju¨lich, NIC Series vol. 32, ed. by G. Mu¨nster, D. Wolf,M. Kremer (NIC, Ju¨lich, 2006), p. 245.7. M. Bachmann and W. Janke, Phys. Rev. Lett. 91, 208105 (2003).8. M. Bachmann and W. Janke, Phys. Rev. Lett. 95, 058102 (2005); Phys. Rev. E 73,041802 (2006).97

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