Induction of Immune Responses and Phosphatidylserine Exposure by TLR9 Activation Results in a Cooperative Antitumor Effect with a Phosphatidylserine-targeting Prodrug

Head and neck cancer is a major cancer type, with high motility rates that reduce the quality of life of patients. Herein, we investigated the effectiveness and mechanism of a combination therapy involving TLR9 activator (CpG-2722) and phosphatidylserine (PS)-targeting prodrug of SN38 (BPRDP056) in a syngeneic orthotopic head and neck cancer animal model. The results showed a cooperative antitumor effect of CpG-2722 and BPRDP056 owing to their distinct and complementary antitumor functions. CpG-2722 induced antitumor immune responses, including dendritic cell maturation, cytokine production, and immune cell accumulation in tumors, whereas BPRDP056 directly exerted cytotoxicity toward cancer cells. We also discovered a novel function and mechanism of TLR9 activation, which increased PS exposure on cancer cells, thereby attracting more BPRDP056 to the tumor site for cancer cell killing. Killed cells expose more PS in tumor for BPRDP056 targeting. Tumor antigens released from the dead cells were taken up by antigen-presenting cells, which enhanced the CpG-272-promoted T cell-mediated tumor-killing effect. These form a positive feed-forward antitumor effect between the actions of CpG-2722 and BPRDP056. Thus, the study findings suggest a novel strategy of utilizing the PS-inducing function of TLR9 agonists to develop combinational cancer treatments using PS-targeting drugs.

Rejuvenating hepatic tumor microenvironment immunity with a phosphatidylserine-targeting small molecule drug conjugate

We report the design, synthesis and evaluation of a class of phosphatidylserine-targeting zinc (II) dipicolylamine drug conjugates and show that conjugate 16b elicits immune cell infiltration and remodels the "cold" hepatic tumor microenvironment to the inflamed "hot" tumor. Structure-property relationship study via linker modifications and subsequent pharmacokinetics profiling were carried out to improve the solubility and stability of the conjugates in vivo. In a spontaneous hepatocellular carcinoma mouse model, we showed that conjugate 16b exhibited better antitumor efficacy than sorafenib. In particular, significant increase of CD8⁺ T cell infiltration and granzyme B level was observed, providing insights in sensitizing tumors from intrinsic immune suppressive microenvironment. Evaluation of tumor inflammation-related mRNA expression profile revealed that conjugate 16b, through inductions of key gene expressions including STAT1, CXCL9, CCL5, and PD-L1, rejuvenated tumor microenvironment with enhancement in T cell-, macrophage-, NK cell-, chemokines and cytokines'- functions. Our study establishes that an apoptosis-targeting theranostic enables enrichment of multifaceted immune cells into the tumor mass, which provides potential therapeutic strategies in the combination with immune checkpoint blockade treatment.

Chemoproteomic profiling reveals cellular targets of nitro-fatty acids

Nitro-fatty acids are a class of endogenous electrophilic lipid mediators with anti-inflammatory and cytoprotective effects in a wide range of inflammatory and fibrotic disease models. While these beneficial biological effects of nitro-fatty acids are mainly attributed to their ability to form covalent adducts with proteins, only a small number of proteins are known to be nitro-alkylated and the scope of protein nitro-alkylation remains undetermined. Here we describe the synthesis and application of a clickable nitro-fatty acid probe for the detection and first global identification of mammalian proteins that are susceptible to nitro-alkylation. 184 high confidence nitro-alkylated proteins were identified in THP1 macrophages, majority of which are novel targets of nitro-fatty acids, including extended synaptotagmin 2 (ESYT2), signal transducer and activator of transcription 3 (STAT3), toll-like receptor 2 (TLR2), retinoid X receptor alpha (RXRα) and glucocorticoid receptor (NR3C1). In particular, we showed that 9-nitro-oleate covalently modified and inhibited dexamethasone binding to NR3C1. Bioinformatic analyses revealed that nitro-alkylated proteins are highly enriched in endoplasmic reticulum and transmembrane proteins, and are overrepresented in lipid metabolism and transport pathways. This study significantly expands the scope of protein substrates targeted by nitro-fatty acids in living cells and provides a useful resource towards understanding the pleiotropic biological roles of nitro-fatty acids as signaling molecules or as multi-target therapeutic agents.

BPRDP056, a novel small molecule drug conjugate specifically targeting phosphatidylserine for cancer therapy

Zinc(II)-dipicolylamine (Zn-DPA) has been shown to specifically identify and bind to phosphatidylserine (PS), which exists in bulk in the tumor microenvironment. BPRDP056, a Zn-DPA-SN38 conjugate was designed to provide PS-targeted drug delivery of a cytotoxic SN38 to the tumor microenvironment, thereby allowing a lower dosage of SN38 that induces apoptosis in cancer cells. Micro-Western assay showed that BPRDP056 exhibited apoptotic signal levels similar to those of CPT-11 in the treated tumors growing in mice. Pharmacokinetic study showed that BPRDP056 has excellent systemic stability in circulation in mice and rats. BPRDP056 is accumulated in tumors and thus increases the cytotoxic effects of SN38. The in vivo antitumor activities of BPRDP056 have been shown to be significant in subcutaneous pancreas, prostate, colon, liver, breast, and glioblastoma tumors, included an orthotopic pancreatic tumor, in mice. BPRDP056 shrunk tumors at a lower (~20% only) dosing intensity of SN38 compared to that of SN38 conjugated in CPT-11 in all tumor models tested. A wide spectrum of antitumor activities is expected to treat all cancer types of PS-rich tumor microenvironments. BPRDP056 is a first-in-class small molecule drug conjugate for cancer therapy.

Abstract 227: BPRDP056, a novel small molecule drug conjugate specifically targeting phosphatidylserine for cancer therapy

Zinc(II)-dipicolylamine (Zn-DPA) have been described to specifically complex with phosphatidylserine (PS), which has a higher level at the external surface of cancer cells in tumorigenic condition. BPRDP056 is a Zn-DPA-SN38 conjugate designed to provide a PS-targeting drug delivery of cytotoxic payload SN-38 at the tumor microenvironment, thereby, decrease the dosage of SN38, while induce apoptosis in cancer cells. The in vivo therapeutic efficacy of BPRDP056 against the growths of human tumors has been shown significant in mice subcutaneously bearing a tumor type of pancreas, prostate, colon, liver, breast and glioblastoma, as well as in mice with an orthotopic pancreatic tumor. BPRDP056 shrunk tumors at a lower dosing intensity (~20%) of SN38 compared to CPT-11 in all models tested. Micro-Western assays showed that BPRDP056 exhibited apoptotic cell death signal levels similar to those of CPT-11 in the treated tumors in mice. Furthermore, pharmacokinetic and preliminary toxicology studies showed that BPRDP056 has a good stability in circulation with an acceptable therapeutic safety window in mice. BPRDP056 has been demonstrated with a tumor targeting ability and thus increases the cytotoxic payload SN38 concentration in situ for improved efficacy. Its therapeutic spectrum against malignant neoplasm will be expected to cover the PS-rich tumor microenvironment of all cancer types. BPRDP056 is a first-in-class Small Molecule Drug Conjugate for anti-cancer therapy.

Citation Format: Chia-Yu Hsu, Yun-Yu Chen, Chen-Fu Lo, Tai-Yu Chiu, Ching-Ping Chen, Chen-Lung Huang, Chung-Yu Huang, Min-Hsien Wang, Yu-Sheng Chao, Joe C. Shih, Teng-Kuang Yeh, Lun K. Tsou, Chiung-Tong Chen. BPRDP056, a novel small molecule drug conjugate specifically targeting phosphatidylserine for cancer therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 227.

Linker Optimization and Therapeutic Evaluation of Phosphatidylserine-Targeting Zinc Dipicolylamine-based Drug Conjugates

We report that compound 13, a novel phosphatidylserine-targeting zinc(II) dipicolylamine drug conjugate, readily triggers a positive feedback therapeutic loop through the in situ generation of phosphatidylserine in the tumor microenvironment. Linker modifications, pharmacokinetics profiling, in vivo antitumor studies, and micro-Western array of treated-tumor tissues were employed to show that this class of conjugates induced regeneration of apoptotic signals, which facilitated subsequent recruitment of the circulating conjugates through the zinc(II) dipicolylamine-phosphatidylserine association and resulted in compounding antitumor efficacy. Compared to the marketed compound 17, compound 13 not only induced regressions in colorectal and pancreatic tumor models, it also exhibited at least 5-fold enhancement in antitumor efficacy with only 40% of the drug employed during treatment, culminating in a >12.5-fold increase in therapeutic potential. Our study discloses a chemically distinct apoptosis-targeting theranostic, with built-in complementary functional moieties between the targeting module and the drug mechanism to expand the arsenal of antitumor therapy.

Development of Stem-Cell-Mobilizing Agents Targeting CXCR4 Receptor for Peripheral Blood Stem Cell Transplantation and Beyond

The function of the CXCR4/CXCL12 axis accounts for many disease indications, including tissue/nerve regeneration, cancer metastasis, and inflammation. Blocking CXCR4 signaling with its antagonists may lead to moving out CXCR4⁺ cell types from bone marrow to peripheral circulation. We have discovered a novel series of pyrimidine-based CXCR4 antagonists, a representative (i.e., 16) of which was tolerated at a higher dose and showed better HSC-mobilizing ability at the maximal response dose relative to the approved drug 1 (AMD3100), and thus considered a potential drug candidate for PBSCT indication. Docking compound 16 into the X-ray crystal structure of CXCR4 receptor revealed that it adopted a spider-like conformation striding over both major and minor subpockets. This putative binding mode provides a new insight into CXCR4 receptor-ligand interactions for further structural modifications.

Targeting Tumor Associated Phosphatidylserine with New Zinc Dipicolylamine-Based Drug Conjugates

A series of zinc(II) dipicolylamine (ZnDPA)-based drug conjugates have been synthesized to probe the potential of phosphatidylserine (PS) as a new antigen for small molecule drug conjugate (SMDC) development. Using in vitro cytotoxicity and plasma stability studies, PS-binding assay, in vivo pharmacokinetic studies, and maximum tolerated dose profiles, we provided a roadmap and the key parameters required for the development of the ZnDPA based drug conjugate. In particular, conjugate 24 induced tumor regression in the COLO 205 xenograft model and exhibited a more potent antitumor effect with a 70% reduction of cytotoxic payload compared to that of the marketed irinotecan when dosed at the same regimen. In addition to the validation of PS as an effective pharmacodelivery target for SMDC, our work also provided the foundation that, if applicable, a variety of therapeutic agents could be conjugated in the same manner to treat other PS-associated diseases.

Epigallocatechin-3-gallate inhibits bacterial virulence and invasion of host cells

Increasing antibiotic resistance and beneficial effects of host microbiota has motivated the search for anti-infective agents that attenuate bacterial virulence rather than growth. For example, we discovered that specific flavonoids such as baicalein and quercetin from traditional medicinal plant extracts could attenuate Salmonella enterica serovar Typhimurium type III protein secretion and invasion of host cells. Here, we show epigallocatechin-3-gallate from green tea extracts also inhibits the activity of S. Typhimurium type III protein effectors and significantly reduces bacterial invasion into host cells. These results reveal additional dietary plant metabolites that can attenuate bacterial virulence and infection of host cells.

Bromomethylthioindole Inspired Carbazole Hybrids as Promising Class of Anti-MRSA Agents

Series of N-substituted carbazole analogues bearing an indole ring were synthesized as anti-methicillin-resistant Staphylococcus aureus (MRSA) agents from a molecular hybridization approach. The representative compound 19 showed an MIC = 1 μg/mL against a panel of MRSA clinical isolates as it possessed comparable in vitro activities to that of vancomycin. Moreover, compound 19 also exhibited MIC = 1 μg/mL activities against a recent identified Z172 MRSA strain (vancomycin-intermediate and daptomycin-nonsusceptible phenotype) and the vancomycin-resistant Enterococcus faecalis (VRE) strain. In a mouse model with lethal infection of MRSA (4N216), a 75% survival rate was observed after a single dose of compound 19 was intravenously administered at 20 mg/kg. In light of their equipotent activities against different MRSA isolates and VRE strain, the data underscore the importance of designed hybrid series for the development of new N-substituted carbazoles as potential anti-MRSA agents.

Antibacterial Flavonoids from Medicinal Plants Covalently Inactivate Type III Protein Secretion Substrates

Traditional Chinese Medicines (TCMs) have been historically used to treat bacterial infections. However, the molecules responsible for these anti-infective properties and their potential mechanisms of action have remained elusive. Using a high-throughput assay for type III protein secretion in Salmonella enterica serovar Typhimurium, we discovered that several TCMs can attenuate this key virulence pathway without affecting bacterial growth. Among the active TCMs, we discovered that baicalein, a specific flavonoid from Scutellaria baicalensis, targets S. Typhimurium pathogenicity island-1 (SPI-1) type III secretion system (T3SS) effectors and translocases to inhibit bacterial invasion of epithelial cells. Structurally related flavonoids present in other TCMs, such as quercetin, also inactivated the SPI-1 T3SS and attenuated S. Typhimurium invasion. Our results demonstrate that specific plant metabolites from TCMs can directly interfere with key bacterial virulence pathways and reveal a previously unappreciated mechanism of action for anti-infective medicinal plants.

Abstract 4413: Zinc-dipicolylamine directed pharmaceutical delivery system (ZAPS) as an innovative cancer drug delivery platform

Drug delivered by conjugate with antibody is expected to increase drug concentrations at the tumor sites for improved antitumor effects. Showing some successes, the antibody-drug conjugate system has certain limitations such as difficulties in protein characterization, high cost and individual variations in expression of the epitope, which might abrogate the antibody recognition and cause premature drug release arising from antibody-drug linker instability and thus ineffectiveness. Phosphatidylserine (PS) exists in bulk amount in the tumor microenvironment. Studies showed that a small sized Zinc (II)-dipicolylamine (Zn-DPA) molecule was superior in locating PS surrounding the apoptotic sites in vivo. Although Zn-DPA has been constructed incorporating fluorescent dyes as optical tools for imaging the PS-exposed cell membranes to identify tumor sites from healthy cells in animals, drug conjugates with Zn-DPA has never been described. We aimed to design, synthesize and evaluate a Zinc-dipicolylAmine directed Pharmaceutical delivery System (ZAPS), a platform for spatial- and temporal-release of drug specifically at the targeted tumor site. Zn-DPA was conjugated through standard coupling conditions with various linkers to SN-38 and the Zn-DPA-linker-SN-38 conjugates were obtained and investigated for structure-activity relationships on the chemical stability in plasma and cytotoxicity against cancer cells. Promising ZAPS-SN-38 conjugates were evaluated for activities against tumor growths in nude mice. CPT-11 and SN-38 were included for comparisons. ZAPS-SN-38 conjugates were examined for tolerable doses in mice. Among the ZAPS-SN-38 conjugates synthesized, ZAPS001 was found chemically stable in mouse plasma and in vitro active against several cancer cells. ZAPS001 dose-dependently inhibited the growth of Colo205 tumors in nude mice. Zn-DPA-linker001 (ZAPS001 without SN-38) showed no inhibition effect on the tumor growth. Intriguingly, employing only 40% of the SN-38 delivered by CPT-11 (40 mg/kg), ZAPS001 resulted in 8-fold increase in antitumor activity compared to that of CPT-11 given at the same dose regimen. Furthermore, ZAPS001 is also active against pancreatic Mia-Paca2 and BxPC-3 tumors in mice. An increased SN-38 level delivered by ZAPS001 to the tumors in mice was also observed. ZAPS is thus capable of selectively associating with PS-exposing tumor cells/tissues to achieve site-specific delivery of anticancer agents. Our findings strongly support that ZAPS conferred an “in situ dose amplification” effect, i.e., the cytotoxics-induced PS exposure could specifically recruit more ZAPS-drug conjugates to the tumor site and further enhance its therapeutic effect. ZAPS provides spatial and temporal controls to increase drug concentration at targeted disease sites, reduce drug dosage, lessen the toxic side effects, and thus increase therapeutic index of the anticancer drugs.

Citation Format: Lun K. Tsou, Yu-Wei Liu, Yun-Yu Chen, Chen-Fu Lo, Teng-Kuang Yeh, Chien-Huang Wu, Kak-Shan Shia, Joe C. Shih, Brian D. Gary, Koon Y. Pak, Chiung-Tong Chen. Zinc-dipicolylamine directed pharmaceutical delivery system (ZAPS) as an innovative cancer drug delivery platform. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4413. doi:10.1158/1538-7445.AM2015-4413

Stem cell mobilizers targeting chemokine receptor CXCR4: renoprotective application in acute kidney injury

We have discovered a novel series of quinazoline-based CXCR4 antagonists. Of these, compound 19 mobilized CXCR4(+) cell types, including hematopoietic stem cells and endothelial progenitor cells, more efficiently than the marketed 1 (AMD3100) with subcutaneous administration at the same dose (6 mg/kg) in mice. This series of compounds thus provides a set of valuable tools to study diseases mediated by the CXCR4/SDF-1 axis, including myocardial infarction, ischemic stroke, and cancer metastasis. More importantly, treatment with compound 19 significantly lowered levels of blood urea nitrogen and serum creatinine in rats with renal ischemia-reperfusion injury, providing evidence for its therapeutic potential in preventing ischemic acute kidney injury. CXCR4 antagonists such as 19 might also be useful to increase circulating levels of adult stem cells, thereby exerting beneficial effects on damaged and/or inflamed tissues in diseases that currently are not treated by standard approaches.

Function-oriented development of CXCR4 antagonists as selective human immunodeficiency virus (HIV)-1 entry inhibitors

Motivated by the pivotal role of CXCR4 as an HIV entry co-receptor, we herein report a de novo hit-to-lead effort on the identification of subnanomolar purine-based CXCR4 antagonists against HIV-1 infection. Compound 24, with an EC50 of 0.5 nM against HIV-1 entry into host cells and an IC50 of 16.4 nM for inhibition of radioligand stromal-derived factor-1α (SDF-1α) binding to CXCR4, was also found to be highly selective against closely related chemokine receptors. We rationalized that compound 24 complementarily interacted with the critical CXCR4 residues that are essential for binding to HIV-1 gp120 V3 loop and subsequent viral entry. Compound 24 showed a 130-fold increase in anti-HIV activity compared to that of the marketed CXCR4 antagonist, AMD3100 (Plerixafor), whereas both compounds exhibited similar potency in mobilization of CXCR4(+)/CD34(+) stem cells at a high dose. Our study offers insight into the design of anti-HIV therapeutics devoid of major interference with SDF-1α function.

Small molecules aimed at type III secretion systems to inhibit bacterial virulence

The development of new anti-bacterial compounds presents a major challenge to modern medicine as bacterial strains resistant to traditional antibiotics are constantly emerging.

Blocking HIV-1 entry by a gp120 surface binding inhibitor

We report the mode of action of a proteomimetic compound that binds to the exterior surface of gp120 and blocks HIV-1 entry into cells. Using a one cycle time-of-addition study and antibody competition binding studies, we have determined that the compound blocks HIV-1 entry through modulation of key protein-protein interactions mediated by gp120. The compound exhibits anti-HIV-1 replication activities against several pseudotype viruses derived from primary isolates and the resistant strains isolated from existing drug candidates with equal potency. Together, these data provide evidence that the proteomimetic compound represents a novel class of HIV-1 viral entry inhibitor that functions through protein surface recognition in analogy to an antibody.

Protein surface recognition by porphyrin-based receptors

Protein surface recognition is largely unexplored owing to the large solvent exposed surface and lack of proper molecular scaffolds to match the binding residues. This review describes the design, synthesis, and fluorescence binding studies of functionalized porphyrins aimed at targeting surface residues of proteins through complementary recognition. The pattern of lysine residues surrounding the heme-edge of horse heart cytochrome c has been targeted by tetraphenylporphyrin and tetrabiphenylporphyrin receptors that bind with nano- and sub-nanomolar affinity. Other designed porphyrin-based receptors also recognize potassium channel as a target. The strategies for protein surface recognition offer a new use for porphyrins as molecular scaffolds.

Alkynyl-farnesol reporters for detection of protein S-prenylation in cells

Protein S-prenylation is a lipid modification that regulates membrane-protein and protein-protein interactions in cell signaling. Though sites of protein S-prenylation can be predicted based upon conserved C-terminal CaaX or CC/CXC motifs, biochemical detection of protein S-prenylation in cells is still challenging. Herein, we report an alkynyl-isoprenol chemical reporter (alk-FOH) as an efficient substrate for prenyltransferases in mammalian cells that enables sensitive detection of S-farnesylated and S-geranylgeranylated proteins using bioorthogonal ligation methods. Fluorescent detection alleviates the need to deplete cellular isoprenoids for biochemical analysis of S-prenylated proteins and enables robust characterization of S-prenylated proteins, such as effectors that are injected into host cells by bacterial pathogens. This alkynyl-prenylation reporter provides a sensitive tool for biochemical analysis and rapid profiling of prenylated proteins in cells.

Visible fluorescence detection of type III protein secretion from bacterial pathogens

Type III protein secretion is essential for many gram-negative bacterial infections of host cells and an attractive target for new antibacterial drugs. Here, we describe a bacterial protein effector-carboxypeptidase G2 (CPG2) reporter system for fluorescence and visible detection of type III protein secretion in Salmonella typhimurium. This system provides a general method for measuring protein expression and secretion as well as a high-throughput and quantitative assay for analyzing type III protein secretion inhibitors.

Discovery of a synthetic dual inhibitor of HIV and HCV infection based on a tetrabutoxy-calix[4]arene scaffold

A potential anti-HIV and HCV drug candidate is highly desirable as coinfection has become a worldwide public health challenge. A potent compound based on a tetrabutoxy-calix[4]arene scaffold that possesses dual inhibition for both HIV and HCV is described. Structural activity relationship studies demonstrate the effects of lower-rim alkylation in maintaining cone conformation and upper-rim interacting head groups on the calix[4]arene play key roles for its potent dual antiviral activities.

Clickable fluorescent dyes for multimodal bioorthogonal imaging

Bioorthogonal ligation with functionalized fluorescent dyes enables visualization of nuclei acids, proteins and metabolites in biological systems. Bright and modular azide- and alkyne-functionalized dyes are therefore needed to expand the fluorescence imaging capabilities of bioorthogonal ligation methods. We describe the concise synthesis of clickable fluorescent dyes based on 2-dicyanomethylene-3-cyano-2,5-dihydrofuran fluorophores and demonstrate their utility for multicolor imaging of azide- and alkyne-modified proteins as well as FRET studies.

Combined solid/solution phase synthesis of large surface area scaffolds derived from aminomethyl-benzoates

A set of macrocycles was generated by solid phase synthesis of linear trimers of 5-aminoacyl-3-aminomethyl-benzoates followed by resin cleavage and solution phase macrocyclization. These scaffolds can serve as useful building blocks for molecular recognition studies, especially where differentially functionalized molecular platforms spanning large surface areas are required.

Synthetic non-peptide mimetics of alpha-helices

Proteins in nature fold into native conformations in which combinations of peripherally projected aliphatic, aromatic and ionic functionalities direct a wide range of properties. Alpha-helices, one of the most common protein secondary structures, serve as important recognition regions on protein surfaces for numerous protein-protein, protein-DNA and protein-RNA interactions. These interactions are characterized by conserved structural features within the alpha-helical domain. Rational design of structural mimetics of these domains with synthetic small molecules has proven an effective means to modulate such protein functions. In this tutorial review we discuss strategies that utilize synthetic small-molecule antagonists to selectively target essential protein-protein interactions involved in certain diseases. We also evaluate some of the protein-protein interactions that have been or are potential targets for alpha-helix mimetics.

Simple cation-pi interaction between a phenyl ring and a protonated amine stabilizes an alpha-helix in water

Cation-pi interactions have been proposed to be important contributors to protein structure and function. In particular, these interactions have been suggested to provide significant stability at the solvent-exposed surface of a protein. We have investigated the magnitude of cation-pi interactions between phenylalanine (Phe) and lysine (Lys), ornithine (Orn), and diaminobutanoic acid (Dab) in the context of an alpha-helix and have found that only the Phe...Orn interaction provides significant stability to the helix, stabilizing it by -0.4 kcal/mol. This interaction energy is in the same range as a salt bridge in an alpha-helix, and equivalent to the recently reported Trp...Arg interaction in an alpha-helix, despite the fact that Trp...guanidinium interactions have been proposed to be stronger than Phe...ammonium interactions. These results indicate that even the simplest cation-pi interaction can provide significant stability to protein structure and demonstrate the subtle factors that can influence the observed interaction energies in designed systems.